NBSIR 88-3708

Tracer Gas Techniques for

Studying Building Air Exchange

Andrew K. Persily

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

February 1988

1913-1988

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 88-3708

TRACER GAS TECHNIQUES FOR

STUDYING BUILDING AIR EXCHANGE

Andrew K. Persily

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

February 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Abstract

A variety of procedures have been developed that employ tracer gases to study

the air exchange characteristics of buildings. These procedures enable the

examination of several features of building air exchange including ventilation

rates, air movement within buildings, and building envelope airtightness. This

paper reviews tracer gas measurement techniques that have been used to study air

exchange in buildings. Background information is discussed such as the

instrumentation used in these tests, building features that influence their

application, and the fundamental theory of tracer gas measurement. Several

specific applications are discussed including air exchange rate measurement in

large buildings, low-cost procedures for measuring air exchange rates in large

numbers of buildings, techniques for evaluating the performance of air

distribution systems, and pressurization testing of envelope airtightness in

large buildings. A detailed bibliography is also included to facilitate a more

thorough examination of the topics discussed.

Key Words: airflow measurement; building performance; infiltration; measurement;

tracer gas; ventilation.
TABLE OF CONTENTS

1 . INTRODUCTION

2 . IMPORTANT FACTORS IN AIR EXCHANGE EVALUATION.

2 . 1 Building Characteristics ...............................................

2 .Instrumentation

3 . FUNDAMENTALS OF TRACER GAS MEASUREMENTS .....................................

3 . 1 Decay

3 . 2 Constant Concentration

3.3 Constant Injection

3 . 4 Multi-Chamber Techniques

3.4.1 Decay

3.4.2 Constant Concentration..........

3.4.3 Constant Injection

4 . AIR EXCHANGE RATE MEASUREMENT APPLICATIONS

4 . 1 Tracer Gas Measurement in Large Buildings ..............................

4 .

Low Cost Measurement Procedures

4.2.1 Decay

4.2.2 Constant Injection

5 . OTHER TRACER GAS APPLICATIONS

5 . 1 Air Distribution Evaluation ............................................

5 .

Ventilation System Airflow Rates .......................................

5 .

Pressurization Testing of Large Buildings

5 . 4 Qualitative Evaluation Techniques ......................................

6

REFERENCES ........

n
1. INTRODUCTION

This paper reviews tracer gas measurement techniques for studying air exchange

in buildings. These techniques are used to examine three basic aspects of

building air exchange including air exchange rates, interior air movement, and

building envelope airtightness. There are numerous motivations for

investigating these three aspects of building air exchange. The measurement of

building air exchange rates is important as these rates relate to energy

consumption and indoor air quality. Their measurement is useful for determining

whether the outdoor air intake rate is compatible with design specifications,

and whether relevant ventilation standards are being complied with. Tracer

gases can also be used to examine interior air movement patterns, which are

important as they relate to thermal comfort within the occupied space. These

interior air movement measurement procedures can also be used to evaluate the

performance of a building's air distribution system in providing ventilation air

to the occupants and removing internally generated contaminants. Finally, these

tracer gas procedures can be used to verify the isolation of special-use spaces,

where hazardous substances are in use, from the rest of a building's interior.

The third area of building air exchange appropriate to tracer gas

measurement is the determination of the airtightness of a building's envelope.

Envelope airtightness is important as it relates to energy consumption due to

uncontrolled air infiltration through the building shell. The existence of such

infiltration influences interior thermal comfort due to drafts, and may lead to

material, and possibly structural, damage to the building envelope due to

moisture condensation. Air infiltration due to envelope leakage is often a significant fraction of the total building air exchange rate and therefore is

important in understanding the indoor air quality of a building. Finally, the

airtightness of the building envelope interacts with a building's mechanical

ventilation system, affecting its ability to control air exchange rates and air

distribution.

While there have been previous reviews of tracer gas measurement techniques

(Alexander et al 1980; Harrje et al 1981 and 1982; Hitchin and Wilson 1967; Hunt

1980; Lagus 1980; Sherman et al 1980), this paper concentrates on more recent

developments such as the study of large buildings, evaluations of air

distribution effectiveness, and multi-tracer and multi-chamber techniques. The

first section of this review discusses several of the basic issues related to

evaluating building air exchange using tracer gases such as relevant building

characteristics and tracer gas instrumentation. The second section provides a discussion of the fundamentals of tracer gas measurement including the mass

balance theory on which the measurements are based and the three basic

approaches, i.e. decay, constant concentration and constant injection. Multichamber and multi-tracer theory and applications are also discussed. The third

section presents two applications of air exchange rate measurement procedures

including measurements in large buildings and low-cost procedures that are

useful for air exchange measurements in large numbers of buildings. The fourth

and final section discusses other tracer gas applications including the

evaluation of air distribution effectiveness, measurement of airflows within

ventilation systems, pressurization testing of large buildings, and qualitative

tracer gas evaluation techniques.

1
2. IMPORTANT FACTORS IN AIR EXCHANGE EVALUATION

In conducting a tracer gas evaluation of air exchange in any building there are

several issues that must be considered* These include the characteristics of

the building being studied and the instrumentation that will be used in the

investigation* This section discusses these issues* In addition, there are

some questions of terminology that need to be addressed at this stage. In this

paper the term infiltration refers to uncontrolled air leakage into a building

through unintentional openings in the building envelope, i.e., leaks, due to

pressure differences across these openings. These pressure differences are

caused by weather effects, wind and temperature differences between indoors and

outdoors, as well as by mechanical equipment operation, both fans and combustion

appliances. Exfiltration refers to airflow out of the building through these

same leaks. The term ventilation refers to intentional outdoor air intake

through a ventilation system, with mechanical ventilation being that ventilation

induced by a mechanical system. Natural ventilation is unpowered airflow

through intentional openings such as open windows and vents. The term air

exchange rate refers to the total airflow into a building due to all of these

processes, and in general they are all important in determining the net air

exchange rate of a building. In many buildings, particularly residential, there

is no mechanical ventilation except for bath and kitchen exhaust fans, and all

the air exchange is due to infiltration and natural ventilation.

2.1 Building Characteristics

In order to apply any tracer gas evaluation procedure to a building one must

consider several features of that particular structure in order to determine

exactly what questions to ask about the building and how to conduct the

measurements. It is very important to examine the building plans, the

mechanical equipment specifications, and the building itself before making these

decisions. The basic physical characteristics of the building, including floor

area, volume, height, and number of stories, are obviously important. Other

important aspects of a building' s physical layout include the manner in which

the building is divided into zones, and the existence, size and layout of

special-use spaces such as lobbies, atria, kitchens, cafeterias, conference

rooms, and laboratories. Such special-use spaces often have dedicated

mechanical ventilation systems and unique ventilation needs. The existence and

design of ventilation and air distribution equipment is another important

building feature that must be considered. These equipment issues include the

number of fans, their role in ventilation and distribution (e.g. supply, return,

exhaust, mixing), the amount of air they are intended to move, the manner in

which they are controlled, and the particular zone within the building that they

serve. In addition, the location of outdoor air intakes and exhaust vents, and

their proximity to one another, are important factors. All of these features of

a building must be studied and understood before the measurements are planned

and conducted. This process of developing an understanding a building and its

ventilation system requires a significant amount of effort but is absolutely

essential in order to conduct a worthwhile investigation.

In addition to understanding the above features of a particular building,

it is also important to understand and recognize the complex interactions

between the ventilation equipment, building envelope, and interior configuration

in determining a building's air exchange characteristics.' It is impossible to

consider these building systems in isolation because they interact so strongly.

Measurements and modelling in mechanically ventilated office buildings have

2
revealed the importance of these interactions (Persily and Grot 1985),

specifically the influence of mechanical ventilation equipment operation and

control on envelope infiltration rates.

