APPLICATION GUIDE

AIR SYSTEMS – ENERGY SERIESEnergy Recovery Wheels

Supercedes: Nothing Form 102.20-AG6 (305)

INTRODUCTION

As fossil fuel reserves are depleted, the cost of energycontinues to rise. According to the US Department ofEnergy, the cost of energy used in US commercial build-ings increased by more than 200% between 1979 and1995 and conservative DOE estimates predict an addi-tional 46% increase between 2001 and 2025. Given thisand the fact that, on average, HVAC systems consume39% of the energy used in commercial buildings, en-ergy-efficient HVAC systems represent potentially sig-nificant savings in building operating costs.

Increased ventilation rates, which are required to satisfythe ventilation standard ASHRAE 62.1-2004, mean agreater expenditure of energy to condition outside air.One way that savings can be realized in an HVAC sys-tem is through the use of exhaust air energy recovery.Exhaust air energy recovery can take many forms—ro-tary heat exchangers, heat-pipes, plate heat exchangers,etc., but all of the devices operate on the same principle—they use exhaust air to condition supply air through atransfer of energy. This application guide examines therotary heat exchanger—also called an energy wheel, orenergy recovery wheel—and the benefits of incorporat-ing such a device in an air handling unit.

PRINCIPLES OF ENERGY RECOVERY

Energy recovery involves a transfer of energy betweenan exhaust airstream and a supply airstream. Figure 1illustrates the heat transfer process of an energy recov-ery wheel where OA is outside air; SA is supply air;RA is return air from the conditioned space and EA isexhaust air.

As the two airstreams pass through the energy recov-ery wheel, the rotation of the wheel facilitates the trans-fer of energy from the higher energy airstream to thelower energy airstream. This means that the exhaustair preheats the supply air in the winter and precoolsthe supply air in the summer. Some systems use en-ergy recovery wheels to reheat supply air after it hasbeen cooled—an effective means of humidity control.

Some energy recovery wheels transfer only sensibleenergy, while others transfer sensible and latent (i.e.total) energy.

Sensible Heat TransferWhen sensible heat is transferred, the dry-bulb tem-perature of the colder airstream increases and the dry-bulb temperature of the warmer airstream decreases.No moisture is transferred, so the humidity ratio of thetwo airstreams remains unchanged unless the dry-bulbtemperature of the warmer airstream is decreased be-low its dew point, allowing condensation to occur.

Total Heat TransferThis process involves the transfer of sensible and la-tent heat energy. Latent heat energy is dependent onthe amount of water vapor in the air and therefore totalheat transfer can only occur when water vapor is trans-ferred from one airstream to the other. In an energyrecovery wheel this transfer is accomplished throughthe use of a desiccant which absorbs/adsorbs watervapor from the higher vapor pressure airstream andreleases it to the lower vapor pressure airstream. Onlyenergy recovery wheels and certain types of fixed-plateheat exchangers with permeable membranes can trans-fer latent, and therefore total, energy.

EffectivenessThe ratio of the amount of energy transferred by theenergy recovery device to the difference in energy lev-els of the two incoming airstreams is called effective-ness. The total amount of energy transferred by thewheel is a function of the effectiveness of the wheel,the airflow volumes of the two airstreams and the dif-ference in energy levels between the two airstreams.

FIGURE 1. STANDARD AIRFLOW CONVENTIONS

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FORM 102.20-AG6 (305)

Equation 1 shows the calculation of effectiveness asdefined by ASHRAE Standard 84-1991:

Equation 1: = [Vs • (x1 - x2)] / [Vmin • (x1 - x3)]

Where:

= Sensible, or total effectiveness

x1 = OA temp (°Fdb) or enthalpy (btu/lb.)

x2 = SA temp (°Fdb) or enthalpy (btu/lb.)

x3 = RA temp. (°Fdb) or enthalpy (btu/lb.)

Vs = Supply (or outside) air volume (cfm)

Vmin = The lower of the exhaust or supply airvolume (cfm)

Amount of Heat TransferredThe sensible and total energy transferred by the energyrecovery wheel can be calculated using Equations 2and 3:

Equation 2: Qs = • 1.08 • Vmin • (t1 - t3)

Equation 3: Qt = • 4.5 • Vmin • (h1 - h3)

Where:

Qs = Sensible heat transferred (btu/hr)

Qt = Total heat transferred (btu/hr)

= Sensible, or total effectiveness

Vmin = The lower of the exhaust or supply

air volumes (cfm)

t1 = Outside air temperature (°F)

t3 = Return air temperature (°F)

h1 = Outside air enthalpy (btu/lb.)

h3 = Return air enthalpy (btu/lb.)

