F e b r u a r y 2 0 0 4 A S H R A E J o u r n a l 1 5

By Glenn C. Hourahan, P.E., Member ASHRAE

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About the Author

versized air-conditioning systems significantly degrade the abilityof unitary HVAC equipment to control humidity within residen-

tial and commercial buildings. Additionally, oversizing causes manyother problems such as large temperature differences betweenrooms, occupant discomfort, higher installed cost and excessive part-load operation (see sidebar on Page 16).

This article summarizes key steps inperforming such a procedure.

Step 1: Establish Building Design andCriteria Requirements

Before undertaking a load calculation,ascertain the type of HVAC systems (i.e.,ducted vs. ductless equipment, gas-fired

Glenn C. Hourahan, P.E., is vice president ofResearch & Technology for the Air ConditioningContractors of America (ACCA) in Arlington, Va.He has served on various ASHRAE committees,and is a member of the Task Group on Residentialand Small Buildings Applications (TG9.RSBA).

Reasons for designers/contractors toroutinely oversize HVAC equipment in-clude not performing load calculations,performing load calculations incor-rectly, and attempting to avoid customercomplaints. However, to properly sizeHVAC equipment, a rigorous heat gain/heat loss procedure must be observed.

furnaces vs. electric heat pumps, etc.) thatare compatible with the building and thebuilding’s application. This includes de-termining space requirements andoccupant needs. Appearance issues, archi-tectural design concerns, and space con-straints also influence system selection,as well as how the mechanical equipmentresponds to design and building require-ments. The overall building budget andsystem budget also impact the type of sys-tem, zoning, and capabilities of the equip-

The following article was published in ASHRAE Journal, February 2004. © Copyright 2004 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically orin paper form without permission of ASHRAE.

1 6 A S H R A E J o u r n a l a s h r a e . o r g F e b r u a r y 2 0 0 4

ment to be selected. The fuel types avail-able to the site, in addition to their rela-tive costs, also have an impact.

Step 2: Determine the Design LoadsDetermining heat loss/heat gain loads

at design conditions requires examina-tion of several parameters.

Building Construction Parameters

It is critical that building constructionparameters be evaluated carefully, andthat assumptions on the related buildingdetails are verified carefully. Examplesof such deliberations include: buildingenvelope, insulation type/level, glasstype and shading, solar orientation, andduct tightness/location.

Design Conditions

Outdoor design conditions should bethe 1% cooling dry-bulb design pointfor the specific geographic location ofthe building. The indoor design condi-tions should be based on customer needsand requirements. As a default, design-ers should observe the following nomi-nal indoor design conditions:

Winter Heating Design Point: 70°F(21°C) at 30% relative humidity.

Summer Cooling Design Point: 75°F(24°C) at 50% relative humidity.

Full Load vs. Part Load

In performing the load analysis, it isimportant to pay attention to the sensible(i.e., temperature related) and latent loads(i.e., moisture related). It needs to be rec-ognized that the load calculation is basedon the peak-load conditions (sensible +latent). For summer cooling, this gener-ally occurs on a sunny, hot day, and thepeak sensible load occurs at the peak dry-bulb condition. However, what happensin the evening when the sun sets? What ifit is raining? It is reasonable to expectsome summer hours when the outdoorcondition may be in the low 80°Fs andrelative humidity is 100% (it’s raining!).

Since the design methodology resultsin equipment sized for peak dry-bulb tem-perature (the hot, sunny afternoon), theair conditioner has excess capacity whenoperating at non-peak, part load duty (theother 99% of the time). Constant volume

unitary equipment, in being oversized fora part-load condition, easily satisfies thethermostat and cycles off long before mois-ture removal can be affected. Cycling re-sults in a warmer coil temperature withless latent capacity than a colder coil.Hence, just when latent removal capabil-ity may be needed most, it is least avail-able. The tradeoff is whether it is preferableto be a little warm on the hottest days, ormaintain setpoint on the design day,thereby providing poorer humidity con-trol almost all of the rest of the time.

Step 3: Do Not Add Safety FactorsOnce a load calculation has been de-

termined, and the sensible and latent loadsestablished, it is important not to ruin thegood work by arbitrarily adding safetyfactors. Routinely adding “just-in-case”safety factors of 25% to 100% is unac-ceptable for most unitary applications.The practice results in equipment cycling(and hence a warmer average coil tem-perature), and may result in too much air-flow (again increasing the coiltemperature and decreasing latent capac-ity). Additional ways to ruin an otherwisegood load determination include:

• Overly conservative assumptionson the building construction details.Purposely using “loose” or “conserva-tive” design criteria to increase the cal-culated cooling requirement isunnecessary and counterproductive forobtaining proper loads.

