homemade fresh air machine (hrv), january 2004

A Homemade Heat Recovery Ventilator (HRV)

This data is extracted from the book:

ENERGY CONSERVATION IN HOUSINGA Collection of Data onEnergy-Efficient Housing Approaches

by: David L. Meinert

The printed version of this book was published by Vantage Press New York / Los Angeles in 1990 and re-printed in 1992. As of 1996, it was out of print.I made an electronic version, which I re-edited in 2003

++++++++++++++++ NOTE: This Google blog did NOT put any of the drawings / graphics necessary to understand a lot of the information.

However, I have the complete (PDF) and Word documents of the text and graphics on this site. Go to Home of this site, and look at Files which lists the book text in PDF format, as well as additional files on other insulation and energy saving topics.

I also have the complete (PDF) documents of the text and graphics on a Yahoo web-site.

The Yahoo site includes some photos of an attic and basement insulation project. Use this link to connect to it.

http://groups.yahoo.com/group/EnergyConservationHousing

(Expect to have to sign in to the Yahoo group to access the data.)

Starting in November 2008, this data has been available on a different site (Multiply.com).

http://energyconshousing.multiply.com/

The main improvement on this Multiply site is the ability to add videos. I added some insulation videos that can be viewed on the Multiply site. The Word and PDF documents are also on this site. However, my home computer has been unable to open those documents on the Multiply site due to some problem with JavaScript. If you run into similar problems, the best source for the complete PDF and Word documents is on the above Yahoo site.

Expect to have to sign-in to the Multiply site to access the data.

Previously the text Word and PDF documents on the Multiply.com site were on an MSN site. http://www.msnusers.com/EnergyConservationInHousing The MSN sites was closed in February 2009.

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Energy Conservation in Housing

The information on these pages describes a fresh air ventilation device.


What is an HRV? It is a ventilation device that blows out exhaust air while it brings in fresh air from outside. It is designed to use the exhaust air to pre-heat the incoming outside air. In this way, in the dead of winter, you can get a continual supply of fresh air into your home, without the cold drafts and heat loss associated with open windows. Such a device is known as a Heat Recovery Ventilator (HRV) or perhaps more descriptively as an air-to-air heat exchanger.

++++++++++++++++ NOTE: This Google site did NOT put any of the drawings / graphics necessary to understand a lot of the information. However, I have the complete (PDF) documents of the text and graphics on a Yahoo web-site.

The Yahoo site includes some photos of an attic and basement insulation project. Use this link to connect to it.

http://groups.yahoo.com/group/EnergyConservationHousing

(Expect to have to sign in to the Yahoo group to access the data.)

Starting in November 2008, this data has been available on a different site (Multiply.com).

http://energyconshousing.multiply.com/

The main improvement on this Multiply site is the ability to add videos. I added some insulation videos that can be viewed on the Multiply site. The Word and PDF documents are also on this site. However, my home computer has been unable to open those documents on the Multiply site due to some problem with JavaScript. If you run into similar problems, the best source for the complete PDF and Word documents is on the above Yahoo site.

Expect to have to sign-in to the Multiply site to access the data.


Previously the text Word and PDF documents on the Multiply.com site were on an MSN site. http://www.msnusers.com/EnergyConservationInHousing

All MSN sites are scheduled to be closed in February 2009.

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The above diagram shows only one heat exchange plate. In actuality more plates are used to get a better recovery of heat. These ventilation devices are sold commercially, in various types.

For the homemade version I designed, I used 10 exhaust chambers alternating with 10 intake chambers similar to the alternating plates in the counterflow heat exchanger diagram shown below.

For the homemade heat exchanger, the heat exchange plates are aluminum, sold as aluminum flashing commonly available in hardware stores. The plates are spaced apart, using wood or plastic lumber strips, to keep the heat exchange plates at an even spacing. As I describe in this text, it is possible to make such a ventilation device, using some commonly available items, found in hardware and building supply stores.

The idea is to circulate the incoming air throughout the house, and to exhaust stale air from the house, such as from bathrooms. In my house, I connected two bathroom exhaust ducts, and added a hallway exhaust duct, to go to the exhaust side of the heat exchanger. For the incoming fresh air, I simply brought it to one point in my house, where it entered into the basement, instead of adding extra ductwork to each room.


Air-to-Air Heat Exchangers: Homemade ModelsA commercial air-to-air heat exchanger can be expensive. There are methods to fabricate

a heat exchanger with the needed materials, mechanical skill, and time. The complete cost of materials for a homemade whole-house heat exchanger can be half of the cost of an equivalent commercial model. Making an air-to-air heat exchanger requires extensive work in the time spent gathering the assorted materials and assembly of all the parts. In addition, it may be difficult to produce final performance comparable to some of the well-designed commercial models. If not properly made, any retail model will be better.

History of homemade heat exchangersAs energy costs rose, new ways were found to build homes having far less space heating

demands. Due to the nearly airtight construction used in these new homes, indoor air tends to rapidly become stale. Before long, techniques were being devised to provide fresh air for energy efficient homes by recovering heat from the exhaust air. Commercial models of air-to-air heat exchangers were not initially available, so individuals and groups began to design heat exchangers for residential use.

In the late 1970s, the Mechanical Engineering Department at the University of Saskatchewan, Canada, designed a heat exchanger using polyethylene vapor barrier as the exchange material. The polyethylene sheeting was wrapped around plywood spacer strips to form a parallel plate counterflow exchanger. Over the years, they designed two different models of this exchanger. The first model used -inch thick strips of pressure-treated plywood. The later model used 5/16 - inch plywood and acoustical sealant to prevent moisture from entering the edge of the wooden spacer strips. In the second model, each exchange plate had a net size of 84" x 21", providing about 340 square feet of internal surface area for the 28 exchange plates. The exchanger was about 8 feet in overall length x 2 foot wide by 10 inches thick.

In 1985, the same group in Saskatchewan published a design (Solplan 6) of a new exchanger made from a rigid plastic sheeting material called coroplast. Coroplast is a double-wall sheet of rigid plastic. The air is made to flow through the spaces within the coroplast for one air stream; air is passed on the outside of the coroplast for the other stream of air. The homemade coroplast exchanger is a crossflow core, with the exhaust air routed in a double pass through the exchanger. The design is far more compact than that of the earlier counterflow model, with the final size of the core about 36 x 36 x 12 inches.

Polyethylene sheeting as used in the original counterflow model lacks rigidity. When the fans force air through the exchanger, the polyethylene exchange plates tend to billow and close off the air passageways unless the plastic is stretched very tightly during installation. Polyethylene exchange plates tend to cause very high internal air resistance when both blowers are operational. This requires much electrical power to get reasonable airflow.


Coroplast is far superior to polyethylene for use as heat exchange plates. Unfortunately, coroplast is not widely available. It is manufactured in Canada in 4 x 8 foot sheets about -inch thick. The size makes it difficult to order coroplast by mail when not locally available. A number of commercially available models use coroplast as the exchange medium, having exchanger cores of single-pass and double-pass crossflow as well as counterflow designs.

A homemade design for air-to-air heat exchangers(as designed by the author of this text)

This text describes a homemade counterflow heat exchanger using aluminum flashing as the exchange plate material. As a heat exchange medium, aluminum is superior to plastics since it has a conductivity hundreds of times higher, allowing good heat transfer with less surface area. Plywood strips can be used as spacers for the aluminum plates. Use plywood about -inch thick, cut in strips 1-inch wide, cut to the proper lengths. Seal the edges of the plywood with acoustical sealant, or equivalent, to prevent moisture from entering the edges of the wood. By its nature, plywood could eventually rot from the moisture condensing in the exchanger core even when sealant protects the wood. Plastic lumber can be used as spacer strips (one can find information on plastic lumber by a search of the Internet). When I designed this heat exchanger in 1988, I used a brand of plastic garden "bender board" which was likely one of the first types of plastic lumber I used it instead of plywood spacer strips. Bender board was available where I lived at that time (California, in 1988), but I never found it again after I moved to the East Coast of the US.

Plastic bender board, intended to be placed in the ground as edging for gardens, is resistant to both cold and moisture. Bender board came in 40-foot rolls, about 3-inch wide x -inch thick. To provide support and spacing for the aluminum sheets, a 1-inch width of the bender board is sufficient. The roll could be cut into 1-inch widths and the needed lengths with a table saw. Each roll then provides about 120 feet of 1-inch strips. The exchanger design is a parallel plate counterflow design about 5 feet in length, with 21 aluminum exchange plates.


Homemade air-to-air heat exchanger core materials list:(Materials list from 1988)

Use some form of material for spacer strips such as plastic lumber. Years ago, when I made my heat exchanger, I found a plastic material, that did the job. It was plastic bender board, intended to be used as edging in gardens. I used 4 rolls of the plastic bender board ("Plastiform" Lawn and Garden Bender Board, durable redwood grained plastic, manufactured by Kerber Associates, Inc., 1260 Pioneer Street, Brea, CA 92621, (714) 871-2451) . . . . about $10 per roll. In the present day, you could use an equivalent amount of plastic lumber to have spacer strips about thick. If you cant get thick plastic lumber, you might still get reasonable heat recovery using the more commonly available 1 thick plastic lumber as spacer strips. Such an exchanger would use less aluminum (due to half the number of exchange plates). (Using 1 thick plastic lumber should make it easier to seal at the ports.)

2 rolls of 50-foot long aluminum flashing. The 14-inch width is perhaps the minimum width that should be used. The 20-inch width will provide 50% more exchange area for a larger-capacity airflow. Flashing is also manufactured in 24- and 28-inch widths, although most hardware stores may not stock the wider sizes.

