EXHAUST AIR HEAT PUMP OPERATION AND PERFORMANCE IN FIVE HOMES

Peter K Downey and John G§ DouglassWashington State Energy Office

Exhaust air heat pumps (EARP) are a promising technology that provide ampleventilation while recovering more heat than air-to-air heat exchanger systems. TheBonneville Power Administration funded a Residential Construction DemonstrationProject For this project, five houses located in the Pacific Northwest that areequipped with EARPs were extensively monitored for one heating season. TheEARPs provided domestic hot water and space heat.

Remote computerized equipment was used to collect and aggregate real time data..Sensors were queried on a 6-second time interval. Data were preprocessed as neededthen aggregated to hourly resolution. Data were transferred to a central computer viamodems and dedicated phone lines.. Over 20 channels were monitored includingelectric power, temperature, humidity, and flow rates within each home and EARP..Heat transfer to water and air were computed. in real time from flow, temperature,and humidity sensors.

Recovered heat and the coefficient of performance were calculated and controlstrategies were analyzed for each unit. System efficiency may be increased throughchanges in control strategiess These changes include coupling the EARP supple­mental space heat condenser thermostat to the main space heat thermostat to pre­vent superfluous heating and/or excess ventilation during thermostat setback periodsand changing the operation strategy when the outdoor temperature drops below theEARP exhaust air temperature$

INTRODUCTION

Since 1986, Bonneville Power Administration hassponsored the Residential Construction Demon...stration Project (RCDP)0 The Washington StateEnergy Office operates the RCDP project in thefour northwestern states.. As subcontractors, energyoffices from the three other northwest states.....ldaho,Montana, and Oregon--carry out planning andimplementation aspects of the project.. The project'smain purpose is demonstration of energy conservinginnovations, but it also contains strong researchcomponents.. Part of this research involved fieldmonitoring of five houses with exhaust air heatpump ventilation systems (EAHP) located inMontana and Washington. These houses are asubsarrrple within the larger RCDP population of

165 new houses that had both heat recovery venti...lators and state-of-the-art envelope construction"

This paper describes each house's physical charac­teristics, occupant characteristics, and EARPinstallation.. As configured, the EARP serves threesystem functions: water heating, space heating, andventilation. EARP system efficiency is effectedthrough system configuration, installation, controldesign, and occupant operation of controls. EARPsystems are evaluated for system performance andoverall efficiency.. Ventilation rates achieved by thesystems are compared to expected ventilation rates..Energy savings due to heat recovery are evaluated.

Building Equipment and Appliances 1.81

Operational parameters examined include the effectofnighttime thermostat setbackon energy consump­tion. Nighttime thermostat setback contributessubstantial energy savings. However, systemefficiency may be improved through a modifiedcontrol scheme. Coupling the supplemental spaceheat condenser thermostat to the main thermostatshould prevent over-ventilation during periods ofnighttime thermostat setback

Two types of EARP systems are monitored. Thefirst type, which is installed. in three houses inWashington State, provides space and water heatand extracts energy from the exhaust air streamonly. This type of heat pump achieved overallcoefficients of performance (COP) between 2.5 and3$1, and ventilated the homes well above therequired 0.25 mechanical air changes per hourduring the heating season. COP is a ratio of theenergy delivered. to water and space heat and theenergy required to operate the system. Mean energyrecovered from exhaust air by these three heatpumps ranged from 3,183 to 4,731 Btu per hour ofoperation for the entire monitoring period"

The second type of EARP, which is installed in twoMontana homes, extracted energy from both exhaustand supply air streams~ This of heat. pumpachieved considerably lower COPs (1,,7 and 1~8)* Thesystems are operat less often, recover less heatper hour of operation (1 and 1,744 Btu!hr), anddo not provide the recommended air change rate..The manufacturer has sincediscontinued productionof this modet

Ventilation operating patterns coincided well withseasonal periods of space heat demand for all typesof systems0 Wintertime mechanical air change ratesfall between 0*09 and air changes per hOUf0 Thehighest readings are from three E units inwestern Washington, while the lowest reac;Ungs arefrom two EAHP units in Montana.. The threeWashington EAHPs operated almost continuously

the heating season, vvith less frequent useduring the summer months"

