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HVAC Systems and Indoor Air Quality 121

Appendix B:HVAC Systems and Indoor Air Quality

This appendix provides informationabout specific HVAC system designs andcomponents in relation to indoor airquality. It also serves as introductorymaterial for building owners and managerswho may be unfamiliar with the terminol-ogy and concepts associated with HVAC(heating, ventilating, and air conditioning)system design. Further detailed informa-tion can be found in ASHRAE manualsand guides and in some of the guidancedeveloped by other trade and professionalassociations. (See Guidelines of CareDeveloped by Trade Associations inSection 5.) Additional information can beobtained using Appendix G or throughdiscussion with your facility engineer.

BACKGROUND

All occupied buildings require a supply ofoutdoor air. Depending on outdoorconditions, the air may need to be heatedor cooled before it is distributed into theoccupied space. As outdoor air is drawninto the building, indoor air is exhausted orallowed to escape (passive relief), thusremoving air contaminants.

The term “HVAC system” is used torefer to the equipment that can provideheating, cooling, filtered outdoor air, andhumidity control to maintain comfortconditions in a building. Not all HVACsystems are designed to accomplish allof these functions. Some buildings relyon only natural ventilation. Others lackmechanical cooling equipment (AC), andmany function with little or no humiditycontrol. The features of the HVAC systemin a given building will depend on severalvariables, including:

■ age of the design■ climate■ building codes in effect at the time of

the design■ budget that was available for the project■ planned use of the building■ owners’ and designers’ individual

preferences■ subsequent modifications

HVAC systems range in complexityfrom stand-alone units that serveindividual rooms to large, centrallycontrolled systems serving multiple zonesin a building. In large modern officebuildings with heat gains from lighting,people, and equipment, interior spacesoften require year-round cooling. Roomsat the perimeter of the same building (i.e.,rooms with exterior walls, floors, or roofsurfaces) may need to be heated and/orcooled as hourly or daily outdoor weatherconditions change. In buildings over onestory in height, perimeter areas at the lowerlevels also tend to experience the greatestuncontrolled air infiltration.

Working with the electricalcomponents of an HVACsystem involves the risk ofelectrocution and fire. Aknowledgeable member ofthe building staff shouldoversee the inspection of theHVAC controls.

122 Appendix B

Some buildings use only naturalventilation or exhaust fans to remove odorsand contaminants. In these buildings,thermal discomfort and unacceptableindoor air quality are particularly likelywhen occupants keep the windows closedbecause of extreme hot or cold tempera-tures. Problems related to underventilationare also likely when infiltration forces areweakest (i.e., during the “swing seasons”and summer months).

Modern public and commercial build-ings generally use mechanical ventilationsystems to introduce outdoor air during theoccupied mode. Thermal comfort iscommonly maintained by mechanicallydistributing conditioned (heated or cooled)air throughout the building. In somedesigns, air systems are supplemented bypiping systems that carry steam or water tothe building perimeter zones. As thisdocument is concerned with HVACsystems in relation to indoor air quality,the remainder of this discussion will focuson systems that distribute conditioned airto maintain occupant comfort.

Roles of the HVAC System Operatorand Facility Manager

The system operator(s) and facilitymanager(s) (or IAQ manager) are amongthe most significant factors in determiningwhether IAQ problems will occur in aproperly designed, constructed, andcommissioned HVAC system. HVACsystems require preventive maintenance andprompt repairs if they are to operatecorrectly and provide comfortable condi-tions. The operator(s) must have anadequate understanding of the overallsystem design and its limitations. TheHVAC system capacity and distributioncharacteristics should be evaluated beforerenovations to the building, changes in itsoccupancy, or changes in the use of an area.

System operators must be able torespond appropriately to occupant com-plaints. For example, if an occupant

complains that it is too cold or too hot andthe observed (measured) conditions areoutside of the ASHRAE comfort zone,then the HVAC system needs to beevaluated. Sometimes the problem can berelieved by fine tuning or repairing theHVAC system, but in some cases thesystem cannot perform as expected, and along-term solution must be investigated.

TYPES OF HVAC SYSTEMS

Single Zone

A single air handling unit can only servemore than one building area if the areasserved have similar heating, cooling, andventilation requirements, or if the controlsystem compensates for differences inheating, cooling, and ventilation needsamong the spaces served. Areas regulatedby a common control (e.g., a singlethermostat) are referred to as zones.Thermal comfort problems can result if thedesign does not adequately account fordifferences in heating and cooling loadsbetween rooms that are in the same zone.This can easily occur if:

■ The cooling load in some area(s) with-in a zone changes due to an increasedoccupant population, increased lighting,or the introduction of new heat-produc-ing equipment (e.g., computers, copiers).

■ Areas within a zone have different solarexposures. This can produce radiant heatgains and losses that, in turn, createunevenly distributed heating or coolingneeds (e.g., as the sun angle changesdaily and seasonally).

Multiple Zone

Multiple zone systems can provide eachzone with air at a different temperature byheating or cooling the airstream in eachzone. Alternative design strategies involvedelivering air at a constant temperaturewhile varying the volume of airflow, ormodulating room temperature with a


supplementary system (e.g., perimeter hotwater piping).

Constant VolumeConstant volume systems, as their namesuggests, generally deliver a constantairflow to each space. Changes in spacetemperatures are made by heating orcooling the air or switching the air han-dling unit on and off, not by modulatingthe volume of air supplied. These systemsoften operate with a fixed minimumpercentage of outdoor air or with an “aireconomizer” feature (described in theOutdoor Air Control discussion thatfollows).

Variable Air VolumeVariable air volume systems maintainthermal comfort by varying the amount ofheated or cooled air delivered to eachspace, rather than by changing the airtemperature. (However, many VAVsystems also have provisions for resettingthe temperature of the delivery air on aseasonal basis, depending on the severityof the weather). Overcooling oroverheating can occur within a given zoneif the system is not adjusted to respond tothe load. Underventilation frequentlyoccurs if the system is not arranged tointroduce at least a minimum quantity (asopposed to percentage) of outdoor air asthe VAV system throttles back from fullairflow, or if the system supply airtemperature is set too low for the loadspresent in the zone.