2.2 Instrumentation

Tracer gas procedures have been used to measure air infiltration and ventilation

characteristics of buildings for at least thirty years. These procedures

require several pieces of equipment and other associated materials including

tracer gases, a tracer gas concentration measuring device, air sampling and

tracer gas injection equipment, and some means of controlling the measurement

procedure and recording the data. Figure 1 is a schematic of a general tracer

gas measurement system showing the major components of such a system.

A variety of tracer gases have been used in the past, and the desirable

properties of tracer gases have been considered by Honma (1975) and by Hunt

(1980) . These include detectability, nonreactivity, nontoxicity and a relatively low concentration in ambient air. Several different detection

devices can be used to measure tracer gas concentration, and the particular

device that is used depends, of course, on the specific tracer gas. Several

techniques used to measure tracer gas concentrations are listed in table 1,

along with some of the gases appropriate to each method. In selecting a tracer

gas detector and designing one's measurement procedure, several characteristics

of the detector need to be considered including the range of detectable

concentrations, the device's analysis time, the required airflow rate through

the device, and the need for expendables such as compressed gas or other

chemicals

Tracer gas measurements require an air sampling system to bring air from

the various air sampling points within the building to the tracer gas detector.

The complexity of this system can cover a wide range from that appropriate to a fully automated tracer gas measurement system to that used in a one-time, manual

measurement. In its most complex form, an air sampling system consists of a series of appropriately sized pumps, air sample tubes running from the pumps to

the air sample locations, and a manifold/valve set-up to enable selection from

among the pump outlets for input to the tracer gas detector. In an automated

system, this selection is accomplished by a control system. In a manual

procedure, air sampling can involve containers, such as air sample bags or

bottles, and a small pump to fill these containers. A series of these

containers are filled during the test and hand-carried to the tracer gas

detector for analysis. A tracer gas injection system is also required and,

similarly, can cover a range of complexity. The injection system includes a supply of tracer gas and a means of injecting the gas at the desired locations

within the building. An automated measurement system involves electronically

controlled valves for turning the injection on and off and injection tubes

running from the tracer gas injection valves to the injection sites. In a manual operation, one can fill syringes from a compressed gas cylinder, carry

these syringes into the building, and empty them at the injection sites.

An automated tracer gas experiment must include some means of controlling

the tracer gas injection and air sampling, as well as recording the measured

tracer gas concentrations. Several micro-computer based data acquistion and

control systems have been designed and used in tracer gas experiments (Condon et

al 1980, Grot et al 1980, Hartmann and Muhlebach 1980, Kumar et al 1979)

Because air exchange is so heavily influenced by weather conditions, it is

3
important to measure and record environmental conditions during the

measurements, including wind speed and direction and indoor and outdoor air

temperatures. In an automated system, devices for measuring environmental

conditions can be installed at the building site and their output recorded by

the data acquisition and control system. In manual measurements, hand held

devices can be used as well as reports from nearby weather stations. Other

building conditions, such as fan operation and damper position, are also

important to air exchange and need to be examined and recorded.

3 o FUNDAMENTALS OF TRACER GAS MEASUREMENTS

There are three basic tracer gas techniques for measuring air exchange rates,

decay, constant concentration, and constant injection. To understand these

methods, one employs a mass balance of the tracer gas released within the

building. Assuming that the tracer gas mixes thoroughly and instantaneously

within the structure, this mass balance equation is

VC(t) = F (t ) - q C(t) (1)

where V is the building volume, C ( t ) is the tracer gas concentration at time t,

C is the time derivative of concentration, q is the airflow rate out of the

building, and F is the tracer gas flow rate. The outdoor tracer gas

concentration is assumed to equal zero.

The air exchange rate I is given by

I = q/V ( 2 )

where I is in air changes per hour

-1

(h

A

) The solution to equation (1) is given

by

C(t) = C0

-It

e F(u) / V ) du (3)

where C0 is the concentration at time t=0. For those cases in which F is

constant, the solution integrates further to

C(t) - C0 e'

:t

+ (F/q) (1 - e _It

) . (4)

3.1 Decay

The simplest tracer gas technique is the tracer gas decay method, which has been

discussed previously by Lagus (1980), and is the subject of a standardized

measurement procedure (ASTM 1983) . In the decay method, one injects a small

amount of a tracer gas into the structure and allows the tracer to mix with the

interior air. After the mixing period one monitors the rate of decay of the

tracer gas concentration within the building. During the decay there is no

source of tracer gas, hence F(t)=0 and the solution to Eq (1) is

C(t) = C0 e~

Ito

(5)

Solving Eq (5) for I yields

I = d/t) In [ (C0 /C (t) ] ( 6 )
where C0 is the initial tracer gas concentration at t=0, when the decay begins.

To minimize errors in the determination of I due to errors in the

measurement of C(t) and C0 , one can measure C periodically during the decay and

fit the data to an equation of the form

lnC(t) = lnC0

- It. (7)

The concentration measuring equipment can be located within the structure, or

building air samples may be collected in suitable containers and analyzed offsite (Grot 1980; Harrje et al 1982)

As with all tracer gas techniques, there are advantages and disadvantages

associated with the tracer gas decay method. The advantages include the fact

that the equation used to determine the air exchange rate is an exact solution

to the tracer gas mass balance equation. Also, because one takes logarithms of

concentration, only relative concentrations are needed, which may simplify the

calibration of one's concentration measuring device. Finally, one need not

measure the tracer gas injection rate, although it must be controlled such that

the tracer gas concentrations are within the range of one's concentration

measuring device.

The most serious problem with the tracer gas decay technique is imperfect

mixing of the tracer gas with the interior air, both at initial injection and

during the decay. Equations (1) and (5) employ the assumption that the tracer

gas concentration within the building is uniform and can be characterized by a single value. If the tracer gas is not well mixed, either within zones (e.g.

floors or rooms) or between zones of a building, this assumption is not

appropriate and the use of Eq (6) or (7) to determine I will lead to errors. It

is extremely difficult to estimate the magnitude of the errors due to poor

mixing, and there has only been very limited analysis of this problem (Hunt

1980) . The only way to determine if there is or there is not good mixing is to

monitor the tracer gas concentration at several locations within the building.

It has been suggested (Dick 1949) that if one obtains different tracer gas decay

rates in different rooms, due to poor mixing, one may obtain an estimate of the

whole building air exchange rate using a volume-weighted average of the

individual room decay rates.

3.2 Constant Concentration

In the constant concentration technique one injects appropriate quantities of

tracer gas in order to maintain a constant concentration within the building.

If the tracer gas concentration is truly constant, Eq (1) reduces to

q (t ) = F(t)/C (8)

This technique is less well developed than the tracer gas decay technique, but

examples of its application do exist (Collet 1981; Bohac et al 1986)

An advantage of the constant concentration technique is that it can avoid

some measurement problems that occur in the tracer gas decay procedure due to

nonuniform mixing of the initial tracer gas injection. Because the tracer gas

injection is continuous, there is no initial mixing period to be concerned with

once the test is underway. There are, however, other serious mixing concerns as discussed below. Another advantage of the constant concentration technique is

5
that one can separately control the tracer concentration in each zone of a building by separately injecting tracer into each zone, and thereby determine

the amount of outdoor air flowing into each zone (Honma 1975; Sinden 1978)

Tracer gas procedures appropriate to multi-chamber analysis are discussed

further below.

This procedure has the disadvantages of requiring absolute concentration

measurement and precise measurement of the tracer gas injection rates. Also,

imperfect mixing of the tracer gas and the interior air causes a delay in the

response of the tracer gas concentration to changes in the tracer gas injection

rate. This delay in the concentration response, due to imperfect mixing, makes

it essentially impossible to keep the concentration constant and therefore Eq

(8) is actually an approximation. The errors induced by these mixing problems

have not been well examined.