1,.82 Downey and Douglass

METHODOLOGY

Description. of Exhaust Air Heat Pump Systems

Three houses in Washington and two houses inMontana are equipped with EAHP systems. Twodifferent systems are used. All units prioritize waterheating above space heating. If neither water norspace heat are required, a timer can be set tooperate the exhaust fan periodically for internals asshort as 15 minutes for ventilation only. This featurecan be overridden by the occupant Schematics ofthese systems are contained in Figures 1 and 2"

The Montana units are balanced ventilation systemsinstalled in a utility closet within the heated spaceeThe evaporator is connected to the compressor/tankunit by refrigerant lines. The evaporator has both asupply air and exhaust air passage. Supply air andrefrigerant flow parallel to each other and counterto exhaust flow. There is a summer option thatchanges the flow to precool incoming supply arreThe flows of this model are complex.. The watertank is heated by a condenser jacket that encirclesthe tank, contacting the metal surface beneath theinsulation~ A backup resistance element actuates toheat the upper region of the tank if high waterusage drops temperature below a set minimum. Sup­plemental space heating is provided by the conden­ser. In summer operation mode, the unit extractsheat from both supply and exhaust air streams fordomestic hot water heating and air conditioning.These heat pumps achieved considerably lower effi­ciencies, were operated less often, and did notprovide the recommended air change rate" Themanufacturer has since discontinued production ofthis modet

The Washington units are unbalanced systemsinstalled in a closet within the heated space unlessotherwise noted in the house descriptions. Makeupair is provided by two through-the-wall vents oneach floor" The unit recovers heat from exhaust airand alternately provides heat to an integral72-gallon domestic hot water (DHW) tank and a

'-oat

Air

Supply Air

&-- iat

RefrigerantCompressor Unit

kwh_tot

KEYExhaust air mass flow in 1bmlhrSupply air mass flow in 1bm/hrRelative humidity upstreamRelative humiditY downstreamIndoor air temperature in degrees FahrenheitOutdoor air temperature in cfegrees FahrenheitExhaust air temperature upstream in degrees FahrenheitExhaust air temperature downstream in aegrees FahrenheitSupply air temperature upstream in degrees FahrenheitSupply air temperature dOwnstream in aegrees FahrenheitDomestio hot water temperature upstream in degrees FahrenheitDomestio hot water temperature downstream in aegrees FahrenheitDomestio hot water mass flow in 1 bmlhrCompressor and fan energy consum,ption in kW/hrTotar unit energy consumption in kWlhr

.... Electric Power

Building Equipment and Appliances 1.83

KEY

Space HeatCondenser

m1 exm2-sprhe-u

Ventilation [lJcting r~?:doattaex uRefrigerant taex-dtas utas-dElectric Power twl utw2-d

u~_ f1 wtrlfl1C':U.~ kWh fan

kwh:totWashington EARP Systems

e-- oat

~ SupplyAir

ExhaustAir

SupplyAir

kwh_tot

Exhaust air mass flow in 1bmlhrSupply air mass flow in 1bmlhrRelative humidiw upstreamRelative humiditY downstreamIndoor air temperature in degrees FahrenheitOutdoor air temperature in cregrees FahrenheitExhaust air temperature upstream in degrees FahrenheitExhaust air temperature downstream in aegrees FahrenheitSupply air temperature upstream in degrees FahrenheitSupply air temperature dbwnstream in Clegrees FahrenheitDomestic hot water temperature upstream in degrees FahrenheitDomestic hot water temperature downstream in aegrees FahrenheitDomestic hot water mass flow in 1 bmlhrCompressor and fan energy consumgtion in kW/hrTotar unit energy consumption in kWlhr

1..84 Downey and Douglass

remote supplemental space heat condenser" Thesupplemental space heat condenser is connected tothe compressor/tank unit by refrigerant lines" Thewater tank is heated by a condenser jacket thatencircles the tank, contacting the metal surfacebeneath the insulation. A backup resistance elementactuates to heat the upper region of the tank ifhighwater usage drops the temperature below a setminimum.

