Indoor Environmental Quality - Chapter 11 pot

Natural ventilation depends on theinflow of air as a result of 1 pressure differences when a building is underclosure conditions and being heated or cooled, 2 pressure-driven flowswhen b

chapter eleven Ventilation

Ventilation has historically been applied and viewed as both a desirable andeffective technique in improving thermal comfort and general air quality andcomfort in buildings It has been used to dilute and exhaust unwanted con-taminants such as combustion by-products, lavatory and cooking odors, heatand moisture, etc from residential and nonresidential buildings Exhaustventilation is widely used in industry to remove contaminants at their source

to reduce worker exposure and health risks General dilution ventilationinduced and delivered by mechanical fans is used in office, commercial andretail, and institutional buildings to maintain acceptable air quality

Ventilation is a physical process that involves the movement of airthrough spaces It has two dimensions When air flows into a space, venti-lation is characterized by mixing, dilution, and partial replacement Whenair flows (is removed) from a space (because it is under negative pressure),

it is replaced by air from a nearby area General dilution ventilation describesthe first case; exhaust ventilation, the second

Ventilation, because it involves the flow of air due to pressure ences, is a natural phenomenon However, natural ventilation is often toovariable or inadequate As a consequence, it is necessary to use systems thatdeliver controlled, mechanically induced ventilation to provide for the needs

differ-of a variety differ-of building spaces

Ventilation in its dilution, displacement, and replacement aspects causeschanges in chemical composition and environmental factors in the air envi-ronment of ventilated spaces These changes may result in reduced overallcontaminant levels or a decrease in concentrations of some substances, with

an increase in others The outcome depends on the nature of air in ventilatedspaces as well as air used for ventilation The same is true for environmentalfactors such as temperature and relative humidity

The desired effect of ventilation, whether it is natural or mechanicallyinduced, is to enhance or protect the quality of air in the space being venti-

lated Ventilation causes an exchange of air within building spaces andbetween building interiors and the outside environment Ventilation is used

to dilute and remove contaminants, enhance thermal comfort, remove excessmoisture, enhance air motion, improve general comfort, and in large build-ings, maintain pressure differences between zones

I Natural ventilation

All buildings are subject to natural forces that result in air exchange withthe ambient (outdoor) environment Natural ventilation depends on theinflow of air as a result of (1) pressure differences when a building is underclosure conditions and being heated or cooled, (2) pressure-driven flowswhen building windows and doors are open, or (3) the continual movement

of air through a building as it enters through some openings and exitsthrough others (Figure 11.1) In the last case, small buildings will experiencerelatively high air exchange rates Maximum exchange rates will depend onwind speed, the position of open windows and doors relative to each other,and prevailing winds Though limited scientific data are available, airexchange rates should be at their maximum when buildings are ventilated

by opening windows and doors when outdoor conditions are favorable.Residential buildings, which include both single- and multi-family struc-tures, are increasingly being provided year-round climate control As a con-sequence, the practice of opening windows and doors for ventilation pur-poses is decreasing Such residences, and those in seasonally cold or warmclimates, are maintained under closure conditions for extended periods oftime (upwards of 9 months or so)

Under closure conditions, ventilation occurs as a result of infiltrationand exfiltration processes These processes involve pressure-driven flows

Figure 11.1 Ventilation air flow through a single-family house under open tions (From USEPA, Introduction to Indoor Air Quality — A Self-paced Learning Module, EPA/400/3-91/002, Washington, D.C., 1991.)

condi-associated with temperature differences (between indoor and outdoor ronments) and the speed of the wind.

envi-Infiltration occurs as a result of the inflow of air through cracks and avariety of unintentional openings (leakage areas) in the building envelope.Infiltration only occurs through leakage areas where internal pressures arenegative relative to those outdoors In residential structures, infiltration typ-ically occurs at the base and during windy conditions on the lee (downwind)side of buildings

When infiltration occurs, it replaces and displaces air in the buildinginterior As a consequence, air must also flow outward through cracks andother leakage areas In residential structures, such air outflows (exfiltration)typically occur through ceiling areas and upper wall locations where internalpressures are positive