3.3 Constant Injection

The third technique for measuring air infiltration and ventilation is referred

to as the constant injection or constant flow technique. In this procedure, one

injects tracer at a constant rate and, setting C0 = 0, Eq (4) reduces to

C(t) = (F/q) (1 - e_It

) (9)

After a sufficient period of time, the transient term reduces to zero, the

concentration attains equilibrium, and one obtains the simple constant flow

equation

q

= F/C. (10)

This relation is valid only for cases in which the air exchange rate is

constant, thus this technique is appropriate only for systems at or near

equilibrium. This technique is particularly useful in areas with mechanical

ventilation or in locations with high air exchange rates. The constant flow

technique avoids the concentration control problems of the constant

concentration procedure. The constant injection procedure requires the

measurement of absolute concentrations and the tracer gas injection rate.

3.4 Multi-Chamber Techniques

The previous theory applies to structures that are modelled as a single zone,

i.e., the tracer gas concentration within the building can be characterized by a single value. In many cases this assumption is inappropriate and a multichamber approach must be used (Sinden 1978) . A variety of multi-chamber

measurement techniques exist, involving the decay, constant concentration, and

constant injection techniques, and the use of one or several tracer gases.

The equations describing the multi-chamber case are similar to Eq (1),

except for the addition of airflow between chambers. A mass balance for each

chamber yields,

ViCi = Qio^i + X< cIijCj “^^ji^i

j 3

where Vj_ is the volume of chamber i, Cj_ is the tracer gas concentration in that

same chamber, Cj. is its derivative with respect to time, qj_j is the airflow rate

6
from chamber j to the chamber i, qj_ 0 is the airflow rate from chamber i to

outdoors, and q-j_j_ = 0. Note that as in the case of Eq (1), we are assuming that

the outdoor tracer gas concentration is zero. For each tracer gas that is used,

there will be one set of these equations, one equation for each zone. In

designing and applying a multi-chamber experiment one must conceptually divide

the building into separate zones, and it is not always obvious how to make these

divisions

3.4.1 Decay

The multi-chamber tracer gas decay method involves releasing the tracer gas as a pulse and then monitoring the concentration response in all of the chambers

(Sinden 1978) . The decay technique can be applied with a single tracer or

several different tracer gases, and can be used to obtain the values of all the

interzonal airflows q-j_j and qj_ 0

. In the single tracer, multi-chamber decay

approach, one injects tracer into a single zone and monitors the concentration

response in all the zones. The measured concentrations are used in conjunction

with Eq (11) to solve for the airflows. Depending on the characteristics of the

airflow between the various zones, one such test may be sufficient to determine

all of the airflows of interest. In general, however, several such tests will

be required with the tracer being injected into a different zone each time. For

this series of measurements to provide meaningful results, the airflows can not

change significantly over the test period, generally an unreasonable

expectation. To minimize the requirement for constant conditions, one can

employ a multi-tracer decay technique in which a different tracer gas is

injected into each zone and the concentration response of all the gases in all

the zones is monitored. Both the single- and multi-tracer approaches make the

assumption that each individual zone is prefectly mixed and that the test begins

with a uniform tracer gas concentration in each zone. These assumptions are

approximations at best and the errors associated with the actual conditions of

imperfect mixing within zones and nonuniform initial conditions are not well

known

Several measurement systems employing the multi-tracer decay method have

been developed, including those of Irwin et al (1984) and Prior et al (1983)

Both systems employ gas chromatograph electron capture detectors, to measure

refrigerants in the first system and perfluorocarbon tracers in the latter.

I' Anson et al (1982) also employs a decay system involving refrigerant tracers.

In figure 2, we show representative data from their paper in which two tracers

are used to study the airflow between the upper and lower levels of a building.

The refrigerant C2 CI2 F 4 is injected downstairs and CCI2 F 2 upstairs. The

movement of tracer between the zones and the subsequent decay is evident in the

data. Analysis of these data yields the airflow rates between the two zones and

from each zone to the outdoors.

3.4.2 Constant Concentration

The constant concentration method can also be applied to study buildings in

which a single zone approximation is inappropriate. As mentioned earlier one

can separately control the tracer gas concentration in each zone of a building

such that all zones are at the same target concentration C' . This technique

involves sampling the tracer gas concentration from, and injecting tracer into,

each zone. The procedure can be used to determine the outdoor airflow rates

into each zone qj_ 0 , but not the interzonal airflow rates qij. Using Eq (11) and

setting Ci(t) = 0 and Cj_(t) = C' , one obtains,

7
F i ~ 3io .

( 12 )

From this equation one obtains values of q-[ 0 for each zone. The total air

exchange rate of the building is determined by adding together all of the qi 0 's.

Of course this procedure involves the same approximations as the single zone

constant concentration procedure, i.e., that the concentration is maintained

constant despite lags between the tracer gas injection and the concentration

response due to imperfect mixing within a zone. Again, the errors associated

with this mixing problem are not well known. As mentioned earlier, there have

been several systems developed that employ the constant concentration procedure

for multi-chamber studies (Collet et al 1981; Bohac et al 1986)

3.4.3 Constant Injection

The constant injection procedure can also be used to study multi-chamber air

exchange by injecting a different tracer into each zone of a building at a constant rate and measuring the equilibrium concentrations of each tracer in

each zone. By employing the appropriate form of Eq (11), one can solve for all

of the interzonal airflow rates. This procedure is most appropriate for

situations in which these airflows are fairly constant since one is assuming

steady-state conditions in solving for the airflow rates. The multi-chamber,

constant injection procedure is less well developed than some of the other

multi-chamber techniques but some examples of its application exist.

Figure 3 shows an example of a constant injection measurement of

ventilation and interzonal airflows in two adjacent, negative pressure

laboratory rooms (Lagus 1984) . The two laboratories are under negative pressure

(as shown in the figure) , and a different tracer gas is injected at a constant

rate into each laboratory. The equilibrium tracer concentrations in each zone

are shown in the figure. The measured airflow rates agree well with the

ventilation rates measured by tracer gas decay. The presence of the refrigerant

CBrFg into the lower laboratory serves as evidence of undesired airflow between

the laboratories.

A long-term averaging, constant injection technique has also been employed

using perfluorocarbon tracers and passive samplers (Dietz and Cote 1982; Dietz

et al 1984a) . Long-term averaging, passive sampling techniques are discussed in

more detail below. In these measurements, passive tracer sources are used to

release a different tracer into each zone at a constant rate. The average

concentration over the measurement period (which can range from hours to months)

is determined in each zone with a passive sampler that collects tracer gas over

the entire sampling period. From the injection rates and the average

concentrations, one determines the airflows of interest. The determination of

the airflow rates from Eq (11) involves an assumption that the concentration is

at equilibrium, which can lead to measurement errors of unknown magnitude.

Also, in solving these equations one uses average values of the terms qijC£.

Since one measures only the average of Cj_, in order to solve for qj_j one is

making the assumption that the average of qijCj_ equals the average of q^j

multiplied by the average of Cj_. This is not a mathematicailly valid assumption

except in very special cases, and therefore the calculations of qj_j are subject

to errors of unknown magnituide. Figure 4 is an example of measured airflows

for a two zone case using this technique (Dietz et al 1984)

8
4. AIR EXCHANGE RATE MEASUREMENT APPLICATIONS

In this section we discuss two applications of air exchange rate measurement.

The first is tracer gas measurements of air exchange in large buildings, while

the second application concerns low cost procedures for studying air exchange in

large numbers of buildings.