Description.s of Houses and Occupants

Site 228. Site 228 is a 2,356 ft2, split-entry housewith two stories, including the heated daylightbasement It is located in Helena, Montana at analtitude of 3,500 feet. Over the 12 months endingApril 30, 1988, 7,422 degree-days (65°P base) wererecorded at the site" Primary space heating is by wallmounted electric resistance baseboard heaters. Zonethermostats are distributed throughout the house"There is no automatic night setback system, butoccupants reported a daily routine for manuallysetting the various thermostats. This house wasmonitored for 14 months from February 1987 toApril 1988e The house was initially occupied by afamily of two adults, a mother and daughter.. Thedaughter moved out in May of 1987.. The occupant(mother) works a normal shift weeke The occupantconsiders the home comfortably ventilated exceptwhen guests smoke.. The occupant indicated thatthere are instances of inadequate hot water whenboth she and her daughter are living at home.. Thereis a strong passive solar contribution in the wintermonths.. The occupant reported that this sometimesmakes the living/dining area too warm.. Smoketesting revealed a large amount ofcrossover leakagefrom the exhaust side into the supply side of theevaporator!heat exchanger component There is alsoconsiderable leakage out of the component cabinetand at the duct connections..

Site 235.. Site 235 is a 1,821 ft2, two-story house witha vented crawl space" It is located in Helena,Montana at an altitude of 3,850 feet Primary space

is by wall-mounted electric resistancebaseboard heaters~ Zone thermostats are distributed

throughout the house and there is no automaticnight setback system. This house was monitored for9 months from December 1986 to September 1987"

Site 439$ Site 439 is a 2,798 ft2, two-story house witha vented crawl space. It is located in Yacolt,Washington at an altitude of 720 feet. Over the12 months ending April 30, 1988, 4,957 degree-days(65°F base) were recorded at the site. Primary spaceheating is by a conventional electric forced airfurnace. Blower door technicians ranked the furnaceducting as the second largest leak (to outdoors) inthe house. Additional space heating is provided bythe supplemental space heat condenser coil of theexhaust air heat pump. This house was monitoredfor 14 months from February 1987 to April 1988&The home is occupied by a family of two adults andthree school age children" Usually, all familymembers are at home during the evenings.. Oneadult works days, five days per week, and the otherworks outside the home intermittently (substituteteaching)" The occupants understood their heatingand ventilation systems and generally operated andmaintained them according to manufacturers' rec­ommendations" Nighttime setback of the main ther­mostat was regularly practiced. Mass flow data forthe first several months of operation are notrecoverable from this site. During that time, thesensor had come loose and true flow data are notestimable from monitored datae Nine hundred thirtyfour hours of space heat data or 8.8 percent of allspace heat data on Site 439 are excluded due to60 Hz power line interference..

Site 449. Site 449 is a 1,967 ft2, two-story house witha vented crawl spacee It is located in Bothell,Washington at an altitude of 500 feet Over the12 months ending April 30, 1988, 4,639 degree-days(65 °F base) were recorded at the site. Primary spaceheating is by a ceiling electric· resistance radiantsysteme Zone thermostats are distributed throughoutthe house and there is no automatic night setbacksystem. This house was monitored for 14 monthsfrom February 1987 to April 1988.

Building EqUipment and Appliances 1.85

The house is occupied by two adults with no chil­dren living at home. The occupants travel frequentlyand the house is often vacant for weeks at a time.They did not appear to have a good understandingof their heating and ventilating systems. At the timeof monitoring equipment installation, the EARPspace heating fan/coil thermostat was found to beset (incorrectly) lower than the space heatingthermostat in the same zone. The occupantsreported that their EARP operated quietly and thatthey never ran short ofhot water. Initially, there wasa perception of inadequate ventilation in the familyroom. This is reasonable because there are noEARP exhaust or space heating registers in theroom and there were no slot vents instaned. Slotvents were placed in the family room when theywere installed throughout the house in April 1988.The water tank thermostat is set for 1300 Pll This iswithin the unit's operating capability, but themanufacturer recommends 120°F because the vaporcycle circuit achieves a better coefficient ofperformance (COP) at lower water temperaturee

