NAT L INST OF STAND & TECH

A 1 1 1 0 b EbOISI

NBSIR 84-2849

Air Flow Calibration of Building

Pressurization Devices

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

QC

100

.U56

NO. 84-2849

1984

April 1984

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 84-2849

NATIONAL BUREAU

C? STANDARDS

AIR FLOW CALIBRATION OF BUILDING

PRESSURIZATION DEVICES

Andrew K. Persily

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

April 1984

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige. S*cr9fry

NATIONAL BUREAU OF STANDARDS. Emast Ambtor, Director

Abstract

Whole building pressurization devices, or blower doors, have been used to

quantify building airtightness and to determine compliance with airtightness

standards. Using pressurization testing in airtightness standards requires

knowledge of the accuracy of the air flow rate measurement techniques employed

by blower doors. The quantitative accuracy of existing air flow calibrations

are not known and have been questioned. The blower doors considered in this

report employ calibration formula relating the air flow rate through the door to

the fan speed and the pressure difference across the door. Such fan speed

calibrations must be done accurately over wide a range of fan speed/pressure

difference combinations and in a physical setting that closely approximates the

manner in which the blower doors are used in the field.

In order to obtain an accurate and well-documented calibration of pressurization

devices, a facility was designed and constructed at the U.S. National Bureau of

Standards. The calibration facility discussed in this work was built to

accurately determine the flow rate through the fan as a function of fan speed,

air density and pressure difference across the fan. This report describes the

calibration facility and the rationale for its particular design. Results from

the calibration of one blower door are presented. The effect of the form of the

calibration equation on the accuracy of the air flow rate determination is also

discussed.

Key Words: air flow measurement; air tightness evaluation; blower doors;

building tightness; calibration; fan pressurization; pressure

testing.

iii
Preface

This study of pressurization devices was conducted for research purposes and is

not an NBS report of calibration. The work was conducted while the author held

a National Research Council Postdoctoral Research Associateship at the National

Bureau of Standards.

iv
TABLE OF CONTENTS

Abstract iii

Preface iv

List of Tables vi

List of Figures vii

1. Introduction 1

0 2 . Pressurization Equipment 1

3. Calibration Techniques and Formulations 2

3.1 Calibration Requirements 2

3.2 Air Flow Rate Measurement 4

w 3.3 Calibration Formulation 6

4. Test Results 7

4.1 Experimental Procedure 7

4.2 Presentation of Results 9

4.3 Comparison to Other Calibrations and Discussion 11

5. Conclusions 12

6 . Acknowledgments 1

3

7. References 14

v
List of Tables

Table 1

Table 2

Table 3

Table 4

Range of Blower Door Flow Rates

Range of NBS Calibration

Comparison of 50 Pa Flow Rate from Equation (9) and a Straight Line

Fit to the Calibration Data

Comparison of Pressurization Test Results Using Equation (9) and

Straight Line Fits to Calibration Data

vi
List of Figures

Figure 1

.

Simple Schematic of General Calibration Chamber

Figure 2. Schematic of NBS Calibration Facility

Figure 3. Dimensionless Flow Rate vs Dimensionless Pressure Difference,

Counter-Clockwise Fan Rotation, NBS Calibration of Blower Door A

Figure 4. Air Flow Rate vs Fan Speed at Several Pressure Differences

Figure 5. Dimensionless Flow Rate vs Dimensionless Pressure Difference,

Counter-Clockwise Fan Rotation, Manufacturer's Calibration of Blower

Door B

Figure 6. Dimensionless Flow Rate vs Dimensionless Pressure Difference,

Clockwise Fan Rotation, Manufacturer's Calibration of Blower Door B

Figure 7. Comparison of Calibrations of Blower Doors A and B

vii
„
1 . Introduction

There are two basic uses of whole-house pressurization devices, diagnostics

for the location of air leakage paths and quantification of the airtightness of

buildings. When used as a diagnostic tool, the pressure difference induced by

blower doors accentuates the air flow through leakage paths, thereby making

their detection easier [1]. Such leakage detection is further enhanced by using

smoke and/or infrared thermography in conjunction with pressurization. Blower

doors can also be used to quantify the airtightness of buildings by measuring

the air flow rate required to induce and sustain a given inside-outside pressure

difference [2,3]. It is this second use of pressurization which concerns us here.

Quantification of building airtightness is useful for the determination of

space conditioning loads and the maintainance of indoor air quality. While

actual infiltration measurement can fulfill these needs as well as, or perhaps

better than, pressurization testing, the characterization of a building's

airtightness requires infiltration measurements over a wide range of weather

conditions, and is an expensive and time consuming procedure. Pressurization

testing requires only a single measurement which takes roughly one hour and is

relatively inexpensive. Airtightness measurement through pressurization can be

used for measuring the effectiveness of shell tightening retrofits, comparing

homes to each other and determining compliance with building tightness

standards. Sweden, in fact, requires new homes to achieve a specific tightness

level as measured by pressurization [ 3 3 > and the American Society of Heating,

Refrigerating and Air Conditioning Engineers is formulating an airtightness

standard for U.S. homes. The use of pressurization testing for airtightness

evaluation implies a relation between pressurization and weather induced

infiltration. This relation has been studied extensively [4-10], however the

ability to predict infiltration from pressurization is limited.

In order to use blower doors as quantitative tools, one must know the

accuracy of the measurements. Previous work has shown pressurization test

results to be reproducible within about 2% over periods of a few weeks [11].

The absolute accuracy of air flow rate measurement of many pressurization

devices has not been well established, especially for devices which employ fan

speed calibrations for determining the air flow rate. Some devices have been

calibrated by their manufacturers and other researchers [12], but the accuracy

of these calibrations has not been carefully examined. In this report,

pressurization equipment is discussed briefly, along with general calibration

requirements. Several calibration strategies are reviewed and the particular

technique used at NBS is described in detail. The formulation and presentation

of these flow calibrations are also discussed. Finally, preliminary results

obtained at the NBS facility are presented.

2. Pressurization Equipment

There are basically two types of blower