4.1 Tracer Gas Measurements in Large Buildings

This section reviews techniques which employ a single tracer gas to measure

infiltration and ventilation in large buildings, arbitrarily defined as structures with floor areas greater than about 5000 ft (500 in ) . These

buildings may have mechanical ventilation systems or they may be naturally

ventilated. The types of large buildings that have been studied include office

buildings, industrial buildings such as warehouses and airplane hangers, stores,

shopping centers, and institutional facilities such as schools and hospitals.

Most of the measurements in large buildings have employed the tracer gas decay

method and this particular application is discussed below, along with other

procedures applied to large buildings.

The basic approach of tracer gas decay measurements in large buildings is

the same as that described earlier. The tracer gas is released into the

building and allowed to mix with the interior air. The decay in the tracer gas

concentration is monitored over time and the air exchange rate is determined

from the decay rate. Several characteristics of large buildings influence the

manner in which the decay method is applied in these structures. First, because

of the large building volumes, the quantity of tracer gas required foj a test

and its cost become important. The expense depends on the cost per m° of tracer

gas, the building volume, and the magnitude of measurable tracer gas

concentrations. Table 2 shows the range in the maximum building volume that can

be measured for one dollar's worth of tracer gas (leased on approximate 1986

prices) . These volumes range from 4 g00 |t° (jjlOO^iTi ) for helium in the 300 parts

per million (ppm) range, to about 10° ft

J (10° in ) for carbon dioxide and

nitrous oxide. The ability to measure SFg, CBrFg and PDCH in the parts per

q trillion (ppt) range yields measurable volumes of 10° to 10

1 ft' (10 to 10

m ) per dollar's worth of tracer gas. From this table it is apparent that

tracer gases such as SFg, refrigerants, and perfluorocarbons analyzed at ppt, or even parts per billion, levels are most appropriate for large buildings.

However, measurements have been performed in large buildings using infrared

adsorption (Potter et al 1983; Zuercher and Feustel 1983) and flame ionization

gas chromatography (Prior et al 1983)

The mixing of the tracer gas injection in these large building volumes is

an important issue. Mixing by diffusion alone is a slow process; however, even

in naturally ventilated buildings there are significant convective mixing

mechanisms. In mechanically ventilated buildings the air distribution system

can be used to mix the tracer gas, but mixing can still require fifteen minutes

to one hour. In naturally ventilated buildings, tracer gas mixing is a more

difficult problem. If the building interior is open with few internal

partitions, then the gas will mix with the air, although it can take a long

time. Fans can be used to mix the tracer, but they will alter the interior air

movement, which may or may not affect the measurement results. The fans may be

used to obtain an initially uniform concentration, and then be turned off during

the decay. The only way to determine if good mixing has been achieved is to

measure the tracer gas concentration at several locations within the building.

9
The attainment of a uniform concentration is also assisted by injecting the gas

at several locations.

In mechanically ventilated buildings fan operation and damper position are

important issues. Most of these buildings have automatic control systems that

turn fans on and off, modulate airflow rates and adjust exhaust, recirculation

and supply damper positions. Thus, to conduct useful tracer gas measurements

and interpret the results, one must be aware of the fan operating schedules and

ventilation control strategies. One may make long term measurements in a building or conduct only a small number, of tests. Long term studies are useful

for examining the dependence of air leakage on weather and the performance of

the ventilation control system. When making long term measurements, one's

equipment, sensors and air sampling lines must be unobtrusive with regard to the

building occupants and the automatic operation of the building equipment.

In this section we present an example of tracer gas decay measurements in

mechanically ventilated office buildings. We also present other examples of air

exchange measurements in large buildings. The example of measurements in a mechanically ventilated office building involves the use of automated equipment

to conduct long term measurements (Grot et al 1980; Grot 1982) . The tracer gas

equipment is generally located in the building's mechanical equipment room where

the main air handlers are located. Figure 5 is a schematic of such a building

with the mechanical equipment room located in a penthouse. Most office

buildings have separate air handlers, for spaces such as lobbies, which may be

located some distance from the main mechanical equipment room. Such an air

handler is shown in the figure. In order to obtain a uniform tracer gas

concentration throughout the building, one must inject tracer into all the

supply fans. This requires the installation of injection tubing that runs from

the measurement equipment to each supply fan.

As seen in figure 5, the tracer gas concentration is measured at several

locations within the building in order to verify that the tracer is indeed well

mixed. For example, one may sample in the building's main return duct, on

individual floors and in the return ducts of any other air handlers. One must

install air sample tubing connecting the measurement equipment to each of the

sampling locations, and use pumps to bring the air to the measurement equipment.

The individual floor sampling locations can be in the return air plenums

immediately upstream of the return air shafts. In order for these sampling

locations to give meaningful concentration measurements, the air handlers must

be operating. In some buildings it may be possible to run the air sampling

tubes into the occupied space in which case the air handlers need not be running

during the tracer gas decay. Since building geometry and air handler

arrangements vary greatly among buildings, tracer injection and air sampling

locations are different for each building.

In office buildings, there are two types of measurements that are of

interest, referred to here as ventilation and infiltration. Ventilation rates

refer to measurements made when the building HVAC system is operating normally

under occupied conditions. In this case the various spill, recirculation and

intake dampers open or close as the control system dictates in response to

indoor and outdoor temperature and humidity, and time of day. Infiltration

rates refer to the measurements obtained when the spill and intake dampers are

closed (including any minimum outdoor air dampers) . These test results give an indication of the airtightness of the building envelope. The operation of the

fans during these measurements may be necessary for mixing, and their operation

10
may affect the test results.

Short term measurements can also be made in such a building using injection

and sampling by hand. The tracer is injected into the supply fans and air is

collected in containers at locations throughout the building. The tracer gas

concentrations in these containers are determined at some later time. This

"manual" technique has a shorter set-up time than the automated procedure

described above, but each infiltration measurement must be made by hand.

Long term automated infiltration and ventilation measurements have been

made in several mechanically ventilated office buildings (Grot 1982; Grot and

Persily 1983) . In these tracer gas decay tests, hourly average infiltration and

ventilation rates were measured for hundreds of hours in each building and the

results were related to indoor-outdoor temperature difference and wind speed.

The infiltration rates of the different buildings were found to exhibit varying

degrees of weather dependence, and a range in the leakage of the building

envelopes was observed. The ventilation rates reflected the strategies used to

control the outdoor air intake rates and also revealed that uncontrolled

envelope infiltration is generally an important portion of these building's

total air exchange rates, even with significant amounts of outdoor air intake.

The tracer gas decay technique has been applied in an eleven-story office

building, employing hand injection of the tracer gas and sampling of the

interior air with polyethylene bottles (Harrje et al 1982) . The tracer gas

concentration in the bottles was later determined at a central location. The

air was sampled on four different floors and in the main return duct of the air

handler. Only a small number of measurements were made in the building, but the

results demonstrated the utility of this "air sample container" technique in

large buildings.

A small number of industrial buildings have also been studied with the

tracer gas decay technique. These buildings are often characterized by large

open volumes, such as warehouses, where tracer gas mixing can take a long time

or require the use of fans. In a study of three large naturally ventilated

single-zone structures in England, a fan was used to mix the tracer gas (Waters

and Simons 1984) . A uniform tracer gas concentration throughout the space was

generally obtained within about twenty or thirty minutes after injection,

although some spatial variation did remain. A series of tracer gas decay

measurements in airplane hangers has also been conducted (Ashley and Lagus

1984).

There are other examples of the use of tracer gas in large buildings

(Potter et al 1983; Zuercher and Feustel 1983)

In one particular application,

a constant injection scheme with gas bag sampling of the interior air was

applied to a laboratory building (Freeman et al 1983)

In these measurements,

tracer gas was injected at a constant rate at twelve locations and the

equilibrium tracer gas concentration was determined from air sample bags filled

at twelve other locations. This "bag sample equilibrium" method was compared to

measurements based on tracer gas decay and the agreement was good when mixing

was thorough.