5 pounds of 1 - inch (5d) galvanized box nails pound of 1-inch (2d) galvanized box nails 2 tubes of silicone sealant One box of staples inch or longer and staple gun. (More nails can alternatively be used instead

of staples, although staples make it easier to hold parts of the exchanger in position during assembly.)

Pop rivets or sheet metal screws for assembling sheet metal sections10 feet of rigid angle metal ( x -inch, to form the rigid metal attachment flange)1 or 2 hose or tubing connectors as condensate drains (3/8 to inch in diameter)4' x 8' sheet of rigid insulation, to 1-inch thick.One large section of sheet metal (perhaps 3 foot x 10 foot in size) cut to specific dimensions to

cover the exterior of the exchanger (alternatively, plywood can be used). Additional remnants of sheet metal (to make endplates and expansion chambers)30 feet of 1 x 1 inch sheet metal angle strip. (Alternatively, 1 x 1 - inch size can be used)Sheet metal duct pipe, 6 inches in diameter. Four duct pipe sections, at least 4 inches long each,

will be needed. Two of the sections should have a tapered end. To directly install duct booster fans in the pipe, two of the sections should be made about 8 inches long. If tapered ends are desired for each connector, then four original duct pipes are needed.

Procedure. Cut the rolls of bender board (or plastic lumber) into 1-inch-wide strips. Bender board strips cut and nail similar to wood, although they are far more flexible and difficult to handle when cutting. Make the following lengths of the 1-inch-wide strips:

51" long strips 80 needed 8" long strips 80 needed 14" long strips 4 needed (for the outer layer on each side)

50" long strips 4 needed (for the outer layer on each side)


I tested my heat exchanger design, and found it had about 72% heat recovery. (The inside house / exhaust temperate is 68 F; the outside temperature is +8 F. The incoming air is pre-heated to 51 F. Inside to outside: 68-8 = 60 T; intake air pre-heat: 51 - 8 = 43 T; 43 60 = 71.6% recovery)


NOTE. Consider using plastic lumber, potentially cut from thick material, to serve as the spacer strips. If -inch thick plywood is used instead of plastic bender board, then 40 strips of 51-inch length and 40 strips of 8-inch length plywood will serve as spacers. Alternatively, if the exchanger is made 3 inches shorter, 48-inch long plywood strips with 51-inch long exchange plates could be used. The edges of the wood must be caulked to block condensed moisture from entering the wood in order to prevent later rotting. Wrapping the plywood strips with 6 mil polyethylene could also prevent moisture from entering the edges of the wood; it will still be necessary to use sealant at the edges of the plywood strips to keep moisture from getting around the polyethylene into the ends of the wood strips. Pressure treated plywood might alternatively be used.

Cut the aluminum flashing into 54-inch lengths (each sheet will be 54 x 14 inches for the small size exchanger). Each aluminum roll will make 11 plates; 21 plates will be needed for the exchanger. There is a one-inch overlap of the aluminum plates at the port ends of the exchanger to allow the aluminum plates to be folded together when sealing the ports; the plates are 54 inches long and the plastiform (or plastic lumber) laminations are only 52 inches long.

Each exhaust lamination is two layers of plastiform thick; each intake lamination is also two layers of plastiform thick. The resultant air space is nearly inch on either side of each exchange plate.

The exchanger is assembled in sandwich fashion: an exhaust lamination, an aluminum exchange plate, an intake lamination, an aluminum plate, an exhaust lamination, an aluminum plate, et cetera. It is difficult to get the first few layers started, since one is connecting exhaust laminations to intake laminations through an aluminum plate, not providing enough thickness for nailing until there are at least 5 layers of plastiform strips. To get the process started, attach an intake and exhaust layer on either side of one aluminum plate by way of inch staples. After 5 layers of plastiform, the 2d nails can be used. Once there are 9 layers of plastiform, the 5d nails can be used.

Using 1 thick plastic lumber should make it easier to seal at the ports. The aluminum could be bent over and nailed to the adjacent plastic lumber, perhaps taking less time and effort than the above-described rolling and flattening of the aluminum plates. (This process is described in the above diagram, and in more detail in the diagram on page 9.)


Continue by alternating the laminations of exhaust and intake plastiform. Always use an aluminum plate in between each double layer of plastiform strips. While staples are convenient in holding the plastiform strips in place temporarily, it will be necessary to use 5d nails every 3 layers of plastiform strips to hold the exchanger core together. The nails should be spaced about every 1.5 inches along the plastiform strips to ensure a fairly good air seal, but do not use nails through the 5" spaces reserved for the ports. Stretch the aluminum plates and plastic strips tightly when stapling in place to prevent bunching up the materials. Continue the assembly until there are a total of 10 intake laminations and 10 exhaust laminations. There are 19 exchange plates between the laminations and 2 plates on the outside of the exchanger, for a total of 21 plates. The 19 internal plates provide the heat exchange surface. The outer plates are secured in place by the 14-inch and 50-inch plastiform strips, using a nailing pattern similar to the earlier layers. Be careful not to nail through the 5-inch spaces reserved for the ports. The thickness of the central core will be about 9 inches. There is a possibility of some air leakage through the layers of plastiform strips; a tight nailing pattern will minimize leakage out the sides. Near the ends of the exchanger where plastiform strips attach at right angles there is more possibility for leakage; the outside of the exchanger should be caulked along these joints. As a finishing touch, the two plastic sides of the exchanger can be covered with aluminum flashing, requiring two more sheets of flashing, about 52 inches long each. With the sides of the aluminum sealed under the final plastiform strips and the ends caulked at the expansion chambers, any air or moisture that leaks between the plastiform strips will be trapped in that space and will not allow further leakage.

The ports are sealed by the following procedure: (1) Cut away a section of aluminum between the ports; (2) fold the aluminum around the plastiform strips to leave a clear opening for each port; and (3) caulk the edges of the folded aluminum to prevent air leakage from the opposing air stream.

The aluminum flashing material might have a coating of oil on it from the factory. If desired, you could clean off the metal surface before or after assembly. After I completed my basic exchanger core, I soaked the exchanger in a large container of soapy water to dissolve the oil and any dirt introduced during assembly. (Actually I filled a large garbage can with soapy water, soaking the core, one end at a time, to dissolve the oil. Then I rinsed thoroughly with a garden hose to remove the soapy water.)


Note: I used the Superbooster fan set-up (shown below) when I first installed my home heat exchanger. By the time I needed replacement fans, that company (Ancor Industries) was apparently out of business. I was able to find a suitable replacement in local hardware stores. (The Tjernlund company makes such duct boosters, WITHOUT the vibration-absorbing foam rubber feature.)


When the central core is completed, an expansion chamber is attached to both ends of the exchanger. Two pieces of sheet metal will be needed, 5 by 50 inches in size. These are wrapped around the port ends of the core to make a rectangular tube, holes are drilled through the sheet metal, and the expansion chamber is nailed to the plastiform perimeter at each end of the exchanger. Another piece of sheet metal (4 x 10 inches) is installed as a septum to divide the exhaust and intake airflows. The septum is formed by making inch folds on two ends. (The final septum size is 4 x 9 inches for a 9-inch thick exchanger core). Where the septum contacts the exchanger between the ports the seal can be made more secure by cutting a score line (with a hacksaw) in the plastiform and setting the septum in the groove before sealing with caulk. All the internal joints of the expansion chamber and septum must be caulked to prevent air leakage and cross-leakage. Attach rigid angle metal to the perimeter edge of the sheet metal and to the septum to allow for attachment of the end plate at a later time. The end plate will hold the two duct pipes and the condensate drain. Rigid insulation covers the central core; sheet metal or plywood covers the exterior of the exchanger. For best results, the exchanger made from 14-inch wide exchange plates should have 6-inch-diameter ducts; 4-inch-diameter ducts will provide sufficient airflow for only 100 cfm capacity. Careful measurements are necessary when making the endplates, since the spacing is very tight.

Once the exchanger is assembled, it must be hooked to the appropriate duct connections for fresh air supply and exhaust air removal; fans are attached to move air through the exchanger. Theoretically, if the home is tightly constructed, when air is exhausted fresh air automatically will be drawn in through the fresh air ductwork of the exchanger; hooking up bathroom and kitchen exhaust fans to the exchanger would serve this purpose. By exhausting air (without active fresh air supply), a negative air pressure is created in the home, potentially increasing radon infiltration as well as driving infiltration through any break in the vapor barrier. In practice, most exchangers have both exhaust and intake fans.

Air movement can be provided by a twin centrifugal blower (both air streams moved by one motor) or by separate axial fans or centrifugal blowers. The fans can be built into the warm end of the exchanger instead of directly attaching the endplate, or a separate blower housing can supply the air to the exchanger through ducts. Commercial heat exchangers use either detached fan modules or built-in fans. Either method is suitable for this exchanger, depending on the preference of the builder. I found that the easiest method is to use "duct boosters" in the duct connections next to the endplate. See the listing of fan and blower suppliers on page 18.

Filters should be installed to reduce dust accumulation in the exchanger. (See page 153 for updated details on filters for this homemade exchanger.) For up to 200 cfm airflow, 10-inch x 10-inch filters, installed in the duct system before air reaches the exchanger core, should provide adequate filtering.