A Stack effect

On calm days or during calm periods during the day, infiltration and tration occur as a consequence of pressure differences associated with dif-ferences between the inside and outside temperature (∆T) During the heat-ing season in seasonally cool to cold climates, warm air rises and creates apositive pressure on ceilings and upper walls of small residential (and non-residential) buildings This upward flow of warm air produces negativepressures, which are at a maximum at the base of the structure Thesenegative pressures cause an inflow (infiltration) of cool or cold air, withmaximum inflows where negative pressures are (in absolute terms) the high-est Infiltration causes air to be drawn in from both the outdoor environmentand from the ground (soil gas)

exfil-An idealized characterization of pressure conditions in a single-familydwelling on a cool day is diagramed in Figure 11.2 As can be seen, an area

of neutral pressure exists between negative and positive pressure ments This is the neutral pressure level or neutral pressure plane (NPP) At

environ-Figure 11.2 Generalized pressure conditions in a small house on a cool, calm day under closure conditions (From Lstiburek, J and Carmody, J., Moisture Control Hand- book, Van Nostrand Reinhold [John Wiley & Sons], New York With permission.)

the NPP, indoor and outdoor pressures are equal In Figure 11.2, the NPP islocated at approximately mid-level, suggesting that leakage areas are uni-formly distributed over the building face The location of the NPP depends

on the distribution of leakage sites In older single-family dwellings, the NPP

is often above mid-height, and in the case of houses with flue exhaust ofcombustion by-products, the NPP may be above the ceiling level duringexhaust operation The construction of residential dwellings has included anumber of energy-conserving measures, most notably tighter building enve-lopes Since leakage areas have been reduced, pressure characteristics havechanged, resulting in reduced infiltration and exfiltration In such houses,the NPP would be expected to occur at mid-height (absent the active oper-ation of combustion exhaust systems)

In tall buildings, the NPP may vary from 30 to 70% of the building height.The inflow and outflow of air discussed above is called the stack effectbecause airflows are similar to those which occur in a smokestack Themagnitude of the stack effect increases significantly with building height.The change in pressure with height in a large building has been reported to

be approximately 0.001” H2O (0.25 pascals) per story Stack effect flowsupward are particularly noticeable in elevator and other service shafts and

in open stairwells Each story, if constructed in an airtight way, can behavemore or less independently, i.e., have its own stack effect

The influence of stack effect on building air exchange rates is, for themost part, proportional to ∆T, the difference between indoor and outdoortemperatures This relationship can be seen in model predictions graphed

in Figure 11.3 for ∆T = 0°F and 40°F As ∆T increases, air exchange increases,with maximum values on cold days Minimum air exchange occurs when

Figure 11.3 Building air exchange associated with different stack effect and wind speed conditions.

indoor and outdoor temperatures are the same or little different from eachother Such conditions exist for brief periods (hours) during diurnal changes

in outdoor temperatures, and for more extended periods on mild overcastdays, and mild days in the fall and autumn in temperate climatic regions.They commonly occur in coastal regions where maritime climates produceoutdoor temperature conditions which in many cases are in the same range

as those indoors

B Wind

Infiltration and exfiltration are also significantly influenced by wind Theeffect of wind on pressures both inside and outside of buildings is a relativelycomplex phenomenon

As wind approaches a building it decelerates, creating significant tive pressure on the windward face As wind is deflected, its flow separates

posi-at a building’s sides as well as its top or roof This produces negposi-ative sures around the sides of the building, the roof, and the leeward side Theeffect of these pressure differences is to cause an inflow of air on the windside and an outflow on all exterior surfaces which are negative relative toindoor pressures The effects of wind on pressure conditions on a rectangularbuilding oriented perpendicular to the wind is illustrated in Figure 11.4 Thedistribution of positive and negative pressures on a building depends onwind speed, building geometry and size, and the incident angle of the wind.The magnitude and distribution of infiltration air is influenced by the type

pres-of building cladding; tightness pres-of the building envelope; and barriers to air

Figure 11.4 Effect of wind on pressure conditions in/on a rectangular building oriented perpendicular to the wind (From Allen, C., Technical Note AIC 13, Air Infiltration Centre, Berkshire, U.K., 1984 With permission.)

movement such as trees, shrubbery, and other buildings Such barriersinduce turbulence that reduces wind speed and alters wind direction.The effect of wind on building infiltration is a squared function of windvelocity or speed (mph, m/s) The modeled effect of wind speed on houseair exchange rates can be seen in Figure 11.3 for a ∆T of 0°F (0°C) and 40°F(22°C) Though the effect of wind speed on air exchange rates is exponential,significant increases in infiltration-induced ventilation are only seen at highwind speeds (>8 mph, 3.56 m/s) As can been seen in Figure 11.3, thecombined effect of stack effect and wind speed on building air exchangerates (ventilation) can be significant.