1.08 = Conversion factor

4.5 = Conversion factor

If the effectiveness of the wheel is known, Equation 1can be solved for x

2 to determine the supply air leaving

enthalpy and/or temperature. In Example 1 the outsideair and return air volumes are equal, or “balanced”(V

s = V

min). Optimal energy transfer occurs at “balanced

flow” conditions, however building exfiltration andexhaust sources such as bathroom fans reduce the re-turn air volume below the supply air volume.This unbalanced flow reduces heat transfer, even thoughit increases the effectiveness factor of the energyrecovery wheel.

Example 1: Calculate the supply air conditions leav-ing a total heat recovery wheel with 12,000 cfm ofoutside air at 94°Fdb and 77°Fwb; and 12,000 cfm ofreturn air at 75°Fdb and 50% RH, if both the sensible& latent effectiveness of the wheel is 60%. Calculatethe reduction in required cooling energy.

At the conditions given above:

x3 = the return air enthalpy = 28.15 btu/lb. = h3

x1 = the outside air enthalpy = 40.38 btu/lb. = h1

From Equation 1:

h2 = h1 - [ • Vmin • (h1 - h3)]/Vs

h2 = 40.38 - [0.6 • 12000 • (40.38 - 28.15)] / 12000

h2 = 33.04 btu/lb.

Also from Equation 1:

t2 = t1 - [ • Vmin • (t1 - t3)]/Vs

t2 = 94 - [0.6 • 12000 • (94 - 75)] / 12000

t2 = 82.6°F.

Therefore, the supply air leaving the heat recoverywheel will be: 82.6°Fdb / 68.9°Fwb

From Equation 3:

Qt = • 4.5 • Vmin • (h1 - h3)

Qt = 0.6 • 4.5 • 12000 • (40.38 - 28.15)

Qt = 396,252 btu/hr = 33 tons

Figure 2 shows the effect of unbalanced airflow on ef-fectiveness.

Example 2 indicates that despite a 7% increase in ef-fectiveness, the overall heat transfer of the wheel de-creased by 10% in relation to the balanced flow condi-tions of Example 1.

FIGURE 2– EFFECT OF UNBALANCED AIRFLOW

FORM 102.20-AG6 (305)

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If a sensible-only energy recovery device, such as aheat pipe, had been used, the energy savings in Ex-ample 1 would have been:

Qs = 0.6 • 1.08 • 12000 • (94-75)

= 147,744 btu/hr

= 12.3 tons

And for Example 2:

Qs = 0.67 • 1.08 • 9600 • (94-75)

= 131,985 btu/hr

= 11.0 tons

The additional energy savings achieved by transferringtotal energy illustrate one of the advantages of usingenergy recovery wheels over a device that only trans-fers sensible energy.

Transfer of Air between AirstreamsInherent in the operation of an energy recovery wheelis a direct transfer of air between the return and supplyairstreams (See Figure 3). This air transfer is due toleakage through the seals separating the airstreams aswell as by the small amount of air carried over in thematrix of the wheel as it rotates from one airstream tothe other.

The air passing from the return airstream to the supplyairstream is defined as the Exhaust Air Transfer Ratio(EATR). The EATR is the percentage of supply airthat originated as return air. This ratio is determinedby measuring the concentrations of a tracer gas in theRA, SA and OA airstreams.

The air passing from the outside airstream to the ex-haust airstream is defined as the Outside Air Correc-tion Factor (OACF). The OACF is the OA volume atPoint 1 divided by SA volume at Point 2. The OACFand EATR are determined for a given condition throughtesting in accordance with ARI Standard 1060-2000.Figure 4 illustrates the effect of this leakage on the air-flow rates of an energy recovery wheel with a balanced,nominal flow rate of 12,000 cfm.

The EATR and OACF are typically calculated by soft-ware provided by the energy recovery wheel manufac-

Example 2: Consider the unit from Example 1, butinstead of balanced flow, the return airflow is only9,600 due to exfiltration and/or exhaust air. What arethe conditions of the supply air leaving the energyrecovery wheel in this case? What is the new heattransfer rate? (Assume the outside air and return airconditions remain the same.)

Vmin/Vmax = 9,600 cfm / 12,000 cfm = 0.8.

From Figure 2: At 0.8 flow ratio, = 67%.

Solving Equation 1 for x2 (h2):

h2 = h1 - [ • Vmin • (h1 - h3)]/Vs

h2 = 40.38 - [0.67 • 9600 • (40.38 - 28.15)] / 12000

h2 = 33.82 btu/lb.

And:

t2 = t1 - [ • Vmin • (t1 - t3)]/Vs

t2 = 94 - [0.67 • 9600 • (94 - 75)] / 12000

t2 = 83.8°F.