4.2 Low Cost Measurement Procedures

The tracer gas measurement techniques described above involve bringing

sophisticated equipment for measuring tracer gas concentration to the building

11
under investigation. Long-term measurements require one to leave this equipment

in the building for extended periods of time. These studies provide immediate

results and much detail, but they require expensive equipment and skilled

professionals for installation. Several low-cost tracer gas measurement

techniques have been developed that are useful for surveying the air leakage

characteristics of large numbers of buildings. In addition, the field

procedures can be handled by less well trained people. These techniques provide

less information than long term studies, but for some applications the data

collected are adequate. The low-cost procedures generally involve some device

for on-site air sampling, with the tracer gas concentration of the air sample

being subsequent ially determined at a central location. Thus, a single tracer

gas measuring device can be used to make air exchange measurements in many

buildings. These techniques have generally been applied to homes, but they can

also be used in large buildings.

4.2.1 Decay Methods

Low-cost tracer gas decay techniques are based on the same principles as other

decay methods, and have been described previously (Grot 1980; Harrje et al 1982;

Tamura and Evans 1983) . They involve injection of the tracer gas into the

building at an appropriate location or locations to facilitate achieving a uniform tracer gas concentration throughout the structure. After sufficient

time is allowed for this mixing, air sample containers are filled at roughly

equal time intervals. The containers are then sent to a central laboratory

where the tracer gas concentration of each sample is measured. From the decay

rate of the tracer gas concentration, one determines the average air exchange

rate during the measurement period. The tracer gas injection and the air

sampling requires careful attention by the field personnel, but the procedures

themselves are not difficult. A variety of specific containers have been used

including 30.5 in" (500 ml) flexible polyethylene bottles (Harrje et al 1982),,

600 in'

3 (10 L) air sample bags (Grot 1980; Persily and Grot 1984), and 1.2 in

(20 mL) evacuated blood sample tubes (Tamura and Evans 1983) . These low-cost

decay methods have generally been applied to homes, but larger buildings have

also been tested. A study of an eleven-story office building using the "bottle

technique" has been mentioned earlier (Harrje et al 1982)

The low-cost of these techniques has enabled several surveys of the air

leakage characteristics of large numbers of homes. In these surveys a small

number of tracer gas decay tests are conducted in each home under a range of

outdoor weather conditions. One study measured air infiltration rates in over

two-hundred homes in low-income areas of the U.S. (Grot and Clark 1979)

Another survey examined more than fifty pasive solar homes (Persily and Grot

1984) . The low-cost of these measurement techniques enables the testing of

large numbers of homes spread over a large geographical area with a single

tracer gas concentration measuring device.

4.2.2 Constant Injection

Low-cost air infiltration measurement techniques based on the constant injection

of a tracer gas into a building have also been developed. In these procedures,

a tracer gas is injected at a constant rate and the interior air is continuously

sampled to determine the average tracer gas concentration in the building over

the sampling period. The average concentration and the injection rate are then

related to the average infiltration rate over this same time period. In the

low-cost versions of this technique, relatively simple devices are used for

12
tracer gas injection and air sampling, while the tracer gas concentration of the

air sample is determined with equipment at a central facility. All long-term

average, constant injection measurement techniques have the problem that they

actually determine the average of the inverse air exchange rate and not the

average air exchange rate. The difference between the average of the inverse

and the actual average depends on the distribution of air exchange rates during

the measurement period. The magnitude of these differences are just beginning

to be studied, but can be on the order of 20% for one-month averaging periods

(Sherman and Wilson 1987) . Longer averaging periods will generally lead to

larger errors.

One version of the constant injection technique is the average infiltration

monitor (AIM) developed at the Lawrence Berkeley Laboratory (Harrje et al 1981)

This system employs suitcase-sized injectors and samplers to enable unattended

measurement of long-term average infiltration rates. The injector contains a pump which slowly releases the tracer gas at a constant rate into the building.

The sampler slowly fills a sample bag with a pump to obtain the average tracer

gas concentration during the measurement period. The concentration in the

sample bag is later determined at a central location. Another technique employs

small passive injectors and samplers in a similar procedure. The Brookhaven

National Laboratory Air Infiltration Monitoring System (BNL/AIMS) employs a perfluorocarbon tracer gas (PFT) which diffuses out at a known constant rate

from a fluoroelastomer plug impregnated with the tracer (Dietz and Cote 1982;

Dietz et al 1984 and 1986) . The passive sampler is a small capillary adsorption

tube which collects the PFT from the building interior during the test. The

sampler is later analyzed to determine the average tracer gas concentration in

the building, and hence the average (inverse) air exchange rate.

The application of these constant injection, long-term averaging techniques

to large buildings requires that several tracer sources be used and that they

are well-distributed throughout the building. Several samplers are also

required. In mechanically ventilated office buildings the intermittent

operation of fans, changing damper positions, and variations in outdoor air

intake rates must be considered when using these techniques.

5. OTHER TRACER GAS APPLICATIONS

In this section tracer gas measurement procedures are discussed that

characterize aspects of building air exchange and air movement other than

building air exchange rates. These procedures include recently developed

techniques to evaluate the performance of air distribution systems and to

measure airflow rates associated with a ventilation system. In addition,

procedures exist to pressure test the airtightness of large building envelopes

and to evaluate building air movement in a qualitative manner.

5.1 Air Distribution Evaluation

Building ventilation systems are designed to satisfy minimum outdoor air intake

levels (ASHRAE 1981) in order to provide a safe and comfortable environment for

the building occupants. Even if the ventilation system is bringing in a sufficient amount of outdoor air, the air may not be well distributed within the

interior space. In this case, not all of the air will be effective in

maintaining acceptable indoor air quality within the space. The concept of

ventilation effectiveness has been developed in order to quantify the air

distribution system's ability to provide freshly conditioned air to the

13
occupants and to remove internally generated pollutants. Many different

definitions of ventilation effectiveness exist (Persily 1985) and can be divided

into those that quantify the distribution of supply air and those that quantify

pollutant removal effectiveness. A great deal of valuable ventilation

effectiveness research has been conducted involving experiments in test rooms

(Sandberg 1981 and 1983; Sandberg and Sjoberg 1983; Sandberg et al. 1982;

Malmstrom and Ahlgren 1982; Skaret and Mathisen 1982 and 1985) . These

experiments have employed test rooms with reconfigurable intake and exhaust

openings, and controllable supply air temperatures and ventilation rates, to

« study the dependence of ventilation effectiveness on these variables. The

procedures employed in these laboratory measurements can also be used to measure

ventilation effectiveness in actual buildings.

Several definitions and theoretical frameworks have been used to discuss

ventilation effectiveness, but there are essentially two basic approaches. The

first type of ventilation effectiveness measures can be referred to as "concentration efficiencies" and are based on relations between gas

concentrations in the supply air, the exhaust air, and the air at various

locations in the space. Efficiencies based on age distributions and residence

times, using approaches of chemical reactor engineering, constitute the second

approach to ventilation effectiveness. For a detailed review of the various

definitions of ventilation effectiveness and the associated measurement

techniques see Persily (1985), as well as the original articles on the material

(Malmstrom and Ahlgren 1982; Sandberg 1981 and 1983; Sandberg and Sjoberg 1983;

Sklret and Mathisen 1982)

The techniques for quantifying air distribution effectiveness in actual

buildings are still being studied. No standard procedures exist yet for typical

North American buildings, but the procedures based on age distribution theory

appear to have potential for being useful and are therefore discussed below with

reference to mechanically ventilated office buildings. Ventilation

effectiveness definitions based on age distributions involve average and local

ages of the interior air, and tracer gases can be used to determine these ages.