There are aspects of the EARP installation thatdegrade system efficiency including: long runs ofsupply dueting from the supplemental space fan/coilrouted through the vented crawl space; exhaustdueting' routed through unheated. attic space oroutside walls; large area uninsulated exhaust inletplenum in the unheated garage; and noticeable leaksin the EAHP casing with air escaping from thespace between the fan and the evaporator" Thelatter may cause a significant energy loss becausethe unit is outside the conditioned space*

Site 450" Site 450 is a 2,183 , two-story house witha vented crawl spacell It is located in Olympia,Washington at an altitude of 100 feet Constructionis timber framing with 6 3/8" stress-skin wall panelsand 8 3/8" stress-skin vaulted roof panelse Themonitoring period did not span an entire heatingseason" Over the 8 months from January 17, 1987through September 17, 1987, 2, 7 degree-days(65 0 F base) were recorded at the sitell Primary spacene~ltulL2 is by wall mounted electric resistance fancoil heaters. Zone thermostats are distributedthr~DtU~~hOl~t the house and there is no automaticnight setback system" The house is occupied by a

1.86 Downey and Douglass

family of two adults with two young children (oneof which was an infant during the monitoringperiod). One adult worked full time on a normalshift; the other did not work outside the home. Theoccupants understood their heating and ventilationsystems and generallyoperated and maintained themaccording to manufacturers' recommendations.

Data Acquisition System Overview

There are two levels of field thermal performancedata acquisition in the RCDP project. All theRCDP houses are instrumented to record thermalperformance with weekly or more resolution. Thisfive-house subsample represents a higher timeresolution experiment with more data points perhouse. The monitoring system for these homesaggregates data from 21 channels every hour.. Dataacquisition system design, installation, and dataretrieval and archiving are performed by asubcontractore

equipment was purchased "off the shelf.It Dataloggers were originally developed for use in remoteunmanned weather stations. They are packaged in atamper-proof box along with a modem connected toa dedicated phone linelO Programming is containedin volatile memory. The program may be reloadedor modified remotely/; An internal battery retainsthe program and data for several hours in the eventof a power interruption. Recorded channels includeindoor and outdoor ambient temperatures as wen asall parameters necessary to perform an energybalance for the heat recovery ventilators. All pointsare queried every 6 seconds and aggregated tohourly resolution and funning averages.. At leastonce per week, the units are interrogated by acentral computer and reset. Hourly data are thenarchived. The monitoring protocol is described indetail by Douglass (1989).

Air flow rates are determined with hot tip airvelocity transducers.. These sensors give an outputproportional to the velocity of air at standardtemperature and pressure. The transducer output isconverted directly to mass flow rate by an algorithmin the data logger, Flow monitoring transducers areinstalled toward the downstream end of the longest

straight dueting run available. To ensure accuracy,a duct traverse was done on installation to find aposition for the transducer probe where the meas­ured velocity accurately represents mass flow"

Humidity is monitored both upstream and down­stream of the heat recovery units in the exhauststreamG These measurements allow latent heat to becalculated. Bulk resistance type relative humiditysensors are used. Accuracy is within 4 percentbetween 30 percent and 90 percent relativehumidity. From 90 percent to 99.9 percent relativehumidity, S percent accuracy applies.. Air and watertemperatures are measured with RTD-type sensorsto within O.soP (0.3° e)" Electrical energy con­sumption is measuredwith power transducers. Thesesense voltage and current and give true power, notvolt amperes to the data logger" Accuracy is withinthe greater of 15 W or 3 percent

Hot water flow rate is sensed with a pulse initiatingnutating (for example, wobble plate) meter" Accur...acy is within 3 percent at flows above 0,,2 gpm(13 Is), but problems occur when there is no flowas discussed below$ Heat transfer parameters arederived by the data logger in real time (6-secondintervals) from other parameters listed aboves BTUsensors are not usede