Therefore, the supply air leaving the energyrecovery wheel will be: 83.8°Fdb / 69.8°Fwb

From Equation 3:

Qt = • 4.5 • Vmin • (h1 - h3)

Qt = 0.67 • 4.5 • 9600 • (40.38 - 28.15)

Qt = 353,985 btu/hr = 29.5 tons

FIGURE 3 – AIR TRANSFER PATHS

FIGURE 4 – EFFECT OF EATR AND OACF

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FORM 102.20-AG6 (305)

turer, but it is important to understand how these fac-tors affect, and are affected by, system design. Themagnitude of the EATR and OACF affects fan sizing,while the positions of the supply and exhaust fans, andthe air pressure drops they develop, affect the magni-tude of the EATR and OACF. Figures 5 through 8 showthe four possible fan arrangements along with the ad-vantages and precautions associated with each.

Proper fan arrangement and control of the static pres-sure minimizes the air transfer caused by leakagethrough the gaskets and seals of the wheel, howeverthese practices will not reduce the transfer of air causedby carryover. As Figure 9 shows, some of the air fromthe return/exhaust path remains in the matrix of thewheel as it rotates to the outside/supply airstream. This“carryover” air mixes with the incoming outside air and

FIGURE 5 – BLOW-THRU SUPPLY/BLOW-THRUEXHAUST

Advantages

• Minimal leakage when supply and exhaust pathstatic pressures are nearly equal.

Precautions

• Direction of leakage depends on relative staticpressures.

FIGURE 6 – BLOW-THRU SUPPLY/DRAW-THRUEXHAUST

Advantages

• Minimizes leakage of air from exhaust path tosupply path.

Precautions

• Leakage from supply path to exhaust path can beexcessive if static pressure differences are not mini-mized.

FIGURE 7 – DRAW-THRU SUPPLY/DRAW-THRUEXHAUST

Advantages

• Minimal leakage when supply and exhaust pathstatic pressures are nearly equal.

• Good air distribution across the face of the wheel.

Precautions

• Direction of leakage depends on relative staticpressure.

FIGURE 8 – DRAW-THRU SUPPLY/BLOW-THRUEXHAUST

Advantages

• None.

Precautions

• Significant leakage from exhaust air path to supplyair path.

• DO NOT USE.

FORM 102.20-AG6 (305)

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enters the supply air path. For most applications, suchas comfort cooling, carryover is of little concern. How-ever, if there were high concentrations of hazardoussubstances such as VOCs or carcinogens in the air, theywould enter the supply airstream via the carryover airand may present a health hazard to occupants. For thisreason, energy recovery wheels should not be usedin applications where high concentrations of haz-ardous substances may be present in the exhaust air,for example laboratories or hospital operatingrooms.

PurgeA mechanical purge section can be used to reduce thevolume of carryover air. (See Figure 10.) Mechanicalpurge isolates a section of the wheel on the boundarybetween the RA/EA path and the OA/SA path at thepoint where the wheel rotates from the RA/EA path intothe OA/SA path.

Placing a block-off over this section of the wheel onthe RA/SA side forces outside air that has traveledthrough the wheel to flow back into it in the oppositedirection. This prevents air from the RA/EA path fromentering the last few degrees of the wheel before it ro-tates into the OA/SA path. The return air that enteredthe wheel prior to the purge section has time to exit thewheel on the exhaust side. The angle of the purge sec-tion - how large a “slice” of the wheel it covers deter-mines how effective it is. The larger the angle, thegreater is the reduction in carryover. Purge should not,however, be counted on to eliminate carryover com-pletely; therefore, even energy recovery wheelsequipped with purge sections should not be usedwhen high concentrations of hazardous substancesare likely in the exhaust airstream.

Frost ControlFrost formation is always a concern when HVAC equip-ment operates in sub-freezing weather. Heat recoveryequipment is no exception, but the likelihood of frostformation is greater on sensible-only heat transfer de-vices, including sensible-only energy wheels, than ontotal energy transfer devices. Consider both processeson the psychrometric chart:

During the heating season a total energy recovery de-vice transfers heat and moisture from the warm, moistreturn air to the cold, dry outside air. This lowers theexhaust air dewpoint - the temperature at which con-densation and frost formation occurs. Figure 12 com-pares typical frost threshold temperatures of total en-ergy recovery wheels and sensible-only heat exchang-ers. Condensation and frost begin to form to the left ofthe respective boundary lines. The dewpoint depres-sion achieved with total energy devices can lower thefrost formation threshold well below 0°F when the in-door air relative humidity is low.