The measured values of these ages are compared to each other or to their values

for idealized reference cases (i.e., perfect mixing or pure plug flow through

the space) to determine various ventilation effectivenesses.

In age distribution theory applied to ventilation effectiveness, one

considers three populations of air parcels for a given ventilated space of

volume V and volumetric air exchange rate q: the air at some specific location

within the space, all of the air contained in the space, and the air leaving the

space. One defines the average age of the air at a specific location by

considering all the air molecules at that location and determining the average

amount of time that has elapsed since these air molecules entered the space or

building. The average age of the air at a point i is denoted as tj_. One

defines the average age of all the air in a given space, denoted by [t], as the

average value of tj_ for all locations in the space under consideration. One may

also consider the age of the air leaving the room, denoted by t n , which is equal

to the inverse of the air exchange rate, i.e. (V/q) , regardless of the airflow

patterns within the space (Sandberg and Sjoberg 1983)

It is revealing to compare the values of the local air age tj_, the average

age of the air in the space [t] and the age of the air leaving the space t n , for

three reference cases. First, if the air within the space under consideration

is perfectly mixed, then all the local ages have the same value throughout the

14
space, equal to [t]=t n

. In addition, contaminant concentrations are identical

throughout the space. For the second case, pure piston flow from the supply to

the exhaust, the value of t-j_ depends on the particular location within the

space, with its value increasing from 0 to t n as i moves from the supply to the

exhaust. The average age of the air in the space [t] is equal to t n

/2. For

pure piston flow, the effectiveness of pollutant removal and outdoor air

distribution are maximized, but thermal comfort may be compromised due to

excessive air velocitites or low air temperatures near the supply vents. In

addition, occupants located immediately downstream of a pollutant source will be

subjected to higher contaminant concentrations than in the case of perfect

mixing. The third case is often referred to as "short circuiting," in which a portion of the supply air flows directly into the exhaust vent without mixing

with the rest of the space air. In this situation, the value of ti again

depends on the location of i. In stagnant zones that are bypassed by the supply

air, tj_ is greater than [t] and t n

. In the regions through which the shortcircuiting flow passes, ti is less than [t] and t n

. The average age of the air

within the space [t] is greater than t n

. Such short-circuiting flow has serious

negative implications for indoor air quality because the ventilation air is only

partially effective in providing outdoor air to the occupants and removing

internally generated pollutants. Depending on the degree of short-circuiting,

the concentrations of internally generated contaminants in the stagnant zones

can be much higher than for the cases of perfect mixing and piston flow.

Comparisons of tj_, [t], and t n serve as the basis for definitions of

ventilation effectiveness. Several such definitions exist, but only two are

presented here, the mean air exchange effectiveness n and the local air exchange

effectiveness ej_. The mean air exchange effectiveness quantifies the overall

air distribution pattern for a space and is given by

n = t n / [t]

(13)

n achieves its maximum value under conditions of pure piston flow for which n=2.

If there is perfect mixing, then n=l, and if there is short circuiting of the

supply to the exhaust, then n is less than 1. The local air exchange

effectiveness quantifies local conditions and is given by

ei = [t]/ti. (14)

ej_ can range from zero to infinity, and in the case of perfect mixing equals 1.0

throughout the space. For pure piston flow, the value of ej_ depends on the

specific location within the space being considered. Near the supply, tj_ is

close to zero and ej_ is much greater than one. In the middle of the room, tj_ =

t n/2 and ej_ = 1.0. And as one approaches the exhaust, tj_ approaches t n , and e^

approaches 0.5. In the case of short circuiting, for a location within one of

the so-called stagnant zones, tj_ > [t] and ej_ < 1, a generally undesirable

situation.

One may employ a tracer gas to measure these ages using several techniques

that differ primarily in the tracer gas injection location and duration

(Sandberg 1983) . One of the measuring techniques involves injecting tracer at a constant rate into the supply airstream and monitoring the build-up in tracer

gas concentration in the exhaust vent and at various locations within the space

until equilibrium is attained. In another technique, one begins with a uniform

tracer gas concentration throughout the space and monitors the decay in

concentration in the exhaust and within the space. The measuring procedures

15
have been used successfully in one- and two-room laboratory test facilities

[Sandberg 1983; Sandberg and Sjoberg 1983; Sandberg et al. 1982; Skaret and

Mathisen 1982, 1985], but the application of these procedures in actual

buildings is much more complex. There are several features of real buildings,

particularly large, mechanically ventilated office buildings, that complicate

the application of age distribution measuring techniques (Persily 1987) . The

theory of age distributions consider the ventilated space as a single zone with

a small number of well-defined supply and exhaust (return) vents. The space is

also assumed to have no airflow into or out of the space except through the

mechanical ventilation system, a concern that was discussed earlier. In

reality, the ventilation of an office space is much more complicated that this

theory assumes. Office spaces communicate freely with adjoining spaces and with

the outdoors through leaks in the building envelope. There are generally many

supply vents serving a space and the number of return air vents is extremely

variable among rooms, with some rooms having no return vents at all. This

situation presents several problems for applying age distribution measuring

techniques. These and other factors complicate the application of age

distribution measuring techniques in modern North American office buildings and

the interpretation of the test results, but research is currently in progress to

develop measurement protocols that are appropriate to these field situations.

5.2 Ventilation System Airflow Rates

As mentioned earlier, tracer gas measurements of air exchange rates in

mechanically ventilated buildings determine the sum of the uncontrolled envelope

leakage and the intentional outdoor air intake through the air handling system.

Tracer gas procedures exist to measure these quantities simulataneously, as well

as other airflow rates of interest in understanding the air exchange

characteristics of mechanically ventilated buildings (Persily and Norford 1987)

These quantities can be measured using two different techniques, a steady-state,

constant injection tracer gas procedure and a procedure combining tracer gas

decay and airflow measuring stations. Both techniques are described below.

The constant injection procedure is illustrated in the schematic in Figure

6. In this schematic Qqa is the rate of intentional outdoor air intake through

the air handling system, Qgg is the recirculation airflow rate, and Qgy is their

sum, the supply airflow rate. Qjn is the rate of uncontrolled air leakage into

the building through the building envelope, and Qgx is the sum of the

uncontrolled air leakage out through the building envelope and the airflow out

of intentional openings such as bathroom exhausts. Qgg is the return airflow

rate and Qgp is the airflow rate through the spill dampers.

In the constant injection measurement procedure, one injects tracer gas at

a constant rate F into the supply airstream as indicated in Figure 6. In this

procedure, the value of the outdoor air tracer gas concentration must be

constant, and for many tracer gases it will equal zero. The equations below are

developed under the assumption that the outdoor concentration is zero, but an

alternative set of equations can be easily developed for an outdoor

concentration that is nonzero but constant. During the test, one measures the

return air tracer gas concentration Cr, the supply air concentration Cg, and the

mixed-air concentration Cjj. must be measured some distance downstream of the

location where the recirculation air meets the new outdoor air in order to

provide the two airstreams an opportunity to mix. Similarly, Cg must be

measured downstream of the tracer gas injection location, after the tracer mixes

with the supply airstream. The tracer gas mixing can be enhanced by releasing

16
the tracer at several locations across a supply duct cross-section. In

employing this measurement procedure, one must wait for the tracer gas

concentrations to reach steady-state conditions, which can take several hours

depending on the net ventilation rate of the building, the amount of

recirculation of the return air, and the extent of air mixing within the

building.