BASIC COMPONENTS OF AN HVACSYSTEM

The basic components of an HVAC systemthat delivers conditioned air to maintainthermal comfort and indoor air quality are:

■ outdoor air intake■ mixed-air plenum and outdoor air control■ air filter■ heating and cooling coils■ humidification and/or de-humidification

equipment

TESTING AND BALANCING

Modern HVAC systems typically use sophisticated, automatic controls tosupply the proper amounts of air for heating, cooling, and ventilation incommercial buildings. Problems during installation, operation, mainte-nance, and servicing the HVAC system could prevent it from operating asdesigned. Each system should be tested to ensure its initial and contin-ued performance. In addition to providing acceptable thermal conditionsand ventilation air, a properly adjusted and balanced system can alsoreduce operating costs and increase equipment life.

Testing and balancing involves the testing, adjusting, and balancing ofHVAC system components so that the entire system provides airflows thatare in accordance with the design specifications. Typical componentsand system parameters tested include:

■ all supply, return, exhaust, and outdoor airflow rates■ control settings and operation■ air temperatures■ fan speeds and power consumption■ filter or collector resistance

The typical test and balance agency or contractor coordinates with thecontrol contractor to accomplish three goals: verify and ensure the mosteffective system operation within the design specifications, identify andcorrect any problems, and ensure the safety of the system.

A test and balance report should provide a complete record of thedesign, preliminary measurements, and final test data. The report shouldinclude any discrepancies between the test data and the design specifica-tions, along with reasons for those discrepancies. To facilitate futureperformance checks and adjustments, appropriate records should be kepton all damper positions, equipment capacities, control types and loca-tions, control settings and operating logic, airflow rates, static pressures,fan speeds, and horsepowers.

Testing and balancing of existing building systems should be per-formed whenever there is reason to believe the system is not functioningas designed or when current records do not accurately reflect the actualoperation of the system. The Associated Air Balance Council recom-mends the following guidelines in determining whether testing andbalancing is required:

■ When space has been renovated or changed to provide for new occu-pancy.

■ When HVAC equipment has been replaced or modified.■ When control settings have been readjusted by maintenance or other

personnel.■ After the air conveyance system has been cleaned.■ When accurate records are required to conduct an IAQ investigation.■ When the building owner is unable to obtain design documents or

appropriate air exchange rates for compliance with IAQ standards orguidelines.

Because of the diversity of system types and the interrelationship ofsystem components, effective balancing requires a skilled technician withthe proper experience and instruments. Due to the nature of the work,which involves the detection and remediation of problems, it is recom-mended that an independent test and balance contractor be used andthat this contractor report directly to the building owner, facility manager,or IAQ manager.

124 Appendix B

The illustration above shows thegeneral relationship between many ofthese components; however, manyvariations are possible.

Outdoor Air Intake

Building codes require the introduction ofoutdoor air for ventilation in most build-ings. Most non-residential air handlers aredesigned with an outdoor air intake on thereturn side of the ductwork. Outdoor airintroduced through the air handler can befiltered and conditioned (heated or cooled)before distribution. Other designs mayintroduce outdoor air through air-to-airheat exchangers and operable windows.

■ supply fan■ ducts■ terminal device■ return air system■ exhaust or relief fans and air outlet■ self-contained heating or cooling unit■ control■ boiler■ cooling tower■ water chiller

The following discussion of thesecomponents (each of which may occurmore than once in any total HVAC system)emphasizes features that affect indoor airquality. It may be helpful to refer to thissection when using the HVAC Checklists.

FIGURE B-1: Typical HVAC System Components

Courtesy Terry BrennanCamroden AssociatesOriskny, N.Y.


Indoor air quality problems can beproduced when contaminants enter abuilding with the outdoor air. Rooftop orwall-mounted air intakes are sometimeslocated adjacent to or downwind ofbuilding exhaust outlets or other contami-nant sources. Problems can also result ifdebris (e.g., bird droppings) accumulates atthe intake, obstructing airflow and poten-tially introducing microbiological contami-nants.

If more air is exhausted than isintroduced through the outdoor air intake,then outdoor air will enter the building atany leakage sites in the shell. Indoor airquality problems can occur if the leakagesite is a door to a loading dock, parkinggarage, or some other area associated withpollutants.

Mixed-Air Plenum and Outdoor AirControls

Outdoor air is mixed with return air (airthat has already circulated through theHVAC system) in the mixed-air plenum ofan air handling unit. Indoor air qualityproblems frequently result if the outdoorair damper is not operating properly (e.g.,if the system is not designed or adjusted toallow the introduction of sufficient outdoorair for the current use of the building. Theamount of outdoor air introduced in theoccupied mode should be sufficient tomeet needs for ventilation and exhaustmake-up. It may be fixed at a constantvolume or may vary with the outdoortemperature.

When dampers that regulate the flow ofoutdoor air are arranged to modulate, theyare usually designed to bring in a mini-mum amount of outdoor air (in theoccupied mode) under extreme outdoortemperature conditions and to open asoutdoor temperatures approach the desiredindoor temperature. Systems that useoutdoor air for cooling are called “aireconomizer cooling” systems. Aireconomizer systems have a mixed air

Above: The air intake in the background islocated too close to the sanitary vents (thestraight pipes to the left and in the centerforeground) and the bathroomexhaust vent (next to the sanitary vent onthe right side). Below: The return airdampers in this mixed-air plenum are open(top), but the outdoor air damper (left) is al-most completely closed. Complaints in thebuilding indicate thatunder-ventilation is a problem.