Figure 11.3 describes the effect of indoor/outdoor temperature ences and wind speed on a single house Because of differences in buildingtightness, distribution of leakage areas, and orientation to the wind, airexchange rates in other houses under similar stack effect and wind speedconditions are likely to be different (though the general form of the rela-tionship will be much the same) These curves are based on the followinglinear model:

where I = infiltration rate (ACH)

A = intercept coefficient, ACH (∆T = 0, v = 0)

B = temperature coefficient

C = wind velocity coefficient

∆T = indoor/outdoor temperature difference, (°F)

v = wind speed (mph, m/sec)Both the temperature and wind speed coefficients are empirically derivedand differ for each building Coefficient differences among buildings arerelatively small

C Infiltration and exfiltration air exchange rates

As indicated above, building air exchange rates associated with tion/exfiltration-induced airflows vary with indoor/outdoor temperaturedifferences (which vary considerably themselves), wind speed, and tightness

infiltra-of the building envelope They may also be influenced by pressure changesassociated with the operation of vented combustion appliances, bath-room/lavatory fans, and leaky supply and return air ducts Each of thesecan increase infiltration and air exchange rates above those associated withcombined stack effect and wind infiltration and exfiltration values Leakysupply/return air ducts may be responsible for upwards of 30+% of infil-tration/exfiltration-related air exchange in residential buildings

In response to energy concerns in the late 1970s and early 1980s, the U.S.Department of Energy supported several studies to evaluate infiltration andexfiltration rates in U.S housing stock The average air exchange rate for

more than 300 U.S houses was measured on a one-time basis As can beseen in Figure 11.5, approximately 80 to 85% of houses tested had a dailyaverage air exchange rate of <1 air change per hour (ACH) Infiltration/exfil-tration rates in low-income housing were on average significantly higher;approximately 40% had infiltration values >1 ACH.

On a population basis, these one-time measurements of tration-induced air exchange were likely to have demonstrated a reasonableestimate of ventilation conditions in housing stock existing at the time (early1980s) Since then, construction practices have changed (tighter buildingenvelopes are now the norm), and significant weatherization measures havebeen implemented to reduce energy losses in low-income housing Weath-erization measures using retrofit tightening of building envelopes in low-income housing have, however, only been moderately effective (on average,

infiltration/exfil-≤25% reduction in building leakage and infiltration-associated air exchange)

It is highly probable that construction practices in the past several decadeshave significantly increased the stock of housing units in North America,northern Europe, and other developed regions and countries which havelower air exchange (and thus ventilation) rates than older houses Decreasingnatural ventilation rates have been a cause for concern among policy makers

in various governmental agencies, utilities which have supported ization measures, public health groups, and research scientists It was and iswidely believed among environmental and public health professionals thatdecreasing natural ventilation rates associated with infiltration/exfiltration-reducing measures are likely to cause an increase in indoor contaminant levelsand health risks associated with increased exposures

weather-D Leakage characteristics

Air exchange rates in buildings associated with thermal and wind-inducedpressure differences are affected to a significant degree by building leakagecharacteristics Typical leakage areas are indicated in Figure 11.6 for a single-

Figure 11.5 Infiltration rates measured in 312 North American houses in the early 1980s (From Grimsrud, D.T et al., LBL-9416, Lawrence Berkeley Laboratory, Berke- ley, CA, 1983.)

story house on a basement Major structure-related leakage areas include thesole plate where the building frame is fastened to the substructure, andcracks around windows, doors, exterior electrical boxes, light fixtures,plumbing vents, and various joints Leakage also occurs through exhaustfans, supply/return ducts, and combustion appliance flues Leakage is par-ticularly pronounced when combustion appliances such as furnaces, hotwater heaters, and fireplaces are in operation Leakage can also occurthrough duct systems which provide heating and cooling when duct runsare in crawlspaces, attics, and garages.