The filter housings can be homemade from sheet metal or plywood, with duct connections on both sides of the filter housing. There are filter accessories from heat exchanger companies for this exact purpose. (See product listings.) In the bathroom and kitchen provide exhaust ducts for the exchanger. In the kitchen use a re-circulating range hood with a grease trap instead of venting directly to the exchanger; the exchanger duct in the kitchen removes the exhaust. The clothes dryer should be vented directly outdoors; lint from the dryer would quickly clog the exchanger. There are some indoor dryer vent diverters sold for use with electric clothes dryers. These diverter switches are used during winter to put the dryer heat (and moisture) in the house instead of putting it outdoors. If this diverter is covered by a nylon mesh, most of the lint can be trapped before getting into the house air. (e.g., Cover the diverter with one "leg" from a section of nylon hosiery to catch most of the lint that would otherwise be expelled.) The moisture released will tend to raise the humidity in the home excessively. If there is an effective heat exchanger system, the moisture will be removed within a few hours. (Authors comment: When I tried venting dryer air inside my house, I found an increase in mold/mildew deposits on paper products stored near the laundry area. This occurred even with the presence of the air-to-air heat exchanger. I believe that venting a dryer indoors it usually a bad idea, even if an air-to-air heat exchanger is used in the house.)

Under no circumstances should a dryer vent diverter be used with a dryer burning fuel for its operation. (The combustion by-products must be expelled outdoors.)

The pre-warmed fresh air should not be routed directly into the cold air duct of the furnace. (It should be ducted no closer than 10 inches from the cold air inlet of the furnace.) If the heat exchanger air is directly ducted into the furnace air plenum, the furnace fan would make the airflow through the exchanger dangerously out of balance, since the furnace fan is far more powerful than the exchanger fans.27 The air from the heat exchanger should be distributed to all rooms of the house for best air circulation, such as by putting ducts through open cavities of internal walls, floors, and ceilings. If the fresh air is brought to only one point, new air will not be obtained in rooms away from the fresh air supply.13

The fans used to run the heat exchanger should be able to move the needed amount of air without overworking the motor. Some axial fans are not strong enough to move air against much resistance, whereas most centrifugal fans are better able to maintain airflow despite moderate resistance. For the size exchanger described in this text, up to 150 cfm will give reasonable efficiency. An airflow rate of 150 cfm will provide 0.5 air changes per hour for a two-story home, 1,125 square feet per floor, with 8-foot ceilings. (150 cfm x 60 minutes = 9,000 cubic feet per hour. 1,125 sq ft x 8 ft ceiling x 2 floors = 18,000 cubic feet of house air. 9,000 cubic feet per hour 18,000 cubic feet per air change = 0.5 air changes per hour.) In a well-sealed house, infiltration may supply 0.1 air changes per hour. Thus to obtain 0.5 air changes the exchanger need supply only 0.4 air changes per hour.

Higher flow rates are possible with this exchanger. However, the port openings will cause significant airflow resistance. There are 10 ports for exhaust and intake, each 5 inches long and 7/16 inches wide. This leaves only 22 square inches of open space for the air to enter the exchanger for each direction of flow. Six-inch duct pipes allow 28 square inches for airflow. (4-inch duct pipe provides only 12.5 square inches cross-sectional area.) Most axial fans cannot force more than 150 cfm through a 22 square inch space, although centrifugal blowers may have the power to nearly double the airflow rate. Unfortunately, substantially increased electrical power is needed to run centrifugal blowers.


Using 20-inch-wide aluminum flashing to make the exchanger, the port openings will be 8 inches long, providing a 35-square-inch area for the 10 laminations of exhaust and intake. This would allow nearly 250 cfm airflow rates with axial fans. Alternatively, using more laminations of the 14-inch-wide exchanger plates will allow higher airflow rates.

Under very cold outdoor conditions, moisture can freeze on the exchange plates in the exhaust air stream; if sufficient ice accumulates, the exchanger will be unable to function. Defrosting can be accomplished by turning off the heat exchanger (requiring a very long time for the core to thaw out) or routing air at room temperature through the exchanger when the cold air fan is off. Solplan 6 suggests that defrosting can be done automatically by having the intake air blower hooked to a 24-hour timer. The timer will shut off the cold air blower on the schedule set on the timer. If the outside temperature is above +14F, no defrosting is needed. With outside temperatures at 0F, a 30-minute shutoff every 24 hours is sufficient. At -40F, a 30-minute shutoff every 12 hours will allow defrosting. During the defrost mode, the warm air from the house is still being exhausted. The heat of the exhausted air will serve to defrost the frozen core.27 Under ordinary conditions (when the house is closed up during cold or very hot weather), the heat exchanger should be run continuously at the flow rate needed to provide fresh air. (Usually 0.4 to 1.0 air changes per hour is sufficient.) Wiring the exchanger to a humidistat is not correct, because the inside humidity level is not necessarily an indication of the level of indoor air pollution.

During mild weather, it may be preferable to open the windows for ventilation, instead of using the heat exchanger. If the exchanger is to be used to pre-cool hot, humid air in summer, there may be condensation on the intake air passageways. For this reason, a condensate drain can be installed on the warm air end of the exchanger for summer use. However, I have found in 14 years of use in my house, in hot, humid summers, that condensation is not an issue on the warm air end of the exchanger.

The final efficiency of the exchanger will be better if the exchanger is made thicker, using more exchange plates. Having 50% more exchange plates will allow 50% more airflow with no loss of efficiency. It is also possible to make single layers of plastiform (exhaust and intake spacers) to separate the exchange plates; doing this will use 41 aluminum plates, doubling the exchange area (and doubling the cost of the aluminum). However, with single layers of plastiform, the spacing will be so tight that sealing the ports (by rolling the aluminum plates) will be very difficult.


The exchanger can also be made wider (20 to 28 inches instead of 14 inches) to provide greater exchange area and airflow capacity. When making the exchanger wider than 14 inches, it is necessary to increase the port length to allow more airflow. (For a 20-inch exchanger width, use 8-inch ports; for 28-inch exchanger width use 10- to 12-inch ports.) There should be at least a 2-inch overlap of the plastiform strips between the ports to allow adequate space for nailing. With a port size larger than 8 inches, it may be very difficult to fold the aluminum plates to make the port openings as described in the diagram "Sealing the ports of the heat exchanger." The force needed to roll that amount of aluminum is probably more than the 3/16-inch steel rods can withstand.

In constructing the basic core of the 14-inch-wide exchanger, the assembly time was about 30 hours and the cost of material was $220, plus the fan costs. See the product listings for possible sources of blowers and fans.

The ductwork of the exchanger system goes through the exterior walls for fresh air intake and exhaust air removal. The sections of duct pipe going through the wall should have a shield to keep out the rain and a screen to keep out insects and birds. Position the fresh air duct away from possible sources of contaminated air (away from car exhaust, fireplace smoke, septic vent pipes, and the exhaust duct of the heat exchanger).

Although 4-inch duct vents (as used for clothes dryers) going through exterior walls can be obtained for several dollars, the 6-inch sizes usually cost substantially more.


If the exchanger and duct connections do not cause equal airflow resistance, flow balancing between the two air streams is necessary. The air streams are balanced by installation of dampers in the exhaust and intake exchanger duct pipes within the house.

Test and adjust flow balance on a calm day. Open one window and seal it with a loose cover of polyethylene sheeting. Turn on the heat exchanger with both dampers fully open.

If the plastic bows or curves outward, the house has positive pressure due to excessive intake airflow. Gradually adjust the damper on the intake airflow until the plastic sheet is limp, curving neither in nor out. At this point the two flows are balanced.

If the plastic curves inward, the house has negative pressure, requiring that the damper on the exhaust side be adjusted to balance the airflow.27

The exhaust air ducts should be located high on the wall, where the most humid and stale air is present. (Cooking and showering give off heat and humidity, which will rise.) According to heat exchanger manufacturers, the fresh air ducts should also be located high on the wall to allow the cooler fresh air to mix better, instead of staying closer to the floor. Alternatively, if you have bathroom exhaust fans already installed, you can route the exhaust ducts into an air chamber leading to the heat exchanger.

When the clothes dryer is in operation, it will induce some imbalance of the airflow between exhaust and intake air of the house since it is adding to house air exhaust and not to air intake. It is possible to reduce this imbalance by venting outside supply air directly to the dryer or to make a small capacity heat exchanger core specifically for the clothes dryer. The dryer fan system will drive both airflows and no additional fans are needed. Such a heat exchanger core must be provided with filters and frequently cleaned to remove collected lint. (Putting such detail into the clothes dryer exhaust system seems to be way too much work.)


Blowers and fans that one could use in a homemade air-to-air heat exchanger. Such items can be purchased from a hardware store, or from local heating, ventilation, or "electric motor" suppliers. It may be possible to locate motors of some wholesale fan/blower manufacturing companies through their distributors or repair people. Local distributors might be found in the Yellow Pages under "Electric Motors," "Heating," or "Ventilation." Below are some manufactures of blowers and fans.

1) Tjernlund Products, 1601 Ninth Street, White Bear Lake, MN 55110-6794; (615)426-2993; 800-255-4208This company markets duct pipe fans typically available in hardware stores (as of September 2003). See comments on page 153 about fan blade shape and airflow problems (and how to resolve them). They are easily adapted for use in the homemade air-to-air heat exchanger.

2) Some other sources for duct booster fans, found by search of the Internet, as of September 2003. A. Smarthome.com has duct boosters for 4 to 12 diameter ducts.B. ESP ENERGY; 1615 Newberry; Racine, WI 53402; (262) 681-9288; 1-888-551-9288. Has in-line duct

fans for 4 to 12 ducts, plus several other similar fan versions.C. Empire Ventilation Equipment Co. 35-39 Vernon Blvd; Long Island City; NY 11106 (718)728-2143.

Has in-line duct fans for 6 to 12 ducts.D. Aero-Flo Industries; P.O. Box 358; Kingsbury, IN 45645-0358; (219) 393-3555. Has 6 in-line duct fans.