Total energy recovery wheels located in climates withextreme winter conditions, and/or where indoor air rela-

FIGURE 9 – AIR TRANSFER DUE TO CARRYOVER

FIGURE 10 – ENERGY RECOVERY WHEEL WITHPURGE SECTION

FIGURE 11 – EFFECT OF TOTAL ENERGY TRANSFER

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tive humidities are high may still require some meansof frost prevention. There are four common methodsof frost prevention - three of which are generally fa-vored over the fourth.

On-Off control is the least expensive and least com-plicated method of preventing frost. When the wheelis not rotating, no energy transfer takes place and themoisture in the exhaust airstream is in no danger ofcondensing and freezing. The drawbacks to this methodare that when the energy recovery wheel is not operat-ing, no energy savings are realized and heating elementsmust be sized for design winter conditions. On-Offfrost prevention control is best suited for mixed airsystems with a low minimum outside air requirementand in climates where the outdoor air temperature dropsbelow the frost threshold mainly during the unoccu-pied period when ventilation is not required.

Bypassing the outside air or a portion of it is anothercommon tactic for preventing frost formation. Thismethod consists of mounting a bypass damper in theOA/SA path that opens to divert some of the outsideair around the energy recovery wheel when outside airconditions are below the frost threshold. This reducesthe heat transfer capacity of the wheel and prevents theleaving exhaust air from reaching saturation. Propermixing is crucial to prevent stratification, which couldfreeze downstream coils in the air handler. The heat-ing elements must be somewhat larger in this type ofsystem than a system in which the wheel operates at100% during winter design conditions, but do not nec-essarily have to be sized for design winter conditions.Bypass systems are best suited for climates with veryfew hours per year below the frost threshold and forsystems that do not include humidifiers. Systems withairside economizers are required to have OA bypass totake full advantage of economizer operation.

Entering air preheat is a widely accepted method offrost control that has the additional advantage of al-lowing the wheel to operate at maximum air volumeduring design heating conditions. The approach to thisfrost control method is to mount a heating device on

either the OA entering side of the wheel (position 1) oron the RA entering side of the wheel (position 3); seeFigure 13. When the heating device is mounted in po-sition 1, the temperature of the outside air is raisedabove the frost threshold temperature of the wheel, pre-venting the exhaust air from getting cold enough to formfrost. When the heating element is placed in position3, enough energy is added to the RA path to preventthe heat transfer to the OA/SA path from lowering theexhaust air temperature below the saturation point.

This method of frost control is well suited to climates

with extreme winter conditions, and for systems thatinclude some form of mechanical humidification. Ifthe preheat device consists of a steam or hydronic coilmounted in the outside airstream, some form of freezeprotection for the coil must be included. No such pre-caution is necessary if a steam or hydronic coil is usedto preheat the return air. Although a heating devicemounted in position 3 (RA) will typically require agreater capacity than a device mounted in position 1(OA) to provide the same degree of frost prevention,adding heat to the return air path increases the sensibleheat transfer to the supply air path. When a preheatelement is used in position 3, caution should be exer-cised so that the temperature of the entering return airdoes not exceed the recommended operating tempera-ture of the energy recovery wheel.

Variable speed control can be used to reduce the rota-tional speed of the wheel, which reduces its ability totransfer energy. Because of all the variables involved,this method of frost control requires a sophisticatedcontrol strategy, which if not followed correctly canactually increase the likelihood of frost formation.Reducing the wheel speed can also lead to significantreductions in performance and the need for supplemen-tary freeze protection for downstream coils. For thesereasons, the previous three methods of frost preven-tion are generally favored over variable wheel speedcontrol.

FIGURE 12 – FROST THRESHOLD TEMPERATURES

FIGURE 13 – PREHEAT FROST CONTROL

FORM 102.20-AG6 (305)

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Capacity ControlMaximum heat transfer is not always necessary, nordesirable. The two most common methods of control-ling the energy transfer rate of the wheel are variablespeed control and bypass of exhaust or outside air.

Variable speed control consists of using a speed con-troller, such as a variable frequency drive (VFD) orsilicon controlled rectifier (SCR), attached to the drivemotor of the wheel. As the rotational speed of the wheeldecreases, the heat transfer capacity also decreases.There are limits to this type of capacity control, how-ever. The reduction in capacity is not proportional tothe reduction in wheel speed; therefore, reducing thespeed of the wheel by 50% may only result in a 10%reduction in energy transfer.

Exhaust or outside air bypass requires a bypass dampermounted in the OA/SA path or the RA/EA path andhas several advantages over variable speed capacitycontrol. Air bypass results in more linear control ofthe energy wheel capacity making the control strategymore reliable. Also, a greater overall reduction in ca-pacity can be achieved by using bypass than by usingvariable speed control, and the pressure drop of thesystem is reduced as the volume of air passing throughthe wheel is reduced.