126 Appendix B

temperature controller and thermostatthat are used to blend return air (typicallyat 74°F) with outdoor air to reach a mixedair temperature of 55° to 65°F. (Mixed airtemperature settings above 65°F may leadto the introduction of insufficient quanti-ties of outdoor air for office space use.)The mixed air is then further heated orcooled for delivery to the occupied spaces.

Air economizer systems have a sensibleor enthalpy control that signals the outdoorair damper to go to the minimum positionwhen it is too warm or humid outdoors.Note that economizer cycles which do notprovide dehumidification may producediscomfort even when the indoor tempera-ture is the same as the thermostat setting.

If outdoor air make-up and exhaust arebalanced, and the zones served by each airhandler are separated and well defined, it ispossible to estimate the minimum flow ofoutdoor air to each space and compare it toventilation standards such as ASHRAE 62-1989. Techniques used for this evaluationinclude the direct measurement of the

outdoor air at the intake and the calculationof the percentage of outdoor air by atemperature or CO

2 balance. Carbon

dioxide measured in an occupied space isalso an indicator of ventilation adequacy.Some investigators use tracer gases toassess ventilation quantities and airflowpatterns. There are specific methods foreach of these assessments. See Appendix Afor more information.

Many HVAC designs protect the coilsby closing the outdoor air damper if theairstream temperature falls below thesetpoint of a freezestat. Inadequateventilation can occur if a freezestat tripsand is not reset, or if the freezestat is set totrip at an excessively high temperature.Stratification of the cold outdoor air andwarmer return air in the mixing plenums isa common situation, causing nuisancetripping of the freezestat. Unfortunately,the remedy often employed to prevent thisproblem is to close the outdoor air damper.Obviously, solving the problem in this waycan quickly lead to inadequate outdoor airin occupied parts of the building.

Air Filters

Filters are primarily used to removeparticles from the air. The type and designof filter determine the efficiency atremoving particles of a given size and theamount of energy needed to pull or pushair through the filter. Filters are rated bydifferent standards and test methods suchas dust spot and arrestance which measuredifferent aspects of performance. See thediscussion of ASHRAE Standard 52-76 onpage 138 of this appendix.

Low efficiency filters (ASHRAE DustSpot rating of 10% to 20% or less) areoften used to keep lint and dust fromclogging the heating and cooling coils of asystem. In order to maintain clean air inoccupied spaces, filters must also removebacteria, pollens, insects, soot, dust, anddirt with an efficiency suited to the use ofthe building. Medium efficiency filters(ASHRAE Dust Spot rating of 30% to

Proper air filtration can play an impor-tantrole in protecting the rest of the HVACsystem and in maintaining good indoor airquality in occupied spaces. Air filtersshould be selected and main-tained toprovide maximum filtration, while notovertaxing the supply fan capability orleading to “blow out” situa-tions with no airfiltration. Shown aboveare roll filter (top) and bag, panel, andpleated filters (below).


60%) can provide much better filtrationthan low efficiency filters. To maintain theproper airflow and minimize the amount ofadditional energy required to move airthrough these higher efficiency filters,pleated-type extended surface filters arerecommended. In buildings that aredesigned to be exceptionally clean, thedesigners may specify the equipment toutilize both a medium efficiency pre-filterand a high efficiency extended surfacefilter (ASHRAE Dust Spot rating of 85%to 95%). Some manufacturers recommendhigh efficiency extended surface filters(ASHRAE Dust Spot rating of 85%)without pre-filters as the most costeffective approach to minimizing energyconsumption and maximizing air quality inmodern HVAC VAV systems that serveoffice environments.

Air filters, whatever their design orefficiency rating, require regular mainte-nance (cleaning for some and replacementfor most). As a filter loads up withparticles, it becomes more efficient atparticle removal but increases the pressuredrop through the system, thereforereducing airflow. Filter manufacturers canprovide information on the pressure dropthrough their products under differentconditions. Low efficiency filters, ifloaded to excess, will become deformedand even “blow out” of their filter rack.When filters blow out, bypassing ofunfiltered air can lead to clogged coils anddirty ducts. Filtration efficiency can beseriously reduced if the filter cells are notproperly sealed to prevent air frombypassing.

Filters should be selected for theirability to protect both the HVAC systemcomponents and general indoor air quality.In many buildings, the best choice is amedium efficiency, pleated filter becausethese filters have a higher removalefficiency than low efficiency filters, yetthey will last without clogging for longerthan high efficiency filters.

Choice of an appropriate filter andproper maintenance are important tokeeping the ductwork clean. If dirtaccumulates in ductwork and if the relativehumidity reaches the dewpoint (so thatcondensation occurs), then the nutrientsand moisture may support the growth ofmicrobiologicals. Attention to air filters isparticularly important in HVAC systemswith acoustical duct liner, which isfrequently used in air handler fan housingsand supply ducts to reduce sound transmis-sion and provide thermal insulation. Areasof duct lining that have become contami-nated with microbiological growth must bereplaced. (See later discussion of ductsand duct cleaning .) Sound reduction canalso be accomplished with the use ofspecial duct-mounted devices such asattenuators or with active electronic noisecontrol.

Air handlers that are located in difficult-to-access places (e.g., in places whichrequire ladders for access, have inconve-nient access doors to unbolt, or are locatedon roofs with no roof hatch access) will be

Pleated medium efficiencyfilters are often preferred overlow efficiency filters becausethey offer added protectionto both the HVAC equipmentand to indoor air quality, yetthey do not clog as easily ashigh efficiency filters.Medium efficiency filters doneed routine maintenance,however, which the filter inthis photo did not receive.

128 Appendix B

Without proper installationand maintenance, rust andcorrosion may accumulate incondensate pans underheating and cooling coils.The rust in the pan indi-catesthat it was installed without apitch or was pitched in thewrong direction, so that waterdid not drain out properly.

more likely to suffer from poor air filtermaintenance and overall poor mainte-nance. Quick release and hinged accessdoors for maintenance are more desirablethan bolted access panels.