3) Ancor Industries, 1220 Rock Street, Rockford, IL 61101, (815)963-7100. (This company apparently out of business, as of 1994.) This company sold fans easily installed in duct pipes. Their Superbooster duct boosters were intended for use in heating and cooling duct pipes to improve airflow to selected rooms. Motors were low-wattage and were made in 120 Volts (AC), 24 Volts (AC), and 240 volts (AC); sound insulated with foam rubber. I found the 6-inch size fans very well suited for the homemade heat exchanger (using 14-inch wide aluminum plates) that I describe on pages 97-109. Below were the sizes and specifications during the time they were made.

Duct size Model Volts Amps Watts total CFM Price Shipping 5 inch 15-0070 115 V 0.37 15 80 $39.95 $3.006 inch 15-0071 115 V 0.40 20 110 $39.95 $3.007 inch 15-0072 115 V 0.44 23 150 $39.95 $3.008 inch 15-0073 115 V 0.46 25 200 $39.95 $3.00

4) Broan Manufacturing Company, P.O. Box 140, Hartford, WI 53027. This company makes various ventilation products. (As of September 2003) Broan makes Heat Recovery Ventilators, and has fans and blowers available as replacement parts for a large variety of ventilation units they market. Below are listed a few of these fan and blowers with some related statistical data (from 1990). To vary motor speeds for these models, model no. 57 solid state infinite speed control, 3 amp capacity ($22.95) is suitable.

Broan Centrifugal blowersPart # total CFM Used For Model # Price Amps 97006021 100 ventilator 360 $58.60 0.797006022 160 ventilator 361 $61.30 1.0

Broan Axial room-to-room fans: Units complete with housingPart # total CFM Used For Model # Price Amps 6 inch fan 90 Ventilating 512 $33.95 1.0 8 inch fan 180 Ventilating 511 $78.95 1.5

Broan Axial range fans: Fan and bracket onlyPart # total CFM Used For Model # Price Amps 97005163 190 range hood 42000 $21.90 0.8 (two speeds)97005161 160 range hood 40000 $21.20


Broan Twin centrifugal blowers ("dual blowers")Part # total CFM CFM/stream Used For Model # Price

Watts 97006152 200 100 range hood 76000 $46.40 225 97005985 360 180 range hood 88000 $66.80 155 97007542 440 220 range hood 89000 $87.40 270 97006023 200 100 Ventilator 362 $75.70 115 97006024 300/310 150 Ventilator 363, 383 $81.60 165 97007074 960 480 Ventilator 366 $167.90 390

5) Northern Tool & Equipment Co., P.O. Box 1499, Burnsville, Minnesota 55337-0499. 1-800-533-5545. This mail-order company occasionally has surplus fans and blowers at a low cost that could potentially be used in a heat exchanger.

Below are listed other types of fans and blowers made by wholesale manufacturers. (as of 1992)

1. Howard Industries, P.O. Box 287, Milford, IL 60953. This manufacturer makes a number of sizes of low-wattage axial fans. The manufacturer sells only to distributors. Write to the manufacturer for a listing of distributors. Below are listed a few of the 115 volt models.

Model # CFM Diameter Watts Price (plus power cord) 2672 70 cfm 4.7" 17 W. $25.74 $1.114315 100 cfm 4.7" 19 W. $25.74 $1.116052 120 cfm 6.72" 10 W. $52.76 $1.115812 180 cfm 6.72" 18 W. $55.54 $1.115804 240 cfm 6.72" 30 W. $42.85 $1.110101 560 cfm 10.0" 37 W. $55.46 $1.11Power cord, 24" long, Model No. 6-170-672, $1.11 each

2) Fasco Industries, Inc., Motor Division Headquarters, 500 Chesterfield Center, Suite 200, St. Louis, MO 63017. The manufacturer sells only to wholesale distributors. It may be possible to locate some of their distributors or repair people through the yellow pages. The manufacturer makes a number of different types of blowers and fans for specific commercial applications. Below are specifications of a few of the many blowers made by Fasco.

75 cfm, Model B75, 115 V, 0.59 amps. 105 cfm, Model 50757-D500, 115 V, 0.55 amps. 120 cfm, Model 50746-D500, 115 V, 0.72 amps. 160 cfm, Model 50755-D500, 115 V, 1.0 amp. 180 cfm, Model B47120, 115 V, 1.95 amp. 212 cfm, Model A212, 115 V, 1.25 amps. 320 cfm, Model 50756-D500, 115 V, 1.3 amps.


Air-to-Air Heat Exchanger -- Update information (as of 1995)In 1988 I designed and constructed a homemade air-to-air heat exchanger. (This is described on

pages 96 to 109). I installed the heat exchanger that same year in our new home, using the Superbooster duct fans as described on pages 109 and 143. After more than 6 years of faithful service, the intake fan motor wore out. Being unable to obtain a replacement Superbooster (the company apparently went out of business), I then replaced the fan with a Tjernlund* fan designed for 6" ducts. I soon found the new fan had a difficult time maintaining adequate airflow. The motor often labored and ran down to very low RPMs. This was particularly the case if the inlet airflow was even slightly restricted. By comparing the Superbooster to the Tjernlund fan, the main apparent difference was with the fan wheel itself. The Superbooster fan wheel was made from a circular aluminum disc, cut and shaped to have 10 pie-shaped blades to move the air. The Tjernlund fan wheel was a plastic disc, molded to have 4 fan blades. The Tjernlund fan wheel caused a larger volume of air to move since the blades struck a much larger area of air per revolution. If the inlet air was restricted (by filters and bends in the ventilation system), the Tjernlund fan ran down. Yet under restricted airflow the Superbooster fan seemed to actually speed up. I found that by reducing the size of the Tjernlund fan blades, I could prevent the motor from being overloaded by reducing its airflow capacity to be better suited for the 6" ducts and filters of the heat exchanger. The below diagram shows how to trim the fan blades. It is necessary to make exact measurements and careful cuts (such as by using a coping saw, and then filing the edges smooth) to keep the fan wheel in balance for when it is reinstalled on the motor.

When I first installed the heat exchanger, I used a screened inlet to keep insects out, and then I used an in-line dust filter. Over time, I found conventional ("furnace") filters are NOT effective in keeping dust out of the heat exchanger; dust also eventually clogs the fan blades and motor. I found that " thick foam rubber (such as used for carpet padding or for thin cushioning) makes a very effective filter for dust. I covered the outside air inlet with " foam rubber and made an additional 10" x 10" in-line filter (using foam rubber) to keep dust out of the exchanger. I also used the same filtering system on the exhaust side, prior to air reaching the exchanger. It is typically necessary to remove the intake filters and clean them (using soap and water) usually every few months since they get progressively clogged with dust over time.

I later obtained filter foam from a store specializing in foam (and foam mattresses). The filter foam I obtained in 1-inch thickness. I now use the filter foam as the in-line air filter. I also use the new filter foam to replace the previous foam rubber, to cover the outside air inlet for the air-to-air heat exchanger duct. I use the filter foam to replace the foam rubber for the inside of the exhaust air plenum, to block dust from ever reaching the heat exchanger on the exhaust side.

I also used this type of filter foam to cover the two intake vents for our whole house ventilation ducts, for the forced air heat system. (Actually I removed the intake grills, and found I could fit filter foam inside that location.) I first covered the internal duct with hardware cloth (which is a wire mesh). I put the filter foam against that wire mesh. The intake air must then flow through the filter foam, before it enters the ductwork. Now when I clean the furnace filter I clean 3 filters: both of the intake filters AND the regular filter inside the air handling unit of the forced air heater / air conditioner. The extra intake filters block additional dust from entering the forced air ductwork of the house.

* Tjernlund Products, 1601 Ninth St., White Bear Lake, MN 55110-6794

Additional Data on Ventilation

Infiltration of Outside AirIn many houses there is a significant amount of leakage of outside air. The outdoor air

leaks into the house through any crack or crevice of the wall, floor, or ceiling or the air passes directly through porous wall sections. This leakage of outside air into a building is referred to as infiltration. When driven by even mild winds (fifteen miles per hour), a conventional home can undergo one to four complete changes of air in one hour.1 Infiltration makes a house feel drafty and uncomfortable in winter and significantly increases the heating bill, since heated air is forced out with infiltration. Exfiltration is the term describing heated air forced out of a building.

By contrast, a well sealed home can undergo one air change in two hours (one-half air change per hour). With the use of a continuous vapor barrier surrounding the living area and properly sealed doors and windows there can be less than one air change every 12 to 24 hours (less than 0.1 air changes per hour).

In the early years of energy efficient buildings, reduction of infiltration and improvement of the level of insulation made homes far more economical to heat. Before long it was found that there were side effects of having a home extremely well sealed. The first effect, obvious within hours, is the tendency for water vapor to collect inside the home. High water vapor content in the air can be suspected when moisture condenses on double-pane windows. Other side effects occur over time as the occupants breathe contaminants and germs trapped in stale inside air.

Such indoor air pollution is typically worse than outdoor air quality. The pollution can come from a variety of sources: carbon monoxide from incomplete combustion of stoves or heaters, carbon monoxide and other by-products from smokers, chemicals emitted from new building materials, excessive water vapors, radon from the soil and some masonry building materials, and other miscellaneous contaminants. Radon, a dangerous radioactive gas from uranium, can be present in soil anywhere. As well as getting into some building materials, it can enter homes through foundation cracks, porous cement, sump pump drains, pipes entering the foundation, and well water. A thoroughly ventilated crawl space below a well-sealed floor eliminates most sources of radon entering the house. Radon is second only to smoking as a cause of lung cancer. 12 If the degree of infiltration is less than 0.25 to 0.5 air changes per hour, then the building will tend to have a higher concentration of contaminants than is healthy. Infiltration is the only source of fresh air in conventional homes. Forceful winds cause fast rates of infiltration; minimal winds result in too little infiltration. Therefore, infiltration is not a reliable way to get a consistent supply of fresh air even in conventional homes. Infiltration with conventional homes is related to the speed of the wind, not to the amount of contaminated air.