Placing the bypass damper in a position to divert ex-haust air in lieu of outside air will minimize the possi-bility of stratification in 100% outside air systems. Thisdamper could also be placed in the exhaust airstreamto control the capacity of the wheel by reducing thevolume of exhaust air available to transfer energy to orfrom. However, in an air hander with an airside econo-mizer, when the system modulates from minimum out-side air operation to 100% economizer mode, somemeans of diverting the outside air around the wheelmust be included to prevent excessive pressure dropand unwanted preheating. In this arrangement theeconomizer bypass damper could be used as a capacitycontrol damper in the minimum outside air mode, al-though proper mixing of treated and bypassed air wouldbe essential to prevent freezing of hydronic coils down-stream of the energy recovery wheel.

ENERGY RECOVERY APPLICATIONS

There are three application categories for air-to-air en-ergy recovery based on the type of system the energyrecovery device serves, and two subcategories based onhow the device is used in the system. The applicationcategories are process-to-process, process-to-comfort andcomfort-to-comfort. Examples of these applications aregiven in Table 1. The subcategories are preconditioningof outside air and tempering of supply air.

Method Typical ApplicationProcess-to-process Dryers and OvensProcess-to-comfort Flue stacks

BurnersFurnacesIncineratorsPaint exhaustWelding

Comfort-to-comfort Swimming poolsLocker roomsResidentialSmoking exhaustOperating roomsNursing homesAnimal ventilationPlant ventilationGeneral exhaust

2000 ASHRAE Handbook - HVAC Systems and Equipment

Table 1. Applications for Air-to-Air Energy Recovery

Process-to-process (PTP) and process-to-comfort (PTC)applications generally only require heating. The en-ergy recovery device transfers the heat generated bythe process from the exhaust airstream back into theprocess supply airstream or building makeup airstream,respectively. Because excess humidity is detrimentalto many process applications, PTP systems typicallyonly require sensible heat transfer. Process-to-com-fort applications may use total energy recovery devicesto humidify dry outside air in the winter and to reducethe likelihood of frost formation. Comfort-to-comfort(CTC) heat recovery systems operate during both theheating and cooling seasons and may transfer sensible-only or total heat from the building exhaust sources tothe makeup airstream.

For PTP and PTC applications a plate-type heat ex-changer may be more appropriate than an energy re-covery wheel. This is due to the fact that in most casescross leakage must be eliminated, also plate exchang-ers may be more resistant to the temperatures of theprocess exhaust and usually only sensible energy re-covery is required. Energy recovery wheels are com-monly used for CTC applications, except in cases wherethe possibility of cross contamination precludes theiruse.

Preconditioning of Outside AirA primary use of exhaust air energy recovery is to pre-condition outside air. Preheating/humidifying in win-ter and precooling/dehumidifying in summer reduces

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annual operating costs and may decrease first cost asthe required heating and cooling capacity is reduced.Figure 14 illustrates one possible configuration of anoutside air preconditioning system.

Example 1 on page 2 illustrates a potential 33 ton (42%)equipment capacity reduction of an outside air precon-ditioning system in the cooling mode. Using the en-ergy recovery wheel to preheat and humidify the out-side air in the heating season can achieve similar sav-ings and capacity reductions.

In systems with airside economizers, the energy recov-ery wheel should be sized for minimum outside air-flow only and provisions should be included to bypassor otherwise prevent operation of the wheel duringeconomizer mode. Operation of an energy recoverywheel during economizer mode increases energy con-sumption.

Tempering of Supply AirUsing an energy recovery wheel to temper supply airis an energy efficient way to control humidity. In thepast, humidity control involved cooling the air belowthe temperature required to satisfy the sensible loadand then reheating it. While this simultaneous heatingand cooling provided fine temperature and humiditycontrol, it wasted a great deal of energy. ASHRAEStandard 90.1-2004 prohibits simultaneous heating andcooling unless, among other specific exceptions, at least75% of the reheating energy is provided from a site-recovered energy source.

Two methods of tempering the supply air are shown inFigures 15 and 16. The parallel arrangement uses theenergy recovery wheel to reheat the supply air duringthe cooling mode by transferring energy from the re-turn airstream to the supply airstream. During the heat-ing mode, the wheel preheats the outside or mixed airby transferring energy to the supply airstream. Thismay help reduce the winter heating load, but any hy-dronic coils upstream of the wheel may require someof form of freeze protection, particularly in 100% out-side air systems.

In the series arrangement, energy is not actually “re-covered”, but simply transferred from the upstream tothe downstream side of the cooling coil. This transferreheats the supply air and preconditions the outside airsimultaneously. Since the goal of a series arrangementsupply air tempering system is dehumidified supply air,sensible-only energy recovery devices. Series type sys-tems as shown in Figure 16 are not used during theheating season.