Filters are available to remove gasesand volatile organic contaminants fromventilation air; however, these systems arenot generally used in normal occupancybuildings. In specially designed HVACsystems, permanganate oxidizers andactivated charcoal may be used for gaseousremoval filters. Some manufacturers offer“partial bypass” carbon filters and carbonimpregnated filters to reduce volatileorganics in the ventilation air of officeenvironments. Gaseous filters must beregularly maintained (replaced or regener-ated) in order for the system to continue tooperate effectively.

Heating and Cooling Coils

Heating and cooling coils are placed in theairstream to regulate the temperature of theair delivered to the space. Malfunctions ofthe coil controls can result in thermaldiscomfort. Condensation on under-insulated pipes and leakage in pipedsystems will often create moist conditionsconducive to the growth of molds, fungus,and bacteria.

During the cooling mode (air condition-ing), the cooling coil provides dehumidifi-cation as water condenses from the air-stream. Dehumidification can only takeplace if the chilled fluid is maintained at acold enough temperature (generally below45°F for water). Condensate collects in thedrain pan under the cooling coil and exitsvia a deep seal trap. Standing water willaccumulate if the drain pan system has notbeen designed to drain completely underall operating conditions (sloped toward thedrain and properly trapped). Under theseconditions, molds and bacteria willproliferate unless the pan is cleanedfrequently.

It is important to verify that condensatelines have been properly trapped and arecharged with liquid. An improperlytrapped line can be a source of contamina-tion, depending on where the line termi-nates. A properly installed trap could alsobe a source, if the water in the trapevaporates and allows air to flow throughthe trap into the conditioned air.

During the heating mode, problems canoccur if the hot water temperature in theheating coil has been set too low in anattempt to reduce energy consumption. Ifenough outdoor air to provide sufficientventilation is brought in, that air may notbe heated sufficiently to maintain thermalcomfort or, in order to adequately condi-tion the outdoor air, the amount of outdoorair may be reduced so that there is insuffi-cient outdoor air to meet ventilation needs.


Humidification and DehumidificationEquipment

In some buildings (or zones withinbuildings), there are special needs thatwarrant the strict control of humidity (e.g.,operating rooms, computer rooms). Thiscontrol is most often accomplished byadding humidification or dehumidificationequipment and controls. In office facili-ties, it is generally preferable to keeprelative humidities above 20% or 30%during the heating season and below 60%during the cooling season. ASHRAEStandard 55-1981 provides guidance onacceptable temperature and humidityconditions. (See also the discussion ofhumidity levels in Section 6.)

The use of a properly designed andoperated air conditioning system willgenerally keep relative humidities below60% RH during the cooling season, inoffice facilities with normal densities andloads. (See the previous discussion of thecooling coil.)

Office buildings in cool climates thathave high interior heat gains, thermallyefficient envelopes (e.g., insulation), andeconomizer cooling may require humidi-fication to maintain relative humiditywithin the comfort zone. When humidi-fication is needed, it must be added in amanner that prevents the growth of micro-biologicals within the ductwork and airhandlers.

Steam humidifiers should utilize cleansteam, rather than treated boiler water, sothat occupants will not be exposed tochemicals. Systems using media otherthan clean steam must be rigorouslymaintained in accordance with themanufacturer’s recommended proceduresto reduce the likelihood of microbiologicalgrowth.

Mold growth problems are more likelyif the humidistat setpoint located in theoccupied space is above 45%. The highlimit humidistat, typically located in theductwork downstream of the point at

Above: Occupants of this buildingcomplained of an inter-mittant fish tank odor.An investigation showed that this water sprayhumidifi-cation system is regularlymaintained. The coils are washed roughlyevery two weeks using fresh tap water,eliminating the need for any use of algacides.Below: Further investigation identified thefact that the maintenance practice wascausing the odor problem. This picture ofthe downstream side of the coils was takenone day after the washing. A high pressurestream of water caused algae in the waterto foam and float for several days,coinciding with the occupant complaints.

130 Appendix B

Fan performance is expressed as theability to move a given quantity of air(cubic feet per minute or cfm) at a givenresistance or static pressure (measured ininches of water column). Airflow inductwork is determined by the size of theduct opening, the resistance of the ductconfiguration, and the velocity of the airthrough the duct. The static pressure in asystem is calculated using factors for ductlength, speed of air movement and changesin the direction of air movement.

It is common to find some differencesbetween the original design and the finalinstallation, as ductwork must sharelimited space with structural members andother “hidden” elements of the buildingsystem (e.g., electrical conduit, plumbingpipes). Air distribution problems canoccur, particularly at the end of duct runs,if departures from the original designincrease the friction in the system to apoint that approaches the limit of fanperformance. Inappropriate use of longruns of flexible ducts with sharp bends alsocauses excessive friction. Poor systembalancing (adjustment) is another commoncause of air distribution problems.

Dampers are used as controls torestrict airflow. Damper positions may berelatively fixed (e.g., set manually duringsystem testing and balancing) or maychange in response to signals from thecontrol system. Fire and smoke damperscan be triggered to respond to indicatorssuch as high temperatures or signals fromsmoke detectors. If a damper is designedto modulate, it should be checked duringinspections to see that it is at the propersetting. ASHRAE and the Associated AirBalance Council both provide guidance onproper intervals for testing and balancing.

Ducts

The same HVAC system that distributesconditioned air throughout a building aircan distribute dust and other pollutants,including biological contaminants. Dirt or

DUCT LEAKAGE

Leakage of air from ducts can cause or exacerbate air quality problems, inaddition to wasting energy. In general, sealed duct systems specified witha leakage rate of less than 3% will have a superior life cycle cost analysisand reduce the likelihood of problems associated with leaky ductwork.Examples of excessive duct leakage leading to problems include:

■ leakage of light troffer-type diffusers at the diffuser/light fixture interfacewhen they are installed in a return plenum. Such leakage has beenknown to cause gross short-circuiting between supply and return,wasting much of the conditioned air. If the “room” thermostat islocated in the return plenum, the room can be very uncomfortable whilethe temperature in the plenum is operating at the control setpoint.