Data. Quality and. Monitoring Problems

Data quality for this set of homes is very high"However, three sensors...·hot water flow, space heatenergy consumption, and exhaust ventilation massflowe-exhibit systematicquality problemse In the caseof exhaust ventilation mass flow, recorded data arelower than actual flow due to fouling of the massflow sensore Systematic errors in hot water flow andspace heat data are moresporadic in naturee

Exhaust Ventilation Mass Flow Errors

Airborne aerosols, dust, and cooking grease tendedto accumulate on the hot tip mass flow sensors andreduce readingse The rate of degradation variedconsiderably depending upon factors includingsystem on time and amount of airborne contami­nantso Contamination reduced the measured air flow

by as much as one half and was seen exclusively inthe exhaust flow sensors as they are exposed tomany more contaminants than fresh supply air flowsensors4P Since much of the proposed analysis isdependent on mass flow data and measured flowrates correspond to actual flow rates, a procedurewas created to correct for contamination of exhaustmass flow sensors"

All sensors were cleaned and verified as accurate byduct traverses at the time of installation, at leastonce during the monitoring period, and again forcalibration at the time of removal. Accordingly, theperiods immediately following cleanings are treatedas benchmark readingse The algorithm contained inEquation 1 was created to adjust monitored data toestimated actual data. This algorithm depends uponbeginning and ending benchmark readings, dailymaximum flow, and system on time. On those dayswhen the daily maximum flow is less than 85 per­cent of the previous day's maximum reading (forexample, the system did not operate for a contin­uous hour), daily maximum flow is assumed to beequal to the previous day's readings The first partof the algorithm adjusts each measurement as aratio of the initial maximum flow to the dailymaximum flow" The second .part of the algorithmadjusts the new flow data as a ratio of the finalmaximum flow to initial maximum flow multipliedby a percent of total system on-timeo For thepurpose of this analysis, exhaust fan energyconsumption is considered analogous to system ontimee

where Ma = Adjusted mass flow Ibmlhour

Mn = Monitored Mass flow for Observation n

in Ibm/hour

Mmax = Daily maximum monitored flow in Ibm/hour

~ = Final Mass flow in Ibm./hour

M1 = Initial Mass flow in Ibm/hour

Tn = System On Time for Observation n in hours

Tf = Final On Time in hours

Heat transfer data are generated in real time fromthe product of air mass flow and enthalpy difference

Building Equipment and Appliances 1.87

between upstream and downstream points in theEAHP evaporator dueting. These heat transfer dataare corrected as a ratio of the adjusted andmeasured mass flow.

While the algorithm corrects for sensor contamina­tion, potential errors in exhaust mass flow readingsare much greater than random errors generated bythe specified inaccuracy ofsensing equipment Theseerrors are propagated with the calculation of heattransfer rates. Moreover, these errors are difficult toquantify.

Space Heat En.ergy Consumption Errors,. Space heat

data contain outlying high values for Site 439 thatoccur during discrete blocks of time41 These valuesare typically two to four times the capacity of theheating system and corresponded to exactly 60 pul­ses per second. Thus the sensor or data logger had.periodically picked up interference from 60power line hum.. When the power transducer sensorswere replaced, and the problem was correctedo

Unfortunately, there is no way to determine if allreadings below the furnace capacity ceiling are good..They could have been influenced the power lineinterference for of the hour, but not enough toexceed the furnace capacityceiling* The extremereadings occur in clusters within single days andgroups of days~ Other groups of days seem to befree of the high readings * Upon this basis, entireblocks of data are excluded from the analysis ifsome data within the block are sus ted..