Based on the tracer gas injection rate F (in units of volumetric flow

rate) , and the equilibrium values of the supply, return and mixed-air tracer

concentrations, one can determine the values of Qqa/ QiN' Qrc anci Qsu as follows. The value of Qsu is determined from the increase in concentration

across the tracer gas injection in the supply airstream location according to

C S

“ CM

= F /QSU ( 15 )

Based on a mass balance of tracer gas at the point where the recirculation air

mixes with the outdoor air, one obtains Qrq from the following equation

Qrccr

+ Q0Acout

= Qsucm ( 16 )

Assuming the outdoor concentration of tracer gas Cout is zero, Eq 16 can be

rearranged to yield

Qrc = ( cm/cr)Qsu < 17 )

The outdoor air intake rate Qqa can be determined from the values of Qsu and Qrq

and a mass balance of airflows where the outdoor and recirculation air mix,

i.e.,

Qoa = Qsu “ Qrc- dS)

The rate of envelope infiltration Qjjg can be determined by tracer gas and

airflow mass balances on the total building system

(Qqa + Qin) c out + F = (Qex + Qsp) cr < 19 )

and

Qex + Qsp

= Qqa + Qin- (2°)

Again assuming Couu equals zero, Eqs 19 and 20 can be combined to yield

Qqa + Qin = Qae = f / cR‘ ( 21 )

Qae as the total air exchange rate of the building. Based on the value of Qqa

obtained from Eq 18, Eq 21 can be used to determine Qjjj. Each determination of

these airflows requires several hours to reach equilibrium, and the damper

positions and fan airflow rates must be essentially constant during the

measurement. When these conditions are altered, one must again wait another few

hours to determine the quantities of interest at the new airflow conditions.

Alternatively, one may use a tracer gas decay/airflow rate measuring

station procedure to determine these same quantities. In this procedure one

conducts a tracer gas decay test to determine the total air exchange rate of the

building, while simultaneously measuring the supply airflow rate with an

appropriate airflow rate measuring device. The tracer gas measurement involves

17
injecting a small amount of tracer into the building, generally into the

building supply airstream, waiting for the tracer to mix thoroughly with the

interior air, and monitoring the decay rate of the tracer gas concentration over

time. The decay rate of the tracer concentration is used to determine the

building air exchange rate = Qqa + Qin* During the tracer gas decay

measurement, an airflow rate measuring station is used to determine the supply

airflow rate. From these measurements, one can determine all of the quantities

of interest.

During the tracer gas decay, one determines the building air exchange rate

by monitoring the return air tracer concentration Cr, which decreases according

to the following equation:

CR = CRoe

_I 1

(22)

where t is time, Cr0 is the return air tracer gas concentration at t=0, and I r is the slope of InCR versus time. I r is equal to Q&r divided by the building

volume V. In this measurement procedure, one also monitors the supply air

tracer gas concentration Cg, which is given by

CS = CSoe

_I

(23)

where Cg0 is the supply air concentration at t=0 and I s is the slope of InCg

versus time. Theoretically X s equals X r ; however, measurement errors can lead

to differences between these two quantities.

Given constant air intake and recirculation rates, the ratio of Cr and Cg

will be constant and can be used to determine the percent of outdoor air intake,

i.e., Qqa/Qsu° This determination employs mass balances of tracer gas and

airflow at the point where the recirculation and outdoor air intake airflows

mix. These mass balances take the forms of Eqs 17 and 18. In this procedure

= Cg, and these two equations can be combined to yield

Qoa/QsU = 1 - (C S /CR ) (24)

At this point one employs the airflow rate measuring station to determine

QgU- Such a station can consist of multi-point arrays of pitot tubes,

thermistors or hot wire anemometers, or other airflow measuring devices. Other

airflow rates, such as the return and spill airflows, can also be measured if

this additional information is desired. Whatever airflow measuring devices are

used, they must be employed properly, i.e., factors such as lengths of upstream

ductwork must be considered. In some ventilation systems it may be very

difficult to employ an airflow measuring device in a manner yielding reliable

measurements

From the measured values of Qgg and Eq 24, one determines Qqa* From the

value of determined from the tracer gas decay, one determines Qjjj (= Q&r

- Q0a)

As in the case of the constant injection tracer gas procedure, the damperpositions and fan airflow rates must be essentially constant during the

measurement

Both the constant injection procedure and the tracer gas decay/airflow rate

measuring station procedure are somewhat limited when there are multiple air

handlers serving building zones that exchange air with one another. Only if

there is no mixing between zones can the above analysis be used in each zone to

18
separately determine the envelope infiltration rate into that zone. In the more

general case of some mixing between zones, using the tracer gas decay/airflow

rate measuring procedure, one can only determine the outdoor air intake rate

into each zone and the tracer gas decay rate for each zone. If there is

sufficient mixing between zones, then the tracer gas decay rate will be

identical for all the zones and equal to the total building air exchange rate.

Even if the mixing is not perfect, the volume-weighted average tracer gas decay

rate for all the zones can be used as an approximation of the total building air

exchange rate. Based on the measured outdoor air intake rates for all the zones

and the total building air exchange rate, one can determine the total building

envelope infiltration rate.

5.2 Pressurization Testing of Large Building

The constant injection technique has been used to measure airflow rates in the

evaluation of the airtightness of large building envelopes using pressurization

testing. In pressurization testing, a fan induces a large pressure difference

across the building envelope and the airflow rate required to induce this

pressure difference is measured. The airflow rate associated with a specific

indoor-outdoor pressure difference is a measure of the airtightness of the

building envelope. The airflow can be induced with a large fan which is brought

to the building for the test or with the building's air handling equipment.

Various means exist for measuring the airflow rate through the fan, but a constant injection tracer gas method has been used in some cases. This is a simple technique which does not require the duct lengths and flow straighteners

associated with other flow measurement techniques.

To measure the airflow rate with the constant injection tracer gas method,

tracer gas is injected into the airstream at a constant and known rate. The

tracer injection is generated using a compressed gas cylinder with a flowmeter

such as a critical orifice, a float-type rotameter or an electronic flow

controller. The tracer concentration is then measured as far downstream as possible from the injection point. Under conditions of perfect mixing of the

tracer gas and the airflow, the airflow rate can be determined from the tracer

injection rate and the measured concentration (see Eq (10)).

Several large industrial buildings have been pressure tested using a fan

that was brought to the buildings and employing the flow measurement technique

described above (Lundin 1984)Figure 7 shows a schematic of the flow

measurement equipment used in these tests. The same flow measurement procedure

was applied to seven modern office buildings in which the building supply fans

were used to pressurize the structure (Persily and Grot 1984a)

Figure 8 shows

a schematic of the test arrangement including the fan operating conditions,

damper positions, tracer gas injection location and tracer gas concentration

measurement point.

5.4 Qualtitative Evaluation Techniques

It should be noted that one does not require the full solutions to Eq (1) to

obtain useful information regarding a building's air exchange characteristics

with tracer gases. A significant amount of qualitative information can be

obtained using tracer gases in buildings. For example, the existence of the

movement of air from one location to another can be verified by releasing tracer

gas at the first location and measuring the concentration at the second. This

simple procedure can be used to examine the occurence of reentrainment of

19
exhaust air into an outdoor air intake, movement of air from one location within

a building to another, or to verify the isolation of a special-use space from

the rest of a building. All such applications require extreme care regarding

the tracer gas injection to insure that the tracer is released only at the

intended location, such that any tracer detected elsewhere is due only to

airflow and not to unintentional tracer release.

20
6. REFERENCES

ASHRAE, "Ventilation for Acceptable Indoor Air Quality," Standard 62, American

Society for Heating, Refrigerating, and Air-Conditioning Engineers, Inc.,

1981.

ASTM, "Standard Practice for Measuring Air Leakage Rates by the Tracer Dilution

Method", E 741-83, American Society for Testing and Materials, 1983.

Alexander, D.K., Etheridge, D.W., Gale, R. "Experimental Techniques for

Ventilation Research, " in Air Infiltration Instrumentation and Measuring

Techniques , proceedings of the First Air Infiltration Centre Conference,

Berkshire, UK, 1980.