It is necessary to have a good vapor barrier to keep moisture from condensing inside the insulation of walls, floors, and ceilings. A properly installed vapor barrier will block much of the infiltration. It may then be necessary to ventilate the house to provide the needed fresh air.


VentilationVentilation means bringing fresh air into a building and exhausting the stale air to the

outdoors in a controlled fashion. Fresh air is easily provided by opening windows, which increases the heating bill and causes drafts in cold weather. Another approach is to bring outside air in through the furnace cold air duct, allowing the air to be heated on its way inside the house. Specific areas of the house should be ventilated to remove moisture, odors, and contaminants. Usually the kitchen, laundry room, and bathrooms should have air exhausted to the outdoors. Exhaust fans have a difficult time getting enough replacement air to vent adequately in a tight house. Instead of opening a window to feed the exhaust fan, it is preferable to provide the needed fresh air using a special type of whole-house ventilator. The air-to-air heat exchanger

A whole-house ventilation system provides fresh air while stale air is exhausted. All exhaust air is vented to one point and fresh air is brought in at another point. By bringing hot stale air in close proximity to the incoming cold fresh air, it is possible to exchange the heat between the two streams of air. In this way, the outgoing air heats the incoming air. Since this heat is extracted from the outgoing air, it is a way to get fresh air without having to heat the air to reach room temperature. Such a device is known as a Heat Recovery Ventilator (HRV) or perhaps more descriptively as an air-to-air heat exchanger.

Having a slow rate of continuous ventilation will provide fresh air as well as exhausting contaminants from the house. The efficiency of the heat exchanger is a measure of its heat recovery capability. The efficiency rating can range from 50% to 90% in models commercially available. It is also possible to construct a homemade heat exchanger, although it takes a significant amount of work. The commercial exchangers tend to be somewhat expensive, although they pay back the cost over time when compared to merely opening a window for ventilation or having leaky doors or windows to allow infiltration. A properly arranged vent plan for an air-to-air heat exchanger places exhaust ducts in the bathrooms and kitchen while providing fresh air ducts for all other rooms. This could eliminate the need for individual bathroom and kitchen exhaust fans; the heat exchanger fans remove contaminated air and supply fresh air. Another approach is to route the exhaust fan ducts from the bathrooms and kitchen into an exhaust air chamber, which then leads into exhaust chamber of the air-to-air heat exchanger.


Air supply ducts can be made by use of inside wall, ceiling, and floor cavities or by putting actual ducts through these spaces, depending on requirements of the local building code. Initially it was felt that supplying fresh air at one point in the house would allow even ventilation. Although this may work for open floor plans, individual rooms do not receive an adequate supply of fresh air.

It might seem a good idea to add the fresh air to the cold air inlet of the forced air heating system. There are two problems with this: 1) The fan power of the furnace is much stronger than the heat exchanger fans. This would cause a tremendous imbalance of the airflow through the exchanger. 2) In an energy efficient home, the forced air ventilation system runs so infrequently that it would not adequately circulate ventilation air. Some heat exchanger companies recommend turning the forced air fan to the "on" position whenever the exchanger is operational so that fresh air from the heat exchanger is properly distributed. This technique will suffice, but much electricity is required for the frequent operation of the high-powered furnace blower. In the long run, it is more cost-effective to install fresh air ducts for the heat exchanger vent system instead of using the ducts of the heating system to mix the fresh air within the house.

If one retrofits a heat exchanger in an existing house, duct booster fans mounted in the forced air heating system ducts could be used to provide adequate mixing of the house air. Duct boosters have low power consumption (e.g. 25 watts for 200 cubic feet per minute airflow); if strategically placed in the heating ducts, two of these fans operating continuously might provide adequate mixing of house air. The duct boosters could maintain continual airflow throughout the house, mixing the air without continual operation of the high-powered furnace blower.


Part ThreeHOUSE VENTILATION

Fresh Air for TightlyConstructed Homes

Evaluating Air-to-Air Heat ExchangersIntegral to the fresh air needs of new, well-insulated homes is ventilation to replace

infiltration (which has been minimized). Ventilation of cold outside air into the home is unpleasant if the air is not heated in advance. When the outdoor temperatures are low, the most economical method of getting fresh air is by use of an air-to-air heat exchanger. This device has been also termed a Heat Recovery Ventilator (HRV). In this text I will use the terms air-to-air Heat Exchangers, or Heat Exchangers or Heat Recovery Ventilators (HRVs) all to describe the same type of device.

Heat exchangers extract heat from the outgoing air and transfer it to the incoming air. Although schematic diagrams of the exchanger make it seem simple and inexpensive, quite the opposite is the case. As of 1989 prices, heat exchangers cost from $400 to $1,500. The price of an exchanger is not necessarily related to heat recovery efficiency.

A heat exchanger is a relatively new product designed to fill a new need. Many companies market various types of heat exchangers. It is difficult to find the most suitable model with so many varieties to chose from.

Counterflow and crossflow heat exchangersThere are two types of fixed plate heat exchangers: crossflow and counterflow. These

usually have parallel surface membranes between which air is passed. Intake air is on one side of the plate, and exhaust air is on the other side.

The counterflow exchanger routes the exhaust and intake air through the exchanger in opposite directions.


By having a long course through which the air can travel, the heat is readily exchanged between the two streams of air. A short distance of air stream overlap usually means lower heat recovery. Some counterflow models have very long heat exchange cores, up to 8 feet in length, while other counterflow models have barely a 2-foot core length. A short counterflow model can end up very efficient if it has a lot of surface area for heat transfer. A long counterflow model might have less surface area over a much longer distance.

The typical crossflow heat exchanger schematic diagrams show the air streams passing at right angles to each other; significantly more than 60% efficiency cannot be expected. A single-pass crossflow exchanger would require enormous amounts of heat-transfer area and a very slow airflow rate to get high efficiency. Using the crossflow design, higher heat recovery is more effectively obtained by a double-pass crossflow core.


Double-pass crossflow models, routing the air in two passes through the heat exchanger core before exiting, give better efficiency. Single-pass crossflow models can be expected to get 50 to 55% heat recovery, although some companies claim their single-pass models get 75% heat recovery. If the exchanger is made to have the air routed through in a double-pass, the first pass recovers 53% of the heat and the second pass through the other heat exchange element recovers 53% of the remaining heat (0.53 x 0.47 = about 0.25). This results in about 78% heat recovered for the two passes (0.53 + 0.25 = 0.78).

By lengthening the separation between the ports, the crossflow and counterflow models begin to resemble each other. Indeed, there can be combinations of the crossflow and counterflow designs.

The final efficiency of a heat exchanger is determined by a number of factors: the

arrangement of the heat exchange plates, the total surface area available for heat exchange, and the rate of airflow. A fast airflow rate will usually decrease efficiency. The actual details of the internal airflow through the exchanger will help determine the effectiveness. The heat exchange plates should be impervious to air and moisture. There should be essentially no cross-leakage, no mixing of intake and exhaust airflows.

Single-pass crossflow heat exchangers can be expected to have lower efficiency than double-pass crossflow or most counterflow exchangers. Counterflow and crossflow models can obtain better efficiency by an increase of exchange area and by having a longer distance of air travel within the exchanger core. Related to these fixed plate designs are exchangers using tubes for the heat transfer surface.


Heat pipe type of heat exchangers

The heat pipe exchanger transfers heat from one stream of air to the other by way of conductive pipes extending from the exhaust to the intake air streams. Inside the heat pipes is some form of refrigerant fluid, such as Freon, to transfer the heat from one side to the other.

Rotary heat exchangers

The turning wheel picks up the heat from the outgoing stream and transfers it to the cold stream about one-half rotation later. Redrawn from "Heat-Recovery Ventilators," Consumer Reports (October 1985).

The rotary heat exchanger has a slowly turning heat recovery wheel that picks up heat from one stream of air and transfers it to the opposing stream about one-half rotation later. The rotary types range from small home models with a 16-inch-diameter exchange wheel, to huge industrial exchangers with up to a 13-foot-diameter exchange wheel.


The rotary exchange wheel transfers heat between the air stream as well as transferring some moisture (and its latent heat). Recovery of moisture, therefore, increases the overall recovery of heat. Rotary types of heat exchangers must be engineered carefully to prevent cross-leakage of air. As the wheel rotates from the exhaust air to the clean air, some of the contaminated air can re-enter the building.

Most heat exchanger types recover "sensible heat" (heat that can be sensed by touch or measured by a thermometer). "Latent heat" is the heat of fusion or vaporization of water. If a home has air that is too dry, using a humidifier to add moisture to the air will consume additional heat to change the water into the vapor state. As water vapor is recovered by an exchanger, the latent heat of vaporization is also recovered. These types of devices have been termed Energy Recovery Ventilators (ERVs). Not only do they recover sensible heat but latent heat as well.

With very tightly constructed homes the objective is to remove contaminated air and typically to remove excess moisture. Most heat exchanger companies emphasize that their models allow no cross-leakage of the two air streams and no moisture transfer. Rotary heat exchanger companies and other ERV products emphasize that exchanging moisture with the incoming air is a good feature of their product. It is hard to know whether moisture removal or recovery is the better feature for all applications. If the inside air tends to be too dry, then moisture recovery is preferable. However, if the inside air is already too humid, recovery of water vapor does more harm than good. Heat Recover Ventilators (HRVs) are designed to recover sensible heat and to exhaust accumulated moisture (which is most important for cold climates). Energy Recovery Ventilators (ERVs) are designed to recover latent heat as well, which is apparently more useful for air conditioning in hot, humid climates.