The size of leakage areas and their distribution in a building determinethe magnitude of infiltration and exfiltration air exchange when measuredunder similar environmental conditions They also determine the nature ofair flow patterns into and out of buildings

Building leakage potentials are commonly assessed using ization or blower-door techniques By pressurizing buildings with a faninstalled into a test door, the overall leakage potential of residential buildingscan be determined to identify leakage areas that need to be caulked or sealed.Such leakage characterization is commonly conducted in weatherizationprograms which target low-income housing

fan-pressur-II Measuring building air exchange rates

Building air exchange rates (ACH) associated with wind and thermallyinduced pressure flows, as well as those associated with mechanical venti-

Figure 11.6 Air leakage sites on a single-family house with basement substructure.

lation, can be measured using tracer gas techniques Tracer gases used insuch measurements are characteristically unreactive, nontoxic, and easilymeasured at low concentrations Sulfur hexafluoride and perfluorocarbonsare commonly used to measure air exchange rates because they can bedetected and quantitatively determined in the parts per billion range (ppbv).

On occasion, nitrous oxide (N2O) or carbon dioxide (CO2) are used Carbondioxide has the advantage of being measured in real time on relativelyinexpensive continuous monitors It has the disadvantage of being produced

by humans Thus it cannot be used when building spaces are occupied.Silicon hexafluoride, perfluorocarbons, and N2O are collected using one-timesampling techniques and require analysis on sophisticated, expensive instru-ments Perfluorocarbon measurements are usually made using permeationtubes as sources and passive samplers as collectors As such, air exchangemeasurements based on perfluorocarbons typically provide 7-day averages.The concentration decay method (previously described in Chapter 8) isthe most widely used air exchange measuring technique It involves initialinjection of a tracer gas into a space or building with the assumption thatthe tracer gas is well mixed in building/space air The decrease, or decay as

it is often described, of the tracer gas concentration is measured over time.From these measurements the air exchange rate I in ACH can be determinedfrom the following exponential equations:

Q = ventilation rate (m3/hour)

e = natural log base

Let us assume that the initial tracer gas concentration was 100 ppbv and

at the end of 2 hours it was 25 ppbv We could then calculate I as follows:

I = (ln 100/25)/2 (11.5)

I = 0.69 ACH

In perfluorocarbon determinations of air exchange, perfluorocarbons areinjected at a constant rate The air exchange I is calculated from the ventila-tion rate (Q), which is the ratio of the rate of injection (F) to the measuredconcentration C

Since Q is expressed in m3/sec or cubic feet per minute (CFM), it must bemultiplied by appropriate time units and divided by the volume of the space

to obtain ACH values

III Mechanical ventilation

Most large commercial, office, and institutional buildings constructed indeveloped countries over the past three decades are mechanically ventilated.Use of mechanical ventilation is often required in building codes and repre-sents what can be described as good practice for building system designersand architects Increasingly, buildings are being designed to provide year-round climate control To ensure optimum operation of heating, ventilating,and air-conditioning (HVAC) systems, windows are sealed so they cannot

be opened by occupants to provide ventilation The availability of outdoorair for space ventilation depends on the design and operation of HVACsystems as well as air that enters by infiltration and exfiltration processes.Mechanical ventilation is used in buildings to achieve and maintain acomfortable and healthy indoor environment Two ventilation principles areused to accomplish this goal; general dilution and exhaust ventilation Bothprinciples are used in most buildings General dilution ventilation is thedominant ventilation principle used to ventilate buildings Local exhaustventilation is used for special applications: removing lavatory and kitchenodors, combustion by-products, combustion appliance flue gases, etc