In both of the supply air tempering arrangements thecooling system is designed to accommodate the peakcooling (i.e. sensible) design load, which, as Figure 17shows, occurs at a higher dry bulb temperature thanthe design dehumidifying condition. The energy re-covery wheel is not used during peak sensible coolingconditions because it is not desirable to add heat to thesupply air when sensible cooling is the main concern.Therefore, using an energy wheel for supply air tem-pering will not reduce the design capacity of the cool-ing system.

Supply air tempering is used at the peak dehumidify-ing (i.e. latent) design load when the sensible load inthe space is lower and the latent load is higher than the

FIGURE 14 – OUTSIDE AIR PRECONDITIONING

MIXED AIR

100% OUTSIDE AIR

FIGURE 15 – SUPPLY AIR TEMPERATING (PARALLEL)

FORM 102.20-AG6 (305)

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MIXED AIR

100% OUTSIDE AIR

FIGURE 16 – SUPPLY AIR TEMPERATING (SERIES)

FIGURE 17 – COOLING AND DEHUMIDIFYING DESIGNCONDITIONS

cooling design condition. Typically when the sensiblecooling load decreases, the supply air temperature in-creases (in a constant volume system) or the supply airvolume decreases (in a VAV system). However, due tothe high latent load in the space, a temperature increaseor airflow decrease results in unacceptable humiditylevels in the space. To maintain acceptable humidityand temperature levels the supply air is “overcooled”to dehumidify it, then reheated to the temperature re-quired to satisfy the sensible load. Supply air temper-ing accomplishes this process while reducing or elimi-nating the need for mechanical reheat, which satisfiesthe requirements of ASHRAE Standard 90.1-2004.

MAINTENANCE

Energy recovery wheels generally require little main-tenance. The maintenance staff should conduct regularinspection of the drive system, checking for proper belttensioning and motor operation, and adjusting or re-placing loose or worn belts when needed. Personnelshould consult product IOM manuals for the proceduresand frequency of drive system maintenance recom-mended by the manufacturer.

CleaningDry particle build-up in the heat exchange matrix israre. Due to the laminar flow characteristics of thewheel, small particles that enter the matrix typicallypass through while larger particles that lodge on thesurface of the wheel are blown free when they passinto the counter-flow airstream. Build-up of oil or tar-based aerosols on the surface of the energy wheel orwithin the matrix presents a greater concern. Thesecontaminants can adhere to the surface of the wheeland reduce airflow by obstructing the air or, in the caseof total-energy wheels, reducing the latent effective-ness of the wheel by clogging the water adsorbing poreson the surface of the desiccant.

The wheel should periodically (no less than once peryear) be inspected for contamination. In environmentssuch as cooking facilities, bars and restaurants with highsmoking rates and industrial welding applications whereoil or tar-based contamination is more likely, inspec-tions should be conducted more frequently. Inspectionand cleaning may be required as frequently as everythree months in industrial applications.

Regardless of the application, the wheel should becleaned whenever contamination is detected. Dry par-ticles can generally be removed by vacuuming, how-ever personnel should consult IOM manuals for instruc-tions on the removal of oil or tar-based films. Differ-ent manufacturers use different desiccants and materi-als of construction, and what works well for one wheelmay damage or destroy another. Many manufacturersconstruct their wheels of multiple, independently re-movable segments to facilitate the cleaning process.

Design (0.4%) Design (0.4%)Cooling Conditions Dehumidifying Conditions

Location DB WB DP DB WB DPPhoenix, AZ 110 70 46.1 82 74.1 71

Los Angeles, CA 85 64 51 75 69.5 67Boston, MA 91 73 65.2 80 74.2 72

Houston, TX 96 77 69.7 83 79.2 78Seattle, WA 85 65 53.2 71 63.9 60

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Care should be taken in the design of the system toallow sufficient access space for the inspection andcleaning processes.

INDUSTRY STANDARDS

The HVAC industry has published several documentsthat deal with the testing, rating and use of exhaust airenergy recovery devices.

ASHRAE Standard 84-1991ASHRAE Standard 84-1991, The Method of TestingAir-to-Air Heat Exchangers is an ANSI approved stan-dard that was published in 1992. The purpose of thisstandard is to establish a uniform method of testingair-to-air heat exchangers. The standard specifies thetest set-ups and test equipment required. It also speci-fies the data that must be collected, the calculationsthat can be used and the acceptable procedures for re-porting test results.