The October 1985 issue of Consumer Reports included a review of air-to-air heat exchangers. The Consumer Union obtained specific models of exchangers to test for efficiency of heat recovery at two temperatures (5 F and 45 F). Airflow capability, cross-leakage, type of recovery unit, size, price, and overall ratings of five whole-house units and two window models were compared. The rated efficiencies were from 15% to 71% heat recovery. The best three units were marginally acceptable (38% to 71%) even at their lowest airflow rates (42 to 89 cubic feet per minute). Prices of the best three units ranged from $540 to $1,161. The window/wall models had approximately 50% efficiency, with an airflow capacity too low to be effective for a superinsulated house. The performance of one exchanger seemed extremely poor (15% to 19%). In general, Consumer Reports recommended obtaining fresh air by using an exhaust fan or opening windows; the cost of heating the ventilation air did not justify use of a heat exchanger. A heat recovery ventilator was recommended only under certain conditions: for extremely tight houses in extremely cold climates or where unusual problems existed, such as with radon pollution or chemical contaminants in the home.25

The problem with new, tightly constructed homes is that exhaust fans alone cannot result in proper air quality. Since fresh air must be introduced, the house is much more comfortable when the air is pre-warmed. Calculations from the section "Heating Costs for the Year" demonstrate the cost of heating fresh air with and without use of a heat exchanger. The figures show the cost of heating ventilation air without a heat exchanger to be about $80 per year using gas heat. (Compare examples 3 and 4 for Duluth, Minnesota.) However, the same size house without a heat exchanger, having a full-length basement, and using 1.0 air changes per hour would have an annual cost for heating ventilation air of over $300 per year. (Calculations are based on gas heat @ $0.44 per 100 cubic foot; if only electric heat is available, the annual costs would be over $1,000/year.) The following factors affect the cost of heating ventilation air: the house size, the ventilation rate, the coldness of the climate, and the cost per BTU of heat. In the most extreme conditions, an air-to-air heat exchanger can recover sufficient heat from exhaust air to save hundreds of dollars per year, based on gas heating costs.


Considerable variation may exist between claimed and actual efficiency of heat exchanger types. Different companies market similar heat exchangers; if one company claims 75% efficiency and a different company's comparable model claims 55% efficiency, it leads one to doubt the claims.

Power consumption of the heat exchanger fans should also be considered. For heat exchanger models operating continuously for 180 days per year, 65 watts power consumption will use about $20 worth of electricity; 300 watts power consumption will use about $90 worth of electricity (at $0.07 per kilowatt-hour).

Most companies provide accessories to improve exchanger performance. Some models compensate for incomplete heat recovery by adding an electric pre-heater. With such a device, the air going into the house is heated to 100% of room temperature by electrical resistance heat after it leaves the heat exchanger. One rotary model preheats air going to the exchanger core to prevent core freezing when outside temperatures are too low. Many models have a defrost cycle, and most provide a way for condensed moisture to be removed from the exchanger.

The exchanger selected should be able to supply the fresh air needs of the home (about 0.5 air changes per hour) on the lower speed fan setting. As an example: a single-story 1,500 square foot house with 8 foot ceilings has 12,000 cubic feet of air, which is one air change for this size house; half of that is 6,000 cubic feet. An exchanger with a 100 cubic feet per minute (cfm) capacity would move 6,000 cubic feet per hour. Larger quantities of contaminated air are removed by switching the exchanger to the high-speed fan setting, especially useful when combustion is taking place (cooking, smoking, or fireplace use). Contaminated air is exhausted from the bathrooms and kitchen as fresh air is introduced by ductwork into the other rooms of the house. Ventilation standards from the American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc. (ASHRAE), recommend exhaust ventilation of 50 cfm capacity for bathrooms and 100 cfm for kitchens; this is usually supplied by manually operated exhaust vents, used when the need arises. Continuous ventilation by heat exchanger ducts provide less peak airflow although better effective ventilation, since the occupants do not shut off the ventilation system. ASHRAE further recommends a continuous fresh air supply of 10 cfm to each room of the house.13 This is best achieved by providing exhaust vents for the bathroom, kitchen, and laundry and fresh air ducts for all other rooms of the house.

Most heat exchanger companies provide installation instructions for the heat exchanger, including flow balancing. If the exhaust and intake ductwork causes unequal resistance to airflow, the house will be slightly de-pressurized (if outflow is greater) or pressurized (if inflow is greater). Dampers in the ductwork must then be adjusted to equalize the airflow. If unequal, there will be increased rates of infiltration every time a window or door is opened, as well as driving infiltration through any breaks in the vapor barrier.

Although there are heat exchanger models on the market that are well-designed, efficient, and reasonably economical, it may be difficult to find the most suitable model.

The following pages detail an exhaustive mailing research project I did on air-to-air heat exchangers in 1988. Less than 2 years later, I re-contacted the companies and found many had moved, changed names, and changed products. Some had apparently completely gone out of business, as I could find no forwarding address. (At this time in life, it is possible to conduct an Internet search for such products, for those companies that list their products on the web.)

Being a do-it-myself person, I felt I could do a good job designing and making my own. In 1988, I figured how to make such a homemade model from hardware store products.


Since late 1988 (though 2003), I have had my homemade heat exchanger in use in my home. I have placed my recipe for my version in the section of this book on homemade air-to-air heat exchangers.

Summary of Data on CommerciallyAvailable Heat Exchangers

Comments about 1989 data: The following pages list a number of companies marketing or manufacturing air-to-air heat exchangers. During a 24-month period (1987-1989), I had witnessed significant changes in the availability of products from some of the companies. New products were added, formerly available products were deleted, companies changed hands, and some companies went out of production. This is not intended to be an all-inclusive list of every exchanger type available; the listed data are subject to change over time and with market forces.

Abbreviations: s.p. crossflow = single-pass crossflow; d.p. crossflow = double-pass crossflow; Cross/counter = exchanger may be rated by the company as one model or the other, although the airflow pattern, judged by diagrams received from the company, appears to be a combination of the two different types of flow, i.e. a short counterflow model resembles a crossflow model and an elongated crossflow model may be partially counterflow. Power Used = electrical power in watts (W) or amperes (A) used by the fan motors with the fan on the high setting. Some of the smaller exchanger models use one motor to turn two centrifugal blower wheels for the exhaust and intake air streams. In larger-capacity units, invariably two motors are needed and the power consumption increases. Some of the models locate the blower motors within the air stream, so heat given off by the motor is added to the fresh air stream, raising the exchanger effectiveness. The designation N/A = a value not applicable. N/A is used to refer to cores only, which have no fans, thereby no power consumption quantity can be assigned. Most airflow rates listed are values with no resistance from ductwork; as ductwork is added, resistance is added. One exchanger rated at 140 cfm reduces to 119 cfm with a moderate amount of air resistance (e.g. 0.4" static pressure). Efficiency ratings are from company literature. Some companies explain the efficiency at various airflow rates, outside temperatures, and inside relative humidity; other companies state only one efficiency rating. One heat exchanger company claims 70% efficiency, but when tested by Consumer Reports was given an efficiency rating of 38 to 55%. 25 The buyer should be cautious and not necessarily accept heat recovery claims as completely accurate.

September 2003 comments: I did a search for Heat Recovery Ventilators on the Internet. I also did a search of the companies I listed from 1989. Of the 17 listed companies for 1989, there were still several of the same names, still manufacturing Heat Recovery Ventilators (HRVs). There may be others of these companies from 1989 that do not have a functional web-site, or have changed their name. I found more than a dozen other HRV companies (within the first 100 web-sites I searched), which are listed below. There were over 4,000 entries that I found on my Internet search for Heat Recovery Ventilators. These Internet entries included manufacturers, suppliers, installers, and general information on the topic.

I believe that by studying my text on this subject (and also reviewing the data on the HRVs from 1989), you can understand the theories of how air-to-air heat exchangers work. This can help you understand the basic types and forms of these devices, and be better able to know what you are looking at when investigating the products of specific companies.

Heat Recover Ventilators (HRVs) are designed to recover sensible heat and to exhaust accumulated moisture (which is most important for cold climates).

Energy Recovery Ventilators (ERVs) are designed to recover latent heat as well, which is apparently more useful for air conditioning in hot, humid climates.


The following are HRV and ERV companies I found on my Sept 2003 Internet search.

1. Airxchange; 85 Longwater Drive; Rockland, MA 02370; Ph: 781-871-4816. Markets rotary HRVs. (See this company under my 1989 listings.)

2. American Energy Exchange, Inc.; 5737 East Cork Street; Kalamazoo, MI, 49048; Ph: 269-383-9200. Markets large capacity HRVs (1,000 cfm to 30,000 cfm heat wheel recovery (rotary version), flat plate versions (single and double-pass crossflow HRVs with aluminum cores) and heat pipe versions.

3. American Aldes Ventilation Corporation. (See this company under my 1989 listings also.) Markets single-pass and double-pass crossflow HRVs with aluminum cores.

4. Broan. Markets single-pass crossflow HRVs. Phone: 1-800-558-1711; Broan-NuTone, LLC.; P.O. Box 140; Hartford, WI 53027. (See my listings for this company under ventilation products, pg 142-144.)

5. Bryant Heat Recovery Ventilators. (No technical details of the products were shown on the web-site.) (esshvac.com)

6. Carrier makes two different versions of ERVs for residential use. These appear to be single-pass crossflow cores, designed for enthalpic energy and moisture recovery. Carrier is a widely-know company with dealers throughout the USA.