• Failure to observe room and build-ing diversity factors. The required ca-pacity is not necessarily the sum of thepeak individual room loads. It must beobserved that buildings with large lev-els of solar glass load — especially ifthese windows are predominantly on oneside of the structure — will have roomswith large loads that peak at differenttimes than other rooms.

• Upsizing equipment in the belief thatbigger is better. This is a problem withcustomers who think buying a larger unitfor nearly the same money is a goodvalue. Designers need to explain the ben-efits of using properly sized equipment.

Step 4: Verifying System CapacitiesIn verifying capacities and making the

• Marginal part load temperature control• Large temperature differences between

rooms• Degraded humidity control• Drafts and noise• Occupant discomfort/dissatisfaction• Larger ducts installed• Increased electrical circuit sizing• Excessive part load operation• Frequent cycling (loading/unloading)• Shorter equipment life• Nuisance service calls• Higher installed costs• Increased operating expense• Increased installed load on the public

utility system• Increased potential for mold growth• Potential to contribute to asthma and other

respiratory conditions

Effects ofEffects ofEffects ofEffects ofEffects ofOversizingOversizingOversizingOversizingOversizing

••••• Load determination was not made· Prior experience is used to “guess”

the load· Simple replacement of “like” for

“like”: - Assumes that the original installa-

tion was correct - Ignores whether building functions

have changed - Ignores building upgrades

(lighting, insulation, etc.)· Use of obsolete and inadequate “rules

of thumb”••••• Incorrect observance of procedures

· Mistakes in the load calculation· Overly conservative assumptions on

building attributes· Use of safety factors· Designers compensate for air distri-

bution problems by oversizing equipment• Safeguarding callbacks

· Designers seek to minimize occupantcomplaints on days that exceed designconditions

Reasons forReasons forReasons forReasons forReasons forOversizingOversizingOversizingOversizingOversizing

F e b r u a r y 2 0 0 4 A S H R A E J o u r n a l 1 7

final equipment selection, it is essentialto observe all manufacturers’ sizing, se-lection, and application guidelines. As pre-viously noted, be sure that the equipmentcan meet the sensible and latent coolingrequirements without being oversized. Forcontrolling moisture within the building,it is crucial that designers verify that theselected equipment has the capability tohandle the latent load at full load opera-

tion (peak dry bulb conditions) and partload operation (peak wet-bulb conditions).

Step 5: Considerations if EquipmentCannot Satisfy Latent Requirements

If the selected unitary equipment isunable to satisfy the full-load and/or part-load latent requirements, equipmentmanufacturers offer innovative solutionsfor modulating latent capacity:

• Modified control strategies that en-gage the cooling equipment based on hu-midistat demand. However, humidistatcontrol is likely to require reheat to pre-vent overcooling in the conditionedspace. Other control sequences can per-mit the evaporator to operate at a lowertemperature.

• Optimized equipment using multi-speed/variable-speed indoor fan units en-able lower supply air temperatures.Reduced airflow increases opportunitiesto remove moisture from the airstream.

• Hybrid equipment that uses wrap-around heat pipes or air-to-air heat ex-changers. The intent is to reheat the drysupply air leaving the dehumidifying coilwith heat recovered from the air enteringthe coil. This approach allows the cool-ing coil to wring out as much moisture aspossible from the preconditioned air.

For humid applications, or where a highlevel of humidity control is needed, de-signers should consider independentlycontrolling temperature and humidity:

• Whole building dehumidificationequipment (perhaps interconnected to theprimary fan and using the same duct sys-tem as the air-conditioning system) canindependently control moisture loads.

• Dedicated outdoor air systems canbetter reduce the moisture loads that arisedue to the introduction of warm, moistair for ventilation requirements since theoutdoor air is preconditioned beforemixing with the indoor return air.

Both approaches eliminate a major partof moisture load, and allow the primarycoil to do a better job of sensible cooling.

ConclusionCorrect sizing and selection of unitary

equipment, appropriately controlled, areimportant steps to maintain proper humid-ity levels. This requires configuring theHVAC system to meet sensible and latentloads, not only at the design conditions(full load), but also over a broad range ofoff-design conditions (part loads).

Subsequent steps for controlling build-ing humidity levels include ensuring theselected building systems (HVAC equip-ment, ductwork, envelope, etc.) are prop-erly installed, commissioned, maintained,and serviced over the building’s life.

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