■ leakage of supply ductwork due to loose-fitting joints and connectionsor “blow outs” of improperly fabricated seams

■ leakage of return ducts located in crawl spaces or below slabs, allowingsoil gases and molds to enter the ductwork

which water vapor is added, is generallyset at 70% to avoid condensation (with apotential for subsequent mold growth) inthe ductwork. Adding water vapor to abuilding that was not designed forhumidification can have a negative impacton the building structure and theoccupants’ health, if condensation occurson cold surfaces or in wall or roof cavities.

Supply Fans

After passing through the coil sectionwhere heat is either added or extracted, airmoves through the supply fan chamber andthe distribution system. Air distributionsystems commonly use ducts that areconstructed to be relatively airtight.Elements of the building construction canalso serve as part of the air distributionsystem (e.g., pressurized supply plenumsor return air plenums located in the cavityspace above the ceiling tiles and below thedeck of the floor above). Proper coordina-tion of fan selection and duct layout duringthe building design and construction phaseand ongoing maintenance of mechanicalcomponents, filters, and controls are allnecessary for effective air delivery.


dust accumulation on any components ofan air handling system — its cooling coils,plenums, ducts, and equipment housing —may lead to contamination of the airsupply.

There is widespread agreement thatbuilding owners and managers should takegreat precautions to prevent dirt, highhumidity, or moisture from entering theductwork; there is less agreement atpresent about when measures to clean upare appropriate or how effective cleaningtechniques are at making long-termimprovements to the air supply or atreducing occupant complaints.

The presence of dust in ductwork doesnot necessarily indicate a current microbio-logical problem. A small amount of duston duct surfaces is normal and to beexpected. Special attention should begiven to trying to find out if ducts arecontaminated only where specific prob-lems are present, such as: water damage orbiological growth observed in ducts, debrisin ducts that restricts airflow, or dustdischarging from supply diffusers.

Problems with dust and other contami-nation in the ductwork are a function offiltration efficiency, regular HVAC systemmaintenance, the rate of airflow, and goodhousekeeping practices in the occupiedspace. Problems with biological pollutantscan be prevented by minimizing dust anddirt build-up, promptly repairing leaks andwater damage, preventing moistureaccumulation in the components that aresupposed to be dry, and cleaning thecomponents such as the drip pans thatcollect and drain water.

In cases where sheet metal ductworkhas become damaged or water-soaked,building owners will need to undertakeclean-up or repair procedures. Forexample, in cases where the thermal lineror fiberboard has become water-soaked,building managers will need to replace theaffected areas. These procedures shouldbe scheduled and performed in a way thatdoes not expose building occupants to

increased levels of pollutants and shouldbe carried out by experienced workers.Correcting theproblems that allowed theductwork to become contaminated in thefirst place is important. Otherwise, thecorrective action will only be temporary.

The porous surface of fibrous glass ductliner presents more surface area (whichcan trap dirt and subsequently collectwater) than sheet metal ductwork. It istherefore particularly important to payattention to the proper design, installation,filtration, humidity, and maintenance ofducts that contain porous materials. Inaddition, techniques developed forcleaning unlined metal ducts often are notsuitable for use with fibrous glass thermalliner or fiberboard. Such ducts mayrequire a special type of cleaning tomaintain the integrity of the duct (i.e., noheavy brushing tools that might fray theinner lining) while removing dirt anddebris.

More research on both the efficacy andthe potential for unintended exposures tobuilding occupants from various cleaningtechniques is needed before firm guidancecan be provided regarding duct cleaning.

Pay attention to worker safety whenworking with air handling systemsincluding during duct cleaning. Anyworker who may potentially breathe ductcontaminants or biocides should wearsuitable protective breathing apparatus.Workers who are doing the duct cleaningshould be encouraged to also look forother types of problems, such as holes orgaps in the ducts that could allow contami-nants to enter the ventilation airstream.

Building managers can obtain moreinformation on the issue of HVACcontamination and cleaning from theprofessional standards developed by sometrade associations (See Guidelines of CareDeveloped by Professional and TradeAssociations in Section 5 and refer toAppendix G for a list of organizationswith expertise and materials on theseissues.)

132 Appendix B

1. Any duct cleaning should bescheduled during periods whenthe building is unoccupied toprevent exposure to chemicalsand loosened particles.

The air handling unit should not beused during the cleaning or as an airmovement device for the cleaningprocess. The National Air DuctCleaning Association recommendsthat the system should be run toallow at least eight air changes in theoccupied space when duct cleaninghas been completed.

2. Negative air pressure that willdraw pollutants to a vacuumcollection system should bemaintained at all times in theduct cleaning area to preventmigration of dust dirt, andcontaminants into occupiedareas.

Where possible, use vacuumequipment or fans during cleaningand sanitizing to make sure thatcleaning vapors are exhausted to theoutside and do not enter the occu-pied space.

3. If it is determined that theductwork should be cleaned,careful attention must be givento protecting the ductwork.

When gaining access to sheet metalducts for cleaning purposes, it isessential to seal the access holeproperly in order to maintain theintegrity of the HVAC system.Access doors are recommended ifthe system is to be cleaned periodi-cally, and all access holes should beidentified on the building’s mechani-cal plans.

Particular attention is warrantedwhen cutting fibrous glass ducts, andmanufacturers’ recommendedprocedures for sealing should befollowed stringently. Use existingduct system openings wherepossible because it is difficult torepair the damage caused by cuttingnew access entries into theductwork. Large, high volume

vacuum equipment should only beused with extreme care because highnegative pressure together with limitedairflow can collapse ducts.

4. Duct cleaning performed withhigh velocity airflow (i.e., greaterthan 6,000 cfm) should includegentle, well-controlled brushing ofduct surfaces or other methods todislodge dust and other particles.