7. Chester Dawe. Markets at least one type of HRV (KMH-150 Heat Recovery Ventilator). Many locations in Canada. One such address: 1297 Topsail Road; P.O. Box 8280; St. John's, NF; A1B 3N4; (709) 782-3104

8. Chris Smith HVAC, Inc. Markets HRVs with single-pass aluminum crossflow cores. (cshvac.com)9. Cleanaire. Markets single-pass crossflow HRVs. Avon Electric Ltd.; Christchurch, New Zealand. Ph

0800 37924710. Eco Air 56 Bay Road; Taren Point, NSW 2229; Australia. Markets counterflow HRVs with

aluminum core. Ph: 61 2 9526 213311. Fantech; 1712 Northgate Boulevard; Sarasota, Florida 34234; 1-800-747-1762. Markets single pass

crossflow HRVs with polypropylene core12. Grantair Technologies; 1470, Rome Blvd.; Brossard; (Quebec) J4W 2T4; Canada. Markets

residential HRVs and other products.13. Heatilator Home Products; Hearth & Home Technologies; 1915 W. Saunders Street; Mt. Pleasant, IA

52641; (877-427-8368). Markets single-pass crossflow HRVs with aluminum cores and models of ERVs.

14. Honeywell makes two different versions of single-pass crossflow HRVs with aluminum cores and crossflow ERVs. These are marketed and installed by various companies, which can be found by Internet search under Honeywell HRVs.

15. Kiltox Damp Free Solutions; 27 Park Row; Greenwich SE109NL; United Kingdom. Markets HRVs.16. Lifebreath appears to be single-pass crossflow HRVs with aluminum core. Indoor Air Quality

Distributors; 83 Galaxy Blvd., Unit 19 ; Toronto, Ontario M9W 5X6; Canada (416) 674-7525; 1-877-839-3036

17. Newtone Home Heat Recovery Ventilators, ph 800-525-7194. Appears to be single-pass crossflow cores with enthalpic transfer (moisture absorbing/transmitting exchange plates).

18. Nu-Air Ventilation Systems; Newport, Nova Scotia; Canada, B0N 2A0; (902) 757-1910. Markets HRVs, which appear to be single-pass crossflow cores (aluminum or plastic, depending on the model).

19. Raydot, Inc.; 145 Jackson Avenue; Cokato, MN 55321; 800/328-3813 or 320/286-2103. Markets HRVs for agricultural, industrial, and residential uses. (See this company under my 1989 listings.)

20. RenewAire (formerly Lossnay). Markets HRVs, which appear to be single-pass crossflow HRVs with moisture absorbing/transmitting exchange plates. Sound Geothermal Corporation; Rt. # 3 Box 3010; Roosevelt, UT 84066; ph 435-722-5877

21. Summeraire. Markets residential HRVs. Appears to be single-pass crossflow design.22. Venmar Heat Recovery Ventilators; Thermal Associates; 21 Thomson Ave.; Glens Falls, NY 12801;

1-800-654-8263; 518-798-5500. Markets HRVs, which appear to be single-pass crossflow, with a polypropylene core.

23. Xetex, ph. 612-724-3101. Markets flat plate heat exchangers typically with aluminum cores and rotary models. (See this company under my 1989 listings.)


Summary of Data on Commercially Available Heat Exchangers (January 1989)

Company Type of Maximum % Heat Power 1989Model no. core capacity recovery used Length Height Width Price

ACS - Hoval (Numerous aluminum crossflow cores also available in many sizes)PC-130-140 s.p. crossflow 140 cfm 60-75% 120 W 47" 18" 12" $920PC-130-250 s.p. crossflow 250 cfm 60-75% 213 W 47" 18" 12" $1137PC-230-140 d.p. crossflow 140 cfm 75-90% 120 W 67" 18" 12" $1238PC-230-250 d.p. crossflow 250 cfm 75-90% 213 W 67" 18" 12" $1457

Air Changer Marketing: See Memphremagog listings

AirXchangeModel 570 rotary 70 cfm 75-80% 55 W 22" 13" 7.5" $438Model 502 rotary 200 cfm 75-80% 145 W 29" 17.5" 10" $578

American AldesVMP H3/5 cross/counter 140 cfm 70% 1.75 A 53.5" 20" 11.5" $979VMP H4/8 cross/counter 180 cfm 70% 1.75 A 53.5" 20" 11.5" $1015

Aston Industries (many sizes of aluminum cores are available)Thermatube 2300

(core only) counterflow 200 cfm 70% N/A 57.5" 25" 8" $220Aston 2000 exhaust ventilator

(blower only) N/A 1.3 A 18" 14" 12.5" $290Aston 2412

(core only) s.p. crossflow 150 cfm 52% N/A 12" 12" 12" $290Two Aston 2412cores (no blower) d.p. crossflow 150 cfm 77% N/A $580

Berner Air ProductsAQ Plus+ counterflow* 165 82% 370W 28" 17" 11" $820* This is not a standard counterflow unit. see the text narrative on Berner for details.

CargocaireLarge industrial-sized heat exchangers available in the rotary style

Crown Industries (EZE-Breathe exchangers formerly sold by Ener-Quip, Inc.)RHR 100 cross/counter 100 cfm 70% 48 W 46" 8.3" 14" $682RHR 200 cross/counter 200 cfm 70% 58 W 46" 11.3" 18" $764RHR 400 cross/counter 400 cfm 70% 72 W 46" 14.3" 26" $999

Des Champs LaboratoriesSeries 175 window model 75 cfm $357EZV-210 counter/cross 150 cfm 75% 0.8 A 46" 19" 14" $735EZV-220 counter/cross 240 cfm 73% 1.5 A 46" 19" 14" $795EZV-240 counter/cross 430 cfm 72% 3.0 A 49" 19" 18" $970EZV-310 counterflow 145 cfm 85% 0.8 A 58" 19" 14" $805EZV-320 counterflow 220 cfm 84% 1.5 A 58" 19" 14" $880EZV-340 counterflow 415 cfm 83% 3.0 A 61" 19" 18" $1100

EnermatrixEMX 10 s.p. crossflow 103 cfm 75% 2.11 A 28" 18" 13" $399EMX 15 s.p. crossflow 103 cfm 75% 2.11 A 28" 18" 13" $429EMX 20 s.p. crossflow 121 cfm 75% 2.11 A 28" 18" 13" $479EMX 25 d.p. crossflow 250 cfm 80% 2.42 A 60" 18" 13.5" $899


Company Type of Maximum % Heat Power 1989Model no. core capacity recovery used Length Height Width Price

Memphremagog and Air Changer MarketingDR-150 counterflow 120 cfm 76% 84 W 60" 25" 15" $950DR-275 counterflow 200 cfm 78% 260 W 66" 30" 15" $1130

Mountain Energy & Resources, Inc. (makes heat pipe exchangers similar to QDT, Ltd.)MER-150 heat pipe 160 cfm 70% 100 W 24" 24" 7" $585MER-300 heat pipe 235 cfm 70% 180 W 26" 32" 13.5" $1085

NewAireHE-1800c s.p. crossflow 70 cfm 73% 55 W 18" 18" 13" $420HE-2500 s.p. crossflow 110 cfm 78% 120 W 30" 20" 12" $535HE-5000 s.p. crossflow 210 cfm 78% 240 W 30" 20" 21" $795

QDT, Ltd. SAE-150 heat pipe 150 cfm 70% 236 W 29" 22" 12.5" $629

RaydotRD-225-H counterflow 225 cfm 63-82% 240 W 96" 17" 8" $846RD-150-H counterflow 150 cfm 66-82% 150 W 96" 17" 8" $728RD-90-H counterflow 90 cfm 71-86% 90 W 92" 9" 9" $629RD-225-V counterflow 225 cfm 61-78% 240 W 14" 59" 14" $867RD-150-V counterflow 150 cfm 63-79% 150 W 14" 59" 14" $752

Snappy ; Standex Energy SystemsMA 110 s.p. crossflow 110 cfm 77% 50 W 17.5" 17.5" 7.5" $550MA 240 s.p. crossflow 240 cfm 70% 100 W 22" 23" 8.5" $597

Star Heat ExchangersNova counterflow 70 cfm 65% 34 W 25" 16" 7.5" $307Model 165 counterflow 165 cfm 80% 66 W 39" 12.5" 15" $620Model 200 counterflow 200 cfm 80% 66 W 39" 25" 15" $770Model 300 counterflow 300 cfm 80% 132 W 39" 25" 15" $960

XetexHX-50 s.p. crossflow 51 cfm 62% 74 W 11.5" 19" 7" $395HX-150 d.p. crossflow 119 cfm 80% 80 W 18" 24" 12.5" $788HX-200 d.p. crossflow 182 cfm 80% 120 W 25.5" 24.5" 12.7" $902HX-250 d.p. crossflow 279 cfm 80% 125 W 18" 32" 22" $1242HX-350 d.p. crossflow 377 cfm 80% 157 W 25.5" 40" 22" $1533

Air-to-Air Heat Exchangers: List of Selected Commercial Companies

January 1989ACS-Hoval, 935 Lively Boulevard, Wood Dale, IL 60191-2685, (312) 860-6800 or (800)323-5618. Markets a number of sizes of crossflow heat exchangers. The exchangers and cores are constructed of aluminum plates. The joints are sealed so as to prevent any cross-contamination of the two air streams. The standard high-efficiency models are a single-pass crossflow design rated from 60-75%. The ultra-high-efficiency models route the air through the exchanger in a double-pass crossflow pattern, raising the efficiency to between 75 and 90%. The standard efficiency models are rated at 140 cfm (PC-130-140, 120 watts) and 250 cfm (PC-130-250, 213 watts). Both standard models have total dimensions of 47" x 18" x 12" (including blowers).