ARI Standard 1060-2000Published by the Air-Conditioning and RefrigerationInstitute in 2000, ARI 1060 is the Standard for RatingAir-to-Air Energy Recovery Ventilation Equipment.The purpose of this standard is similar to that ofASHRAE 84-1991 in that it establishes “definitions;test requirements; rating requirements; requirements formarking and nameplate data; and conformance require-ments.” The test procedures referenced in ARI-1060are the ones that are specified in ASHRAE 84-1991.

There are a few differences between the standards how-ever. For example, ASHRAE 84-1991 includes run-around loop heat exchangers - that is a closed loop sys-tem consisting of two or more heat transfer coilsmounted in different airstreams. ARI 1060 excludesthis type of heat exchanger and instead applies only toheat pipes, plate heat exchangers and rotary heat ex-changers. Also, the ratings outlined in ARI 1060 onlyapply to the performance of the energy recovery de-vice, as it is used to precondition outside air, not as it isused to recondition, or temper supply air.

The most important difference between the two stan-dards is that ARI administers a certification programin conjunction with Standard 1060; ASHRAE has nosuch certification program. This means that any en-ergy recovery equipment manufacturer can apply forlisting in ARI’s Air-to-Air Energy Recovery Ventila-tion Equipment Certified Product Directory. Manu-facturers who are listed in the ARI directory have cer-tified that their equipment has been tested using theprocedures stipulated in Standard 1060 and that thepublished ratings of the equipment conform to the re-quirements of the standard.

To verify the ratings of a certified manufacturer, ARIrandomly selects samples of the rated equipment on a

regular basis and has the samples tested in accordancewith Standard 1060 at an independent laboratory thatis under contract to ARI. Manufacturers whose equip-ment passes these tests are permitted to use the ARI1060 certification seal shown in Figure 18 on their cer-tified models, specification sheets and other literatureand advertising that pertains to equipment ratings.

Specifying ARI 1060 certified energy recovery equip-ment has several advantages. It facilitates the com-parison of certified products from various manufactur-ers by standardizing the ratings methods used by thosemanufacturers. It also adds credibility to publishedperformance data by ensuring that it is based on stan-dardized test procedures rather than estimates and cal-culations, as may have been the case in the past. Pur-chasing non-certified energy recovery equipment ex-poses the customer to the possibility of receiving equip-ment that will not perform as specified.

ASHRAE Standard 90.1-2004Unlike the previous two standards, ASHRAE 90.1-2004, Energy Standard for Buildings Except Low-RiseResidential Buildings does not regulate the performanceof energy recovery equipment, but rather when it shouldbe used. The purpose of this standard is to provideminimum requirements for the energy efficient designof buildings. As previously mentioned, exhaust airenergy recovery provides the opportunity to conserveenergy in the HVAC system.

Section 6.5.6.1 of Standard 90.1-2004 requires exhaustair energy recovery in “Individual fan systems that haveboth a design supply air capacity of 5000 cfm or greaterand have a minimum outdoor air supply of 70% orgreater of the design supply air quantity...” Most com-fort cooling applications only require 15-25% minimumoutside air. However, even if energy recovery is notrequired by 90.1, it’s still a good idea because the en-ergy it saves means it has a very short payback time.

Section 6.5.6.1 also requires a 50% or greater effec-tiveness in either the heating or cooling mode based onEquation 1. When the system includes an air econo-mizer, the standard requires that provisions be made toprevent the energy recovery device from increasingenergy usage during economizer mode. Several ex-

FIGURE 18 – ARI 1060 CERTIFICATION SEAL

FORM 102.20-AG6 (305)

11YORK INTERNATIONAL

ceptions to the energy recovery requirement exist, andfor more information on those exceptions, please referto the Standard.

Energy recovery is also referenced in Section 6.5.2 -Simultaneous Heating and Cooling. This section al-lows simultaneous heating and cooling for tempera-ture or humidity control if site-recovered energy (in-cluding exhaust air energy recovery) or solar energyprovides at least 75% of the heat energy used to reheatthe air.

ARI Guideline VThis guideline, titled, Calculating the Efficiency ofEnergy Recovery Ventilation and its Effect on Effi-ciency and Sizing of Building HVAC Systems was pub-lished in 2003. It establishes an industry-approvedmethod of determining the efficiency of an HVAC sys-tem that includes energy recovery equipment. It alsoprovides guidance on how to size heating and coolingcomponents in an energy recovery system.

Like ARI Standard 1060, Guideline V applies only toheat pipes, fixed plate exchangers and rotary heat ex-changers; it excludes run-around loop heat exchang-ers. Unlike ARI 1060, this guideline includes perfor-mance calculations for supply air tempering systems.Since such systems are not certified under Standard1060, ARI advises that results obtained using Guide-line V should not be considered “certified.”