A General dilution ventilation

Ventilating buildings to provide a relatively comfortable, healthy, and free environment is based on the premise that a continual supply of outdoorair can be introduced into building spaces As ventilation air mixes withcontaminated air, contaminant levels are reduced by dilution

odor-In general dilution theory, a doubling of the air volume available fordilution is expected, under episodic or constant conditions of contaminantgeneration, to reduce contaminant concentration by 50% If the volume ofventilation air were to be doubled again, the original concentration would

be reduced to 25% of its original value By decreasing ventilation air required

for dilution, contaminant concentrations would be expected to increase in

an inverse manner to the dilution-induced decreases described above

In the real world, the level of contaminant reduction associated withgeneral dilution ventilation is often less than that described above Contam-inant concentrations are determined by how they are generated (episodic,constant, increasing), ventilation capacity (the HVAC system’s ability tomechanically deliver outside air to dilute contaminants to acceptable levels),and the operation and maintenance of HVAC system equipment Elevatedcontaminant levels may result from high source generation rates, inadequatedesign capacity, reduced outdoor air flows associated with energy-manage-ment practices, and poor operation and maintenance of HVAC systems.Because of the large air volumes required to reduce elevated contaminantlevels, general dilution ventilation is not the most efficient control measure.However, when contaminant sources are diffuse and cannot be easily con-trolled in other ways, it is the control method of choice Such conditions arecommon in many large nonindustrial, nonresidential public access buildings

1 HVAC systems

The ventilation function is incorporated into systems that provide climatecontrol, such as heating and cooling (and sometimes humidification), in mostbuildings where general dilution ventilation is used to reduce contaminantlevels and provide a comfortable and healthful indoor environment TheseHVAC systems include one or more air-handling units (AHUs), supplyducts, diffusers, return air grilles, return air plenums, dampers, exhaust fansand exhaust outlets, intake grilles, mixing boxes, etc A relatively simpleHVAC system design that provides conditioned air to a single space isillustrated in Figure 11.7 In this case, the AHU is a small box suspendedabove ceiling level It consists of a filter, thermal sensors, and heating andcooling coils Air that enters the AHU can be 100% outdoor air, nearly 100%recirculated air, or various mixtures of recirculated (from the conditionedspace) and outdoor air

Outdoor and recirculated air percentages are varied to meet buildingoperating needs In middle latitudes in North America, such systems areoperated with 100% outdoor air during limited periods of spring and autumnmonths when temperatures are favorable for using outdoor air for free-cooling That is, outdoor air is used to cool building spaces in lieu of acti-vating energy-consuming cooling units During cold weather, HVAC sys-tems may be operated on nearly 100% recirculated air to prevent freezing

of system coils In many instances, it is still common (particularly in schoolbuildings) for facilities managers to operate HVAC systems on or near 100%recirculated air as an energy management strategy In the latter case, HVACsystems are not being operated as designed, and building air quality isunlikely to be acceptable Good operating practice would require that at aminimum 15 to 20% outdoor air ventilation be provided during normalbuilding occupancy hours to maintain acceptable air quality In older AHUs,dampers that regulate the percentage of recirculated and outdoor air have

to be manually adjusted In most modern units, such adjustments are madeautomatically by use of thermal or CO2 sensors, or timers.

a Types of HVAC systems. Buildings are climate controlled and tilated with a variety of HVAC systems There are three basic system typesused in buildings, each with its own variations These include all-air,air–water, and all-water systems

ven-i All-air systems In all-air systems, air is conditioned as it passesover heating and cooling coils It is then delivered to occupied spacesthrough a single duct or through individual hot and cold ducts to a mixingbox (and then to occupied spaces) Air flow through these systems may be

at a constant rate (constant-air-volume [CAV] systems), or air flow may bevaried to individual spaces (variable-air-volume [VAV] systems) SimplifiedCAV and VAV system designs are illustrated in Figures 11.8 and 11.9

In CAV systems, temperature is varied to meet heating and coolingneeds Supply air flows at a fixed and constant rate In VAV systems, air isinitially conditioned in an AHU It is delivered to different zones or spacesthrough VAV boxes which regulate air flow and temperature in response toheating/cooling demands of individual zones Both systems provide out-door air for ventilation and are able to control the relative amount of outdoorand recirculated air that spaces are provided Variable-air-volume systemswere developed in response to energy management concerns and, whenproperly installed and operated, can reduce building energy consumption

Figure 11.7 HVAC system providing conditioned and ventilation air to a single occupied space (From USEPA, Building Air Quality Manual, EPA 402-F-91-102, Wash- ington, D.C., 1991.)