Duct cleaning that relies only on a highvelocity airflow through the ducts is notlikely to achieve satisfactory resultsbecause the flow rate at the ductsurface remains too low to removemany particles.

5. Only HEPA filtered (high-efficiency particle arrestor) vacu-uming equipment should be used ifthe vacuum collection unit is insidethe occupied space.

Conventional vacuuming equipmentmay discharge extremely fine particu-late matter back into the atmosphere,rather than collecting it. Duct cleaningequipment that draws the dust and dirtinto a collection unit outside thebuilding is also available. Peopleshould not be allowed to remain in theimmediate vicinity of these collectionunits.

6. If biocides are to be used, thenselect only products registered byEPA for such use, use the productsaccording to the manufacturer’sdirections, and pay careful atten-tion to the method of application.

At present, EPA accepts claims andtherefore registers antimicrobials foruse only as sanitizers, not disinfectantsor sterilizers in HVAC systems. (SeeAppendix F for definitions of antimicro-bials.) There is some question aboutwhether there areany application techniques thatwill deposit a sufficient amount of thebiocide to kill bacteria, germs,or other biologicals that may bepresent. Materials such as deodorizersthat temporarily eliminate odors causedby microorganisms provide only a fresh

smell, and are not intended to providereal control of microbiologicalcontaminants.

7. Use of sealants to coverinterior ductwork surfaces is notrecommended.

No application techniques have beendemonstrated to provide a completeor long-term barrier to microbiologicalgrowth, nor have such materials beenevaluated for their potential healtheffects on occupants. In addition,using sealants alters the surfaceburning characteristics of the ductmaterial and may void the fire safetyrating of the ductwork.

8. Careful cleaning and sanitizingof any parts of coils and drip panscan reduce microbiologicalpollutants.

Prior to using sanitizers, deodorizers,or any cleansing agents, carefullyread the directions on the productlabel. Once cleaned, these compo-nents should be thoroughly rinsedand dried to prevent exposure ofbuilding occupants to the cleaningchemicals.

9. Water-damaged or contami-nated porous materials in theductwork or other air handlingsystem components should beremoved and replaced.

Even when such materials arethoroughly dried, there is no way toguarantee that all microbial growthhas been eliminated.

10. After the duct system hasbeen cleaned and restored touse, a preventive maintenanceprogram will prevent the recur-rence of problems.

Such a program should includeparticular attention to the use andmaintenance of adequate filters,control of moisture in the HVACsystem, and periodic inspection andcleaning of HVAC system compo-nents. (See discussion of PreventiveMaintenance on page 36 in Section5.)

PRELIMINARY RECOMMENDATIONS ON DUCT CLEANING


Return air is frequently car-ried through non-ducted ple-nums. It is more difficult tocontrol leakage of pollutantsinto or out of this type of re-turn air system than a ductedsystem.

Terminal Devices

Thermal comfort and effective contami-nant removal demand that air deliveredinto a conditioned space be properlydistributed within that space. Terminaldevices are the supply diffusers, return andexhaust grilles, and associated dampersand controls that are designed to distributeair within a space and collect it from thatspace. The number, design, and location(ceiling, wall, floor) of terminal devicesare very important. They can cause aHVAC system with adequate capacity toproduce unsatisfactory results, such asdrafts, odor transport, stagnant areas, orshort-circuiting.

Occupants who are uncomfortablebecause of distribution deficiencies (drafts,odor transport, stagnant air, or uneventemperatures) often try to compensate byadjusting or blocking the flow of air fromsupply outlets. Adjusting system flowswithout any knowledge of the properdesign frequently disrupts the propersupply of air to adjacent areas. Distribu-tion problems can also be produced if thearrangement of movable partitions,shelving, or other furnishings interfereswith airflow. Such problems often occur ifwalls are moved or added without evaluat-ing the expected impact on airflows.

Return Air Systems

In many modern buildings the above-ceiling space is utilized for the unductedpassage of return air. This type of systemapproach often reduces initial HVACsystem costs, but requires that the designer,maintenance personnel, and contractorsobey strict guidelines related to life andsafety codes (e.g., building codes) thatmust be followed for materials and devicesthat are located in the plenum. In addition,if a ceiling plenum is used for the collec-tion of return air, openings into the ceilingplenum created by the removal of ceilingtiles will disrupt airflow patterns. It is

particularly important to maintain theintegrity of the ceiling and adjacent wallsin areas that are designed to be exhausted,such as supply closets, bathrooms, andchemical storage areas.

After return air enters either a ductedreturn air grille or a ceiling plenum, it isreturned to the air handlers. Some systemsutilize return fans in addition to supplyfans in order to properly control thedistribution of air. When a supply andreturn fan are utilized, especially in a VAVsystem, their operation must be coordi-nated in order to prevent under- or over-pressurization of the occupied space oroverpressurization of the mixing plenum inthe air handler.

Exhausts, Exhaust Fans, andPressure Relief

Most buildings are required by law (e.g.,building or plumbing codes) to provide forexhaust of areas where contaminantsources are strong, such as toilet facilities,janitorial closets, cooking facilities, andparking garages. Other areas whereexhaust is frequently recommended but

134 Appendix B

achieving a neutral or slightly positivepressure.

Self-Contained Units

In some designs, small decentralized unitsare used to provide cooling or heating tointerior or perimeter zones. With theexception of induction units, units of thistype seldom supply outdoor air. They aretypically considered a low priority mainte-nance item. If self-contained units areoverlooked during maintenance, it is notunusual for them to become a significantsource of contaminants, especially for theoccupants located nearby.

Controls

HVAC systems can be controlled manuallyor automatically. Most systems are con-trolled by some combination of manual andautomatic controls. The control system canbe used to switch fans on and off, regulatethe temperature of air within the conditionedspace, or modulate airflow and pressures bycontrolling fan speed and damper settings.Most large buildings use automatic controls,and many have very complex and sophisti-cated systems. Regular maintenance andcalibration are required to keep controls ingood operating order. All programmabletimers and switches should have “batterybackup” to reset the controls in the event of apower failure.