Guideline V introduces the concepts of Recovery Effi-ciency Ratio and Combined Efficiency to assist in ana-lyzing the performance of the HVAC system. Recov-ery Efficiency Ratio (RER), which is the efficiency ofthe energy recovery component in recovering energyfrom the exhaust airstream, is defined as, “the energyrecovered divided by the energy expended in the re-covery process.” The recovered energy can be relatedto humidification, dehumidification heating or cooling.The energy expended includes the added fan powerrequired to move the air through the heat exchangerand, in the case of an energy wheel, the wheel drivemotor power consumption.

The Combined Efficiency (CEF) is a means of deter-mining the overall efficiency of an HVAC system in-corporating an energy recovery component and a uni-tary device such as a packaged air conditioner or heatpump. Depending on the conditioning device, the CEFcan be expressed in either Btu/(W•h) or W/W. An ex-ample in Appendix C of the guideline shows that a to-tal energy recovery device with an effectiveness of 70%can improve the cooling efficiency of an HVAC sys-tem from an EER of 10 Btu/(W•h) to a CEF of 13.46Btu/(W•h).

LEED

The Leadership in Energy and Environmental Design(LEED(tm)) rating system is the U.S. Green BuildingCouncil’s effort to provide a national standard for“green buildings.” Optimizing building energy usageis a key step in achieving certification and energy re-covery wheels are a key component in reducing HVACsystem energy consumption. For building owners, eco-nomic incentives of LEED certification include higherrental rates than comparable buildings and possible taxbreaks for operating a green building.

SUMMARY

Exhaust air energy recovery technology provides a valu-able opportunity for engineers to reduce the first costsand operating costs of buildings. Owners benefit notonly in these initial and annual savings, but may alsoreceive incentives for operating a “green” building insome areas. Finally, the use of energy recovery reducesthe use of non-renewable resources and promotes acleaner environment. Therefore, whether mandated bystate or local building codes, or not, proper applicationof energy recovery wheels and heat recovery in gen-eral is a win-win proposition.

ReferencesEnergy Information Agency, 2001. Commercial Build-ings Energy Consumption Survey. Washington, DC:U.S. Department of Energy.

ASHRAE. 2000. ASHRAE Handbook - 2000 HVACSystems & Equipment, chapter 44. Atlanta, GA: Ameri-can Society of Heating, Refrigerating and Air-Condi-tioning Engineers, Inc.

ASHRAE. 2001. ASHRAE Handbook - 2001 Funda-mentals, chapter 27. Atlanta, GA: American Society ofHeating, Refrigerating and Air-Conditioning Engineers,Inc.

Besant, Robert W., and Simonson, Carey J. “Air-To-Air Energy Recovery.” ASHRAE Journal, Volume 42,Number 5, May 2000 - pp. 31 - 42.

Besant, Robert W., and Simonson, Carey J. “Air-To-Air Exchangers.” ASHRAE Journal, Volume 45, Num-ber 4, April 2003 - pp. 42 - 52.

ASHRAE. 2004. ANSI/ASHRAE/IESNA Standard 90.1- 2004 Energy Standard for Buildings Except Low-RiseResidential Buildings. Atlanta, GA: American Societyof Heating, Refrigerating and Air-Conditioning Engi-neers, Inc.

ASHRAE 2002. ASHRAE Standard 90.1 User’sManual, chapter 6. Atlanta, GA: American Society ofHeating, Refrigerating and Air-Conditioning Engineers,Inc.

ASHRAE. 2004. ANSI/ASHRAE Standard 62 - 2004 -Ventilation for Acceptable Indoor Air Quality. Atlanta,GA: American Society of Heating, Refrigerating andAir-Conditioning Engineers, Inc.

ASHRAE. 2001. ANSI/ASHRAE Standard 84 - 1991Method of Testing Air-To-Air Heat Exchangers. Atlanta,GA: American Society of Heating, Refrigerating andAir-Conditioning Engineers, Inc.

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P.O. Box 1592, York, Pennsylvania USA 17405-1592 Subject to change without notice. Printed in USACopyright © by York International Corporation 2001 ALL RIGHTS RESERVEDForm 102.20-AG6 (305)Supersedes: Nothing

ARI. 2000. Standard 1060 - 2000 Rating Air-To-AirEnergy Recovery Ventilation Equipment. Arlington, VA:Air-Conditioning and Refrigeration Institute.

ARI. 2003. Guideline V - Guideline for Calculatingthe Efficiency of Energy Recovery Ventilation and ItsEffect on Efficicency and Sizing of Building HVAC Sys-tems. Arlington, VA: Air-Conditioning and Refrigera-tion Institute.

ARI. 2003. Air-To-Air Energy Recovery VentilationEquipment Certified Product Directory. Arlington, VA:Air-Conditioning and Refrigeration Institute.