Outdoor Air Intake

Outdoor Air Damper

Damper Actuators Exhaust Air

Supply Air Diffusers Outdoor Air

Temperature Sensor

Humidifier

Mixing Chamber

Filter

Freeze Stat

Return Air Grille Mixed Air Stat

Heating Coil Cooling Coil and Drip Pan Air Handling Unit Fan

requirements (compared to CAV systems.) Variable-air-volume systems aremore complicated than CAV systems and, as a consequence, are more diffi-cult to operate effectively Not surprisingly, VAV system operation in manybuildings has been plagued with problems One of the most common hasbeen the complete closing of VAV valves and cessation of air flow throughdiffusers when a space’s thermal requirements have been met Under suchconditions, one or more spaces may receive little or no ventilation air.

ii Air–water systems Air–water systems differ from all-air ones inthat air can be reheated or cooled by passing over a fan coil before it entersconditioned spaces A terminal reheat system is illustrated in Figure 11.10.iii All-water systems In all-water, or hydronic systems, heating andcooling occurs in terminal reheat units located in each space The terminal

Figure 11.8 Simplified constant-volume HVAC system design (From McNall, P.E and Persily, A.K., in Ann ACGIH: Evaluating Office Environmental Problems, 10, 77, ACGIH, Cincinnati, 1984 With permission.)

Figure 11.9 Simplified variable-volume HVAC system design (From McNall, P.E and Persily, A.K., in Ann ACGIH: Evaluating Office Environmental Problems, 10, 77, ACGIH, Cincinnati, 1984 With permission.)

reheat unit provides for air circulation in a space, but no ventilation Aseparate duct system must be used to provide outdoor air for ventilation.

iv Unit ventilators In many school buildings constructed in the1960s and 1970s, exterior classroom spaces are heated and ventilated throughmodular units installed beneath windows These units (Figure 11.11) aretypically provided with hot water that passes through a fin tube system Airwarmed by convection is delivered through the top of the unit ventilator(univent) Return air is drawn into the base through a filter It is then mixedwith outdoor air, which is drawn through intake grilles on the buildingexterior before it is heated and delivered Some univent systems areequipped with chilled water lines for air conditioning Unit ventilators mayalso be used to condition interior classrooms or larger spaces In such cases,they are suspended from the ceiling and outside air must be ducted to them.Unit ventilators are relatively simple mechanical systems They are oftenpoorly operated Their operation and ability to adequately ventilate spaces

is compromised by the force of numbers (many individual units in a ing), and dampers (which regulate the percentage of outdoor and recircu-lated air), which typically require manual adjustment Because of poor oper-ation and maintenance, unit ventilators often do not adequately ventilatebuilding spaces

build-b Ventilation standards and guidelines. It is widely accepted by ing designers, owners, research scientists, and policy makers that large,nonresidential, nonindustrial buildings with significant occupant densitiesmust be provided with adequate outdoor ventilation air to provide a com-fortable and relatively odor-free building environment It is important, there-fore, that consensus standards and guidelines be available for use by build-ing design professionals and by state and local governments, which setbuilding codes Such standards and guidelines are used to design HVAC

build-Figure 11.10 HVAC system with terminal reheat unit (From Hughes, R.T and O’Brien, D.M., Ind Hyg Assn J., 47, 207, 1986 With permission.)

systems with sufficient capacity to provide outdoor ventilation rates to attainand maintain acceptable air quality.