Local controls such as room thermostatsmust be properly located in order tomaintain thermal comfort. Problems canresult from:

■ thermostats located outside of theoccupied space (e.g., in return plenum)

■ poorly designed temperature controlzones (e.g., single zones that combineareas with very different heating orcooling loads)

■ thermostat locations subject to drafts orto radiant heat gain or loss (e.g., exposedto direct sunlight)

may not be legally required include:reprographics areas, graphic arts facilities,beauty salons, smoking lounges, shops,and any area where contaminants areknown to originate.

For successful confinement and exhaustof identifiable sources, the exhausted areamust be maintained at a lower overallpressure than surrounding areas. Any areathat is designed to be exhausted must alsobe isolated (disconnected) from the returnair system so that contaminants are nottransported to another area of the building.

In order to exhaust air from the build-ing, make-up air from outdoors must bebrought into the HVAC system to keep thebuilding from being run under negativepressure. This make-up air is typicallydrawn in at the mixed air plenum asdescribed earlier and distributed within thebuilding. For exhaust systems to functionproperly, the make-up air must have a clearpath to the area that is being exhausted.

It is useful to compare the total cfm ofpowered exhaust to the minimum quantityof mechanically-introduced outdoor air.To prevent operating the building undernegative pressures (and limit the amount ofunconditioned air brought into the buildingby infiltration), the amount of make-up airdrawn in at the air handler should alwaysbe slighter greater than the total amount ofrelief air, exhaust air, and air exfiltratingthrough the building shell. Excess make-up air is generally relieved at an exhaust orrelief outlet in the HVAC system, espe-cially in air economizer systems. Inaddition to reducing the effects of un-wanted infiltration, designing and operat-ing a building at slightly positive or neutralpressures will reduce the rate of entry ofsoil gases when the systems are operating.For a building to actually operate at aslight positive pressure, it must be tightlyconstructed (e.g., specified at less thanone-half air change per hour at 0.25pascals). Otherwise unwanted exfiltrationwill prevent the building from ever


■ thermostat locations affected by heatfrom nearby equipment

To test whether or not a thermostat isfunctioning properly, try setting it to anextreme temperature. This experiment willshow whether or not the system is respond-ing to the signal in the thermostat, andalso provides information about how theHVAC system may perform under extremeconditions.

Boilers

Like any other part of the HVAC system, aboiler must be adequately maintained tooperate properly. However, it is particu-larly important that combustion equipmentoperate properly to avoid hazardousconditions such as explosions or carbonmonoxide leaks, as well as to provide goodenergy efficiency. Codes in most parts ofthe country require boiler operators to beproperly trained and licensed.

Both ASME and ASHRAE have maderecommendations of how much combus-tion air is needed for fuel burning appli-ances.

Elements of boiler operation that areparticularly important to indoor air qualityand thermal comfort include:

■ Operation of the boiler and distributionloops at a high enough temperature tosupply adequate heat in cold weather.

■ Maintenance of gaskets and breeching toprevent carbon monoxide from escapinginto the building.

■ Maintenance of fuel lines to prevent anyleaks that could emit odors into thebuilding.

■ Provision of adequate outdoor air forcombustion.

■ Design of the boiler combustion exhaustto prevent re-entrainment, (especiallyfrom short boiler stacks, or into multi-story buildings that were added after theboiler plant was installed).

Modern office buildings tend to havemuch smaller capacity boilers than olderbuildings because of advances in energyefficiency. In some buildings, the primaryheat source is waste heat recovered fromthe chiller (which operates year-round tocool the core of the building).

Cooling Towers

Maintenance of a cooling tower ensuresproper operation and keeps the coolingtower from becoming a niche for breedingpathogenic bacteria, such as Legionellaorganisms. Cooling tower water qualitymust be properly monitored and chemicaltreatments used as necessary to minimizeconditions that could support the growth ofsignificant amounts of pathogens. Propermaintenance may also entail physicalcleaning (by individuals using properprotection) to prevent sediment accumula-tion and installing drift eliminators.

It is important to determineperiodically whether theHVAC controls are correctlycalibrated. In addition, timeclocks must be checked tosee if they are properly setand running. Power failuresfrequently cause time clocksto be out of adjustment.

136 Appendix B

Application

Food and BeverageService

Offices

Public Spaces

Retail Stores, Sales Floors,Showroom Floors

Sports andAmusement

Theaters

Education

Hotels, Motels,Resorts,Dormitories

Dining roomsCafeteria, fast foodBars, cocktail loungesKitchen (cooking)

Office spaceReception areasConference rooms

Smoking loungeElevators

Basement and streetUpper floorsMalls and arcadesSmoking lounge

Spectator areasGame roomsPlaying floorsBallrooms and discos

LobbiesAuditorium

ClassroomMusic roomsLibrariesAuditoriums

BedroomsLiving roomsLobbiesConference roomsAssembly rooms

Occupancy Cfm/person Cfm/ft2

(people/1000 ft2)

70 20100 20100 3020 15

7 2060 1550 20

70 601.00

30 0.3020 0.2020 0.2070 60

150 1570 2530 20

100 25

150 20150 15

50 1550 1520 15

150 15

30 cfm/room30 cfm/room

30 1550 20

120 15

FIGURE B-2: Selected Ventilation Recommendations

SOURCE: ASHRAE Standard 62-1989, Ventilation for Acceptable Air Quality


used because source strength is usually notknown.

Whichever procedure is utilized in thedesign, the standard states that the designcriteria and assumptions shall be docu-mented and made available to thoseresponsible for the operation and mainte-nance of the system.