Domestic Hot Water Flow E:r:rors* Hot water flowdata are of the lowest quality.. Water meters utilizea disk that turns a rotor with a Hall effect(magnetic) switche This generates electrical pulsesthat are counted and scaled into units by thedata logger" When the rotor stops nearthe that triggers a pulse, the sensor canemit electronic noise@ Flow rates as high as6,000 gallons hour are recorded.. Spurious dataare and trelau(~nt]lv tlrrOUl!]1011t

the data set. While high readings arethere is no direct way to

validate moderate level readings. Effort was made toremove extreme but confidence in the

1.88 Downey and Douglass

remaining domestic hot water flow data is very low..Consequently, hot water flow data are not used inthis analysis..

RESULTS

Examination of System Ventilation. Performance

Program design standards require 0.25 mechanicalACH. Table 1summarizes ventilation system capac­ity, annual mean air change rate, and heating seasonair change rate in air changes per hour (ACH)I>

These air change figures are for mechanical ventila...tion onlyI' Air change rate figures are averaged overall hours regardless of system operation. All of thesystems had enough capacity to meet programmaticventilation requirements. However, the twoMontana systems (Sites 228 and 235) would havehad to operate almost continuously to meet pro­grammatic requirements and were not operatedfrequently enough to satisfy those requirements. TheWashington homes were over-ventilated during theheating season.. This over-ventilation is partially dueto insufficient control strategies and is examined inthe evaluation system heating performance below..

Examination of Heating Performance

All heat recovered is used for space or domesticwater heat The total amount of heat recovered bythe EAHP units is calculated based on the tempera­ture and relative humidity differences and mass flowrate across the E .. (Both sensible and latentheat recovery are accounted..) Methodologies usedto calculate total heat recovered are taken fromASHRAE (1989)..

Table 2 summarizes heat recovered, ventilation heatloss, net heat recovered, and the ratio of the meanheat recovered heat per mean exhaust air flowe Notethat Sites 235 and 450 were not monitored for anentire heating season" Most of the hours ofobservation for these two houses occurred duringnon-heating months. Only those hours when thesystems were operated (exhaust flow greater thanzero) are compiled in Table 20 Inclusion of thosehours with no exhaust air flow will reduce meanheat recovered statisticse

Table I.. Mechanical Ventilation Air Changes Per Hour

Exhaust AnnualFlow Mean Air

Capacity ChangesSite ACH ACH228 0.26 0.12235 0.27 0.09439 0.57 0.35449 0.73 0.42450 0.60 0.23

StandardDeviation

ACH0.100.090.220.290.26

Heating SeasonMean Air StandardChanges Deviation

ACH ACH0.14 0.100.09 0.070.46 0.120.56 0.180.55 0.10

Table 2. EARP Heat Recovered Heat Entire Monitoring Period

HeatMean Heat Ventilation Net Heat Recovered

#of Recovered Heat Loss Recovered per Ibm FlowSite Hours Btu/hr Btu/hr Btu/hr Btu/Ibm228 6,442 1,744 2,897 -1,153 5.3235 4,022 1,348 770 578 8.1439 7,381 4,731 3,677 1,054 6.1449 8,329 3,183 3,351 -168 5.5450 2,728 3,710 3,390 320 5.7

Recovered heat only part of EAHP systemperformance" Other factors that impact systemperformance are ventilation heat loss and operatingenergy consumption. Ventilation heat loss is afunction of indoor air temperature, outdoor airtemperature, heat capacity of air, and air flow rate"

hanical ventilation heat loss is calculated withEquation 20 Net heat recovered is the differencebetween heat recovered and ventilation heat loss"

ere net recovered heat is negative, more heat isneeded to warm makeup air than is extracted fromexhaust airB Ventilation heat loss in Table 2 is

to the heat loss that would bea non-heat ventilation

n<nil:.3>'Il"OV"lI'li"'llt'll' at the same exhaust rate.