In North America, ventilation standards have, over the years, been oped by a consensus process by professional or standards-setting organiza-tions The American Society of Heating, Refrigerating, and Air-ConditioningEngineers (ASHRAE) has been the lead organization in the development ofventilation guidelines ASHRAE guidelines are given further authoritativestatus by their acceptance and publication by the American National Stan-dards Institute (ANSI)

devel-i Ventilation Rate Procedure The primary approach to setting tilation standards is to specify ventilation rates Ventilation rates areexpressed as volumetric air flows needed per building occupant, commonlycubic feet per minute (CFM) per person or liters per second (L/s) per person.The Ventilation Rate Procedure is based on the pioneering experimentalwork of Yaglou in the 1930s, who determined the amount of outdoor airneeded to maintain relatively human odor-free environments in buildings

ven-As a consequence of Yaglou’s work, a ventilation guideline of 10 CFM(4.76 L/s)/person was used by building designers in the period 1936–1973

In 1973, ASHRAE, taking energy conservation concerns into account,reduced its consensus standards to 5 CFM (2.34 L/s)/person for nonsmok-ing office and institutional environments Higher ventilation rates wererecommended for other environments such as hotel rooms, taverns, audi-toriums, residential living areas, industrial environments, and where build-ing occupants smoked This standard was widely used until 1989, when itwas revised

Figure 11.11 Unit ventilation (univent) system (From Spengler, J.D., Environ Health Perspect., 107, Suppl 2, 313, 1999.)

Supply air

Return air from room

Floor

Unit ventilator

Exterior wall

Cooling or heating element Filter

Outside air Dampers

Blower

The 1989 ASHRAE ventilation standard/guideline significantlyincreased the amount of outdoor air which was believed needed to provideoffice, commercial, and institutional buildings with sufficient outdoor ven-tilation air for a comfortable and healthful environment The revisedASHRAE ventilation standard reflected the explosion in problem buildingcomplaints which occurred in the late 1970s and the early- to mid-1980s Itwas apparent to many building and indoor air quality (IAQ) professionalsthat the 1973 ASHRAE standard of 5 CFM/person was not adequate andthat increased ventilation was necessary The recommended ventilation ratesfor general office environments became 20 CFM (9.5 L/s)/person, with 15CFM (7.14 L/s)/person in school buildings.

In the Ventilation Rate Procedure, the minimum ventilation rate forbuilding spaces is based on the maximum acceptable bioeffluent levels,using carbon dioxide (CO2) emissions and building concentrations as refer-ence points

Ventilation guidelines are based on the use of the following mass balanceequation:

where Q = ventilation rate (L/s)

Ci = acceptable indoor CO2 concentration

Ca= ambient CO2 concentration

G = CO2 generation rate (0.005 L/s)

Solving this equation when Ci = 0.10% and Ca = 0.0365%:

Q = 0.005/(0.0010 – 0.0003) = 7.14 L or 15 CFM (11.8)Assuming a metabolic CO2 generation of 0.005 L/sec at design adultoccupant capacity, a ventilation rate of 15 CFM/person would be required

to assure that an indoor CO2 guideline value of 1000 ppmv would not beexceeded A ventilation rate of 20 CFM/person was recommended for officespaces by ASHRAE in 1989 Based on the use of the above equation or thegraph in Figure 8.2, a ventilation rate of 20 CFM would, in theory, result inpeak steady-state CO2 levels of 800 ppmv

In the Ventilation Rate Procedure, CO2 concentrations are used as asurrogate for bioeffluents which cause human odor and are believed tocontribute to general discomfort Carbon dioxide itself is unlikely to causediscomfort or health effects at concentrations found in buildings, includingthose that are not well-ventilated Carbon dioxide is used as an indicatorbecause it is the bioeffluent with the highest rate of emission and is relativelyeasily measured Research studies have shown a strong correlation betweenbuilding CO2 levels and human odor intensity

The relationship between CO2 levels and building occupancy can beseen in Figures 11.12a and 11.12b The effect of different ventilation rates on

CO2 levels can also be seen when Figures 11.12a and 11.12b are compared.

In Figure 11.12a, the HVAC system is being operated on 100% outside air;

in Figure 11.12b, it is being operated using a mixture of recirculated andoutdoor air

ii Indoor Air Quality Procedure In 1982, ASHRAE introduced what

it described as the Indoor Air Quality (IAQ) Option It was developed toprovide building system designers with an alternative to the Ventilation RateProcedure described above It recognized that the Ventilation Rate Procedurefocused primarily on bioeffluent control and was not designed to controlcontaminants generated in building spaces from construction materials, fur-

Figure 11.12 Effect of occupancy and ventilation on CO 2 levels in a San Francisco office building (From Turiel, I., et al., Atmos Environ., 17, 51, 1983 With permission.)