Important features of ASHRAE 62-1989 include:

■ a definition of acceptable air quality■ a discussion of ventilation effectiveness■ the recommendation of the use of source

control through isolation and localexhaust of contaminants

■ recommendations for the use of heatrecovery ventilation

■ a guideline for allowable carbon dioxidelevels

■ appendices listing suggested possibleguidelines for common indoor pollutants

Standard 55-1981, “ThermalEnvironmental Conditions forHuman Occupancy”

ASHRAE 55-1981 covers several environ-mental parameters including: temperature,radiation, humidity, and air movement.

The standard specifies thermal environ-mental conditions for the comfort ofhealthy people in normal indoor environ-ments for winter and summer conditions.It also attempts to introduce limits on thetemperature variations within a space. Inaddition to specifications for temperatureand humidity, guidelines are given for airmovement, temperature cycling, tempera-ture drift, vertical temperature difference,radiant asymmetry, and floor temperatures.Adjustment factors are described forvarious activity levels of the occupants,and different clothing levels.

Important features of this standardinclude:■ a definition of acceptable thermal

comfort

Water Chillers

Water chillers are frequently found in largebuilding air conditioning systems becauseof the superior performance they offer. Awater chiller must be maintained in properworking condition to perform its functionof removing the heat from the building.Chilled water supply temperatures shouldoperate in the range of 45°F or colder inorder to provide proper moisture removalduring humid weather. Piping should beinsulated to prevent condensation.

Other than thermal comfort, IAQconcerns associated with water chillersinvolve potential release of the workingfluids from the chiller system. The rupturedisk (safety release) of the system shouldbe piped to the outdoors, and refrigerantleaks should be located and repaired.Waste oils and spent refrigerant should bedisposed of properly.

ASHRAE STANDARDS ANDGUIDELINES

Standard 62-1989, “Ventilation forAcceptable Air Quality”

ASHRAE 62-1989 is intended to assistprofessionals in the proper design ofventilation systems for buildings. Thestandard presents two procedures forventilation design, a “Ventilation Rate”procedure and an “Air Quality” procedure.

With the Ventilation Rate procedure,acceptable air quality is achieved byspecifying a given quantity and quality ofoutdoor air based upon occupant densityand space usage. Examples of the tableslisting the prescriptive amounts of outdoorair for the Ventilation Rate procedure arepresented at the end of this section.

The Air Quality procedure is a perfor-mance specification that allows acceptableair quality to be achieved within a space bycontrolling for known and specifiablecontaminants. This procedure is seldom

138 Appendix B

test and the atmospheric dust spot test.The atmospheric dust spot test is the testused to determine the “efficiency “ of anair cleaner. The values obtained withthese two tests are not comparable. Forexample, a filter with a weight arrestanceof 90% may have an efficiency by theatmospheric dust spot test below 40%.

The weight arrestance test is generallyused to evaluate low efficiency filtersdesigned to remove the largest andheaviest particles; these filters are com-monly used in residential furnaces and/orair-conditioning systems or as upstreamfilters for other air cleaning devices. Forthe test, a standard synthetic dust is fedinto the air cleaner and the proportion (byweight) of the dust trapped on the filter isdetermined. Because the particles in thestandard dust are relatively large, theweight arrestance test is of limited value inassessing the removal of smaller, respi-rable-size particles from indoor air.

The atmospheric dust spot test isusually used to rate medium efficiency aircleaners. The removal rate is based on thecleaner’s ability to reduce soiling of aclean paper target, an ability dependent onthe cleaner removing very fine particlesfrom the air. However, it should be notedthat this test addresses the overall effi-ciency of removal of a complex mixture ofdust, and that removal efficiencies fordifferent size particles may vary widely.Recent studies by EPA, comparingASHRAE ratings to filter efficiencies forparticles by size, have shown that efficien-cies for particles in the size range of 0.1 to1 microgram are much lower than theASHRAE rating.

Important features of this ASHRAEstandard include:

■ definitions of arrestance and efficiency■ establishment of a uniform comparative

testing procedure for evaluating theperformance of air cleaning devices usedin ventilation systems

■ a discussion of the additionalenvironmental parameters that mustbe considered

■ recommendations for summer and wintercomfort zones for both temperature andrelative humidity

■ a guideline for making adjustment foractivity levels

■ guidelines for making measurementsIt should be noted that space tempera-

tures above 76°F but within the summercomfort envelope have nevertheless beenassociated with IAQ complaints in offices.

Note: As of summer 1991, a revisedStandard 55 was nearly ready.

Standard 52-76, “Method of TestingAir-Cleaning Devices Used inGeneral Ventilation for RemovingParticulate Matter”

This standard is intended to assist profes-sionals in the evaluation of air cleaningsystems for particle removal. Two testmethods are described: the weightarrestance test and the atmospheric dustspot test. The standard discusses differ-ences in results from the weight arrestance

This air washer is used toremove particles and water-soluble gaseouscontaminants and may alsocontrol temperature andhumidity in the airstream.Such systems are subjectto severe bacterialcontamination.


■ establishment of a uniform reportingmethod for performance

■ methods for evaluating resistance toairflow and dust-holding capacity

No comparable guidelines or standardsare currently available for use in assessingthe ability of air cleaners to remove gas-eous pollutants or radon and its progeny.

Guideline 1-1989, “Guideline for theCommissioning of HVAC Systems”

This guideline is intended to assist profes-sionals by providing procedures and meth-ods for documenting and verifying theperformance of HVAC systems so thatthey operate in conformity with the designintent. The guideline presents a format fordocumenting the occupancy requirements,design assumptions, and the design intentfor the HVAC system. It provides a for-

mat for testing the system for acceptanceby the owner. In addition, the guidelineaddresses adjustments of the system tomeet actual occupancy needs within thecapacity of the system when changes inbuilding use are made andrecommissioning is warranted.

Important features of this guidelineinclude:

■ definition of the commissioning process■ discussion of the process involved in a

proper commissioning procedure■ sample specification and forms for log-

ging information■ recommendation for the implementation

of corrective measures as warranted■ guideline for operator training■ guidelines for periodic maintenance and

recommissioning as needed

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