Ashley, J.L., Lagus, P.L., "Air Infiltration Measurements in Large Military

Aircraft Hangers," in Measured Air Leakage of Buildings , ASTM STP 904, H.R.

Trechsel and P.L. Lagus, Eds., American Society for Testing and Materials,

Philadelphia, 1986.

Bohac, D., Harrje, D., Norford, L.K., "Constant Concentration Infiltration

Measurement Technique: An Analysis of its Accuracy and Field Measurements,”

Proceedings of the ASHRAE/DOE/BTECC Conference Thermal Performance of the

Exterior Envelopes of Buildings III , ASHRAE SP 49, 1986.

Collet, P.F., "Continuous Measurements of Air Infiltration in Occupied

Dwellings," in Building Design for Minimum Air Infiltration proceedings of

the Second Air Infiltration Centre Conference, Stockholm, 1981.

Condon, P.E., Grimsrud, D.T., Sherman, M.H., Kammerud, R.C., "An Automated

Controlled-Flow Air Infiltration Measurement System, " in Building Air

Change Rate and Infiltration Measurement eds., Hunt, C.M., King, J.C., and

Trechsel, H.R., ASTM STP 719, American Society for Testing and Materials,

1980.

Dick,. J.B., "Experimental Studies in Natural Ventilation of Houses," Journal of

the Institution of Heating and Ventilating Engineers , December 1949.

Dietz, R.N., Cote, E.A., "Air Infiltration Measurements in a Home Using a Convenient Perfluorocarbon Tracer Technique, " Environment International,

Vol.8, 1982.

Dietz, R.N., Goodrich, R.W., Cote, E.A., Wieser, R.F., "Application of

Perfluorocarbon Tracers to Multizone Air Flow Measurements in Mechanically

and Naturally Ventilated Buildings," Brookhaven National Laboratory Report

35249, 1984.

Dietz, R.N., Goodrich, R.W., Cote, E.A., Wieser, R.F., "Detailed Description and

Performance of a Passive Perfluorocarbon Tracer System for Building

Ventilation and Air Exchange Measurement, " in Measured Air Leakage of

Buildings , ASTM STP 904, H.R. Trechsel and P.L. Lagus, Eds., American

Society for Testing and Materials, Philadelphia, 1986.

Freeman, J., Gale, R., Lilly, J.P., "Ventilation Measurements in Large

Buildings," in Air Infiltration Reduction in Existing Buildings proceedings

of the Fourth Air Infiltration Centre Conference, Elm, Switzerland, 1983.

21
Grot, R.A., Clark, R.E., "Air Leakage Characteristics and Weatherization

Techniques for Low Income Housing, " in Proceedings of the DOE/ASHRAE.

Conference on the Thermal Performance of the Exterior Envelopes of

Buildings , Orlando, FL, December 1979.

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22
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24
TABLE 1 TRACER GASES AND MEASUREMENT TECHNIQUES

Technique Gases

Thermal Conductivity Detector

Electron Capture Gas Chromatograph

Flame Ionization Gas Chromatograph

Infrared Absorption

TABLE 2

RELATIVE TRACER GAS COSTS TAKING DETECTABILITY INTO ACCOUNT

Gas

Detectable

Concentration

Gas

Per

Volume

Dollar

Maximum Measureable

Volume Per Dollar

(ppm) ft

3

(m

3

) ft

.3 (m

3

)

He 300 1.4 (0.13) 4 X 10

3

(4 x 10

2

)

co21 7.0 (0.65) 6 X 10

4 (6 x 10

5

)

n2 o 1 2.4 (0.22) 2 X 10

6

(2 x 10

5

)

sf 6

— fi 5 x 10 0.13 (1.2 x 10” 2

) 2 X 10

10

(2

9 x 10 )

5 x 10~ 32 X io

7 (2 x 10

6

)

CBrF 2

*

5 x 10~ 53.7 x

10"2

(3.4 x 10~ 3

) 7 X 10

8

(7 x 10

7

)

PDCH** 5 x 10“ 6

3.0 x 10"3

(2.8 x 10" 4

) 6 X 10

8

(6 x 10

7

)

* bromotrifluoromethane

** perfluorodimethylcyclohexane

Hydrogen

Helium

Carbon Dioxide

Sulfur Hexafluoride

Refrigerants

Perfluorocarbons

Ethane

Carbon Monoxide

Carbon Dioxide

Sulfur Hexafluoride

Nitrous Oxide

Ethane

Methane

25
'
Figure 1 Schematic of

a General Tracer Gas Measurement System
Concentration of C 2CI 2^4 t

"* me

upstairs and downstairs

Concentration of CC 1 2^2 a g ains ^ f ime » upstairs and downstairs

Figure 2 Representative Two Tracer Data (I Anson et al 1982 )
oo

COo

qdo•ST oC\J o

00I o

cr> o

UO.i;PJ;U0DUO3 J9DPJ1 ^ -p

o>

TO

0J

i/i Cl

«3 UJ

Co0 r=C AJ

05

p-H ° H

AJd

a) >

05c• r-4

V -o- 00

0) o-« AJ

<u a US o 00

AJ CO

AJC

QJ

c/5

B 3 • i-4 AJ M CO QJ 05 CUX 3 w O co• r—

• pa] AJ c o

0) QJ ) C c O

i—CQ

l-i aj 3 c AJ

to c

AJ 1—)

C/5 c -o o c o C0

co

QJ

l-i 3

oo 0 r-
WHOLE BLDG: 0.74 ACH

29

3864-

2nd Floor

(2358 m3

) 0.15 ACH

72

2605

* 345

1st Floor

(2610 m3

)

1.33 ACH

2554 34

681

232

~r

177

Basement

(872 m3

) 0.58 ACH

507

LAKE OSWEGO LIBRARY

(all flow rates in m3/h)

F igure 4 Airflow Rates Measured in Multi-Tracer Tests (Dietz et al 1984a)
Schematic of Tracer Gas Measurement in a Mechanically Ventilat

\

////i

—

I////H
AIR HANDLING SYSTEM

Figure 6 Schematic of Ventilation Airflow Rate Measurement Procedcures
AIR TRACER GAS

Figure 7 Schematic of Constant Injection Airflow Measurement System

(Lund in 1984)
co c

Q)

03 Cd E0 ^ c 0 0 o o

cd c W o H O

D

C/3 cd0£
NBS-114A rev. 2 -aci U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication D ate BIBLIOGRAPHIC DATA

SHEET (See instruction s)

REPORT NO.

NBSIR 88-3708 FEBRUARY 1988

4. TITLE AND SUBTITLE

Tracer Gas Techniques for Studying Building Air Exchange

5. AUTHOR(S)

Andrew Persily

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in stru ction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ("Street. City. State . ZIP)

National Bureau of Standards

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi cant

bi bl iography or literature survey, mention it here)

A variety of procedures have been developed that employ tracer gases to examine the air

exchange characteristics of buildings. These procedures enable the examination of

several features of building air exchange including ventilation rates, air movement

within buildings, and building envelope airtightness. This paper reviews tracer gas

measurement techniques that have been used to study air exchange in buildings. Background information is discussed such as the instrumentation used in these tests,

building features that influence their application, and the fundamental theory of

tracer gas measurement. Several specific applications are discussed including air

exchange rate measurement in large buildings, low-cost procedures for measuring air

exchange rates in large numbers of buildings, techniques for evaluating the performance

of air distribution systems, and pressurization testing of envelope airtightness in

large buildings. A detailed bibliography is also included to facilitate a more

thorough examination of the topics discussed.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

airflow measurement; building performance; infiltration; measurement; tracer gas;

ventilation.

13. AVAILABILITY

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I

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20402.

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PRINTED PAGES

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15. Price

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