(ACS-Hoval, continued): The ultra-high-efficiency models are rated at 140 cfm (PC-230-140, 120 watts) and 250 cfm (PC-230-250, 213 watts). Both ultra models have total dimensions of 67" x 18" x 12" (including blowers). ACS Hoval also makes heat exchange cores for other commercial and home applications, with dozens of potential sizes available. Their heat recouperators are made to be mounted in exhaust ducts from ovens and furnaces to recover up to 75% of the heat from these high-temperature exhaust gases. Prices: PC-130-140: $920; PC-130-250: $1,137; PC-230-140: $1,238; PC-230-250: $1,457.

Air Changer Marketing, 1297 Industrial Road, Cambridge, Ontario N3H-4T8, Canada, (519) 653-7129. Markets two different models of counterflow heat exchangers (DR2000 series). See Memphramagog write-up for the details.

AirXchange, Inc., 401 VFW Drive, Rockland, MA 02370, ph (617)871-4816. Manufactures rotary type heat exchangers. Rotary-wheel cores are available in interchangeable sensible and enthalpy (dessicant-coated) versions. Enthalpy wheels are recommended for cooling applications and for heating applications where retention of some humidity is desirable. Model 570: 80% heat recovery; 22 " x 12 5/8" x 7 "; 70 cfm capacity; 55 watts; 4-inch ducts; available in wall-mounted and ceiling-mounted units. Model 502: 75% to 80% heat recovery; 29" x 17 " x 10"; 200 cfm capacity, 145 watts; whole house unit for floor, ceiling, or basement installation; 7-inch ducts; a variety of accessories such as grilles, airflow balancing grids, and intake/exhaust fittings are also available. Depending on the exchanger features and the accessories selected, the prices for these exchangers are: Model 570: $438 - 623; Model 502: $578 - 917.

American Aldes Ventilation Corporation, 4539 Northgate Court, Sarasota, FL 34234, (813) 351-3441. Markets heat exchangers of a combined counterflow and crossflow airflow pattern; 70% heat recovery at 90 cfm; polyvinyl chloride parallel plate core; condensate drain; core size 38.5" long x 20" x 11.5"; 160 square foot exchange area; blower unit is 15" x 15" x 18" with 6-inch ducts and 1.75 amps. Model VMP H3/5: 90 cfm or 140 cfm fan setting. Model VMP H4/8: 130 or 180 cfm fan setting. Other, more sophisticated heat exchangers are available (VMP-I). A simpler heat exchanger is available (VMP-A). The exchanger kits include self-balancing airflow controllers that provide constant airflow and eliminate the need to balance the airflow for the house. The company also carries other types of heat exchanger cores, exhaust ventilators, and numerous accessories. The listed prices typically include most of the accessories needed for installation. Prices: VMP H3/5: $979; VMP H4/6: $997; VMP H4/8: $1015; VMP-A: $667; VMP-I 5/7: $1,395.

Aston Industries, Inc., P.O. Box 220, St-Leonard d'Aston, Quebec, Canada, JOC-1M0, (819)399-2175. Markets aluminum crossflow cores and a counterflow heat exchanger core (Thermatube 2300). The counterflow (thermatube) heat exchanger vents the stale air through glass pipes in the exchanger. The incoming fresh air is made to pass around the glass pipes to pick up the heat. Capacity is up to 200 cfm, with up to 70% heat recovery, has drain for condensate. Dimensions: 57.5" long x 25" x 8". The Thermatube 2300 uses the Aston 2000 ventilator module as the blower source for exhaust; 1.3 amps. It apparently does not use a blower for the fresh air return; the return air must enter by the negative air pressure created by the exhaust fan. The aluminum crossflow cores (Aston 2400 series) can be obtained in 150 different sizes from the smallest size: 12" x 12" x 4" (40 to 160 cfm) to the largest size: 48" x 48" x 84" high (up to 24,000 cfm). The aluminum crossflow cores can be hooked up serially to get a double-pass arrangement, if desired. A basic crossflow core will have about 52% heat recovery. With two cores hooked up in a double pass arrangement, the efficiency rises to 77%. Prices: Thermatube core: $220; Aston 2000 blower: $290; Aston 2400 core: $290.


Berner Air Products Inc., P.O. Box 5410, New Castle, PA 16105, (800) 852-5015 or (412)658-3551. Berner previously marketed rotary heat exchangers, but has switched to a counterflow model, the AQ Plus+. Different from other counterflow exchangers, the AQ Plus+ routes supply air through the exchanger core for a 3-second time period; it then reverses the airflow sending exhaust air through the exchanger core for another 3 seconds. The counterflow element is constructed of aluminum foil with a hydroscopic coating for latent-heat (and moisture) recovery. In addition to the counterflow heat exchanger element, the unit employs three filters to eliminate indoor air pollutants and allergens. The unit continuously filters (and returns) the room air while performing the separate functions of exhausting a portion of the room air and supplying fresh air. The unit is sold as a "through-the-wall" installation. It has a variable speed blower, ranging from 60 to 165 cfm; 370 watts maximum; 27.5" x 17" x 11"; 82% heat recovery on the high setting. Price: AQ Plus+: $820.

Cargocaire Engineering Corporation, Senex Division, 216 New Boston Street, Woburn, MA 01801, (617) 933-9010. Markets large capacity industrial heat recovery systems of 500 cfm to 40,000 cfm; rotary type, with rotary heat exchange element that can be 2.3 to 14 feet in diameter; 75% heat recovery; all units too large for home use.

Crown Industries, 2101 E. Allegheny Avenue, Philadelphia, PA 19134, (215)423-8900. This company markets three different sizes of heat exchangers of a combined counterflow and crossflow design, having aluminum heat exchange plates: EZE-BREATHE heat recovery ventilators RHR 100, RHR 200, and RHR 400 (these exchangers were formerly marketed by "Ener-quip", Inc). All have about 70% heat recovery, with two fans, filters, condensate drain, and necessary controls. RHR 100: 100 cfm airflow; 48 watts; 46" x 8.3" x 14", with 4-inch ducts. RHR 200: 200 cfm airflow; 58 watts; 46" x 11.3" x 18", with 5.5-inch ducts. RHR 400: 400 cfm airflow; 72 watts; 46" x 14.3" x 26", with 9-inch ducts. Prices: RHR 100: $682; RHR 200: $764; RHR 400: $999.

Des Champs Laboratories, Inc., Box 440, 17 Farinella Drive, East Hanover, NJ 07936, (201)884-1460. E-Z-VENT heat exchangers. Markets more than 8 different home models of heat exchangers, counterflow (but a relatively short core length); aluminum heat exchange elements; 2-speed blowers; filters; condensate drains. Series 175 exchangers are rated at 75 cfm for single-room use. Series 200 models: 3 models ranging from 150 cfm to 430 cfm; 72 to 75% heat recovery; 24-inch length of core; with blowers, the full dimensions are: 46" x 19" x 14" (smaller size) to 49" x 19" x 18" (larger size); large amount of exchange area is compacted into the core (286 to 382 square feet). Series 300 models: 3 models ranging from 145 to 415 cfm; 83% to 85% heat recovery; 36-inch core length; with blowers, the full dimensions are: 58" x 19" x 14" (smaller size) to 61" x 19" x 18" (larger size); 430 to 574 square feet exchange area. The company plans to market a new model: EZ Vent II, 24" x 26" x 17", 240 cfm, $695. The company also makes other models for commercial use of 615 to 2,200 cfm capacity. Prices: Series 175: $357; EZV-210: $735; EZV-220: $795; EZV-240: $970; EZV-310: $805; EZV-320: $880; EZV-340: $1,100.

Enermatrix, Inc., P.O. Box 466, Fargo, ND 58107, (701)232-3330. Markets single-pass crossflow and double-pass crossflow heat exchangers, polypropylene core, with filters. EMX-10: 18" x 13" x 28" (including blowers); 75% heat recovery; exhaust motor is of greater airflow than intake (113 cfm versus 90 cfm); 2.11 amps total; has condensate drain for both air streams; 4-inch duct size; no defrost cycle (apparently not needed with the faster outgoing air). EMX-15 & EMX-20 are the same size as EMX-10, but they have balanced airflows between intake and exhaust, auto defrost control. EMX-15: 90 cfm; EMX-20: 113 cfm. EMX-25: separate blower housing (14" x 24" x 11") with 6-inch duct connections to exchanger core (35" x 18" x 13"); 80% heat recovery; airflow is balanced between intake and exhaust (about 250 cfm); 2.42 amps total; condensate drain; variable fan speed control; dehumidistat control; auto defrost control. Prices: EMX-10: $399; EMX-15: $429; EMX-20: $479; EMX-25: $899.


Memphramagog Heat Exchangers, P.O. Box 456, Newport, VT 05855, (802)334-5412. Markets two different models of counterflow heat exchangers (DR2000 series); core made from a polypropylene polymer (coroplast, apparently); condensate drain; core size 50" long x 25" high x 15" thick, having 280 square feet of exchange area; cold air ducts are 6-inches in diameter; automatic defrost. To the basic core is added one of two different warm end panels containing the blowers and controls. The Model 150 has axial fans and 6-inch ducts; 60 cfm or 120 cfm fan settings; 84 watts; this provides 76% heat recovery at 117 cfm and outside temperature of 32 F; dimensions: 10" long x 25" x 15". Model 275 has centrifugal fans with 7-inch oval ducts; 120 cfm or 200 cfm fan settings; 260 watts; this provides 78% heat recovery at 117 cfm and outside temperature of 32F (at -13F, the heat recovery is 57% for these models). Dimensions 16" long x 30" x 15"; the heat of the motors raises the intake temperature further, giving an overall energy performance effectiveness of 81 to 94%. The manufacturer lists that under 2 or 3% cross-leakage can occur with these models. There is apparently no moisture transfer with these models.

homemade fresh air machine (hrv), january 2004

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