Om = MCp(TrTo) (2)

where := Heat needed for make-up air due to mechanical

ventilation in Btu/hr

M == Mass flow rate due to mechanical ventilation in

lbmlh

== Mass heat capacity of air in Btu/lbm 0 F

Ti == Interior temperature in 0 F

= Incoming air temperature in 0 F

COP Calculation" The heating coefficient ofperformance (COP) is calculated with Equation 3"This figure is the ratio of the recovered energy tothe energy consumption by the compressor and fan..Note that the numerator contains both the energyrecovered from the ventilation air stream and theenergy utilized by the EAHP because all waste heatfrom the EAHP will also be recovered8

(3)

where COP == the heating system coefficient of performance

= the energy recovered by the EAHP

= the energy consumed by the EAJIP for

compressor and fan operation

The COP does not include energy required toreheat makeup air" The ventilation standard towhich these houses are designed requires mechanicalventilation of 0.25 air changes per hour. Anyventilation above this rate is considered excess andthe associated heat loss reduces the COP" The meanadjusted heating coefficient ofperformance (ACOP)

Building Equipment and pliances 1.89

where Q = Total space heat required in kW

UA = Heat transfer coefficient in kW/o F

IG = Internal and solar heat gains in kW

T1 = indoor temperature in 0 F

To = outdoor temperature in 0 F

Regression values for the heat loss coefficient andinternal gains, and actual air temperature differ­ential are substituted into Equation 4 to derivespace heat energy consumption. Table 4 containsthe resulting hourly space heat values that aresummed over the calendar year ending on April 30,

between 10:00 p.m. and 11:00 p.m. and turned upbetween 5:00 a.m. and 8:00 a.m. Reheat energy con­sumption is particularly difficult to infer becausethe time duration of reheat is confounded by varyingtemperature differentials. Worse yet, space heatenergy consumption tapers off gradually, before theapparent set point is reached, as if the thermostat issignificantly affected by self heating from it'santicipator. High shell insulation levels also creategradual temperature decay.

An envelope heat loss approach was used to evalu­ate the effects of nighttime thermostat setback. Thisrequired information on overall heat loss coefficientand internal gains and solar heat gains. A set ofwhole days with good space heat data and no tem­perature differentials less than 10 0 F was selected. Aregression of total space heating (supplementalspace heat condenser plus furnace energy consump­tion) against temperature differential was performedfor these days. Calculated heat loss coefficients andinternal gains are included. in Table 4.

is calculated by substituting net recovered energyinto Equation 30 See Table 3..

No attempt is made to compare water heating COPto space heating COP.. This would have requiredinvasive tank instrumentation beyond the scope ofthis study. Moreover, this question has already beenexplored in lab testing conducted by a nationallaboratory (Wallman, Fisk, and Grimsrud 1988)..This research noted significantly higher COP forspace heating than for water heating. This is due tothe lower temperature required for space heating.Revision ofcontrols to give priority to space heatingwas suggested.. The manufacturer has since madesuch modificationso Units with redesigned controlsare currently being monitored in Cycle 2, RCDP.Data acquisition, similar to that reported herein,was completed at the end of April 19900

The magnitude of net heat recovered is a functionof exhaust air and outdoor temperature difference..If the outdoor temperature is less than theexhaust temperature, more heat is required to heatmakeup air than is extracted from exhaust airl>Figure 3 plots the difference between outdoor andexhaust air temperatures versus the total amount ofheat recovered for each of the EARP homes~ As theoutdoor temperature decreases, net recovered heatalso decreaseS0 Energy savings could be increased ifthe EAHP is operated only when the exhaust airtemperature is less than the exterior airtemperature.

Effect of Th.ermostat Setback Nighttimethermostat setback was practiced by occupants intwo of the houses (Sites 228 and 439)~ This is themost significant occupant behavior in these houses~

Space heat thermostat is usually turned down

Q = UA (TrTcJ - 10 (4)

Table 3~ EAHP Coefficient ofPerfonnance and Adjusted Coefficient ofPerformance

#of Mean Standard Mean StandardSite Hours COP Deviation ACOP Deviation228 6,435 1.83 0.37 0.44 0.58235 4,022 1.70 0.32 1.29 0.45439 7,381 3.09 0.58 2.39 1.27449 8,329 2.46 0.48 1.56 1.02

2,728 2.71 0.29 1.93 2.01

1.90 Downey and Douglass

40-20

10000

, "

"*-I .. "row

::r.:"0 5000QJ 'C- C- , 'w.c>'oz~CD0:

"*-I

W0Z

o 20Outdoor Air Temp. - Exhaust Air Temp,

Figure 3.. Outdoor-Exhaust Air Temperature Differential vs.. Net Recovered Heat ... Site #439

-5000

52-Week EstimatedSpace Heat

without SetbackkBtu25,32061,464

GainsBtu/hr5,0228,319

UABtu/F-hr

269889

Evaluation ofEnergy Savings from Nighttime Setback

52-Week ActualSpace Heatwith Setback

kBtu21,45851,903

Site228439

1 ~ The above computation is rerun with indoorair temperature data modified so that it never dropsbelow 0 F, the inferred temperature if setback isnever done,.

The savings from night setback are 18..0 percent and18..4 percent for Sites 228 and 439, respectively..Energy consumption was found to be significantlylower due to setback.. However, thesesavings are limIted by the EAHP control strategy..

The EAHP is set to give priority to domestic hotwater (DHW) if there is demand. for such heat Ifwater heat is not required, priority is given to spaceheat that there is demand. The thermostatfor the supplemental space heat condenser is set ata higher temperature than the main space heatthermostat This is to ensure that the EARP pro­vides space heat before electric resistance heat is

Since the EAI-IP supplemental space heat conden­sers operated. at peak capacity during the setbackhours of winter months, it is assumed that thesupplemental space heat condenser thermostat wasnot set back" Consequently, the main thermostatwas set back to a level well below the EARP..Greater energy savings could have been achieved bysetting back both the main space heat and thesupplemental space heat condenser thermostats. Thesupplemental space heat condenser thermostatshould be set on a temperature differential to themain thermostat.. As the main thermostat is setback, the supplemental space heat condenser ther­mostat would also be set back an equal increment.Further analysis is required to determine morningreheat energy requirements compared to supple­mental space heat condenser requirements under setback and non-set back scenarios. If reheat energy isserved by resistance heating, it may be more

Building EqUipment and Appliances 1.91

economical to run the supplemental space heatcondenser when ventilation is needed and indoortemperature is falling below the morning setting ofthe electric furnace thermostat.. The energy eco­nomics are more complicated when consideringover-ventilating in order to operate the supple­mental space heat condenser..

CONCLUSION

Five homes located in Montana and Washingtonwere equipped with exhaust air heat pump systemsand monitored with hourly data acquisition systems..These systems were evaluated for ventilation andheating performance..

1\vo different types of systems were installed.. TheMontana systems had a complex design and lowerperformance than the Washington systems" Themanufacturer has since discontinued production ofthe Montana systems.. All systems had enough venti­lation capacity to meet programmatic ventilationrequirements. However, the Montana systems werenot operated with sufficient frequency to meetprogram requirements" The Washington homes hadexcess ventilation4>

These systems recovered between 5,422 and 34,922kBtu over the monitoring period and operated withCOPs between 1.70 and 3$09. The Montana systemsoperated with lower COPs and are no longer beingmanufactured.. The three Washington. EARP systemsover-ventilated the structure due to insufficientsupplemental space heat condenser control strate­gies.. Real EARP efficiency could be increased11mltlIJL2 the for space and water heatingto those when outdoor exceedsthe expected exhaust or when ventila-tion is Effectiveness of set backcould be enhanced by coupling the main space heatand supplemental space heat condenser thermostats..

AC OWLEDGMENTS

The research herein is sponsored by theUnited States of Energy, BonnevillePower Administration. The Project was operated bythe State Energy Office.. Under contractto Washington The Fleming Group

1.92 Downey and Douglass

implemented the actual monitoring, includingspecifying verification and quality controlprocedures and selecting all hardware. Energyagency personnel in the states of Idaho andMontana served as site liaison, identifying candidatehouses, performing prewiring inspection, andperforming some equipment maintenanceG

REFE NCES

ASHRAE4> 1989.. ASHRAE Handbook--1989 Funda­mentals.. Chap.. 6.. Atlanta: American Society ofHeating, Refrigerating, and Air-ConditioningEngineers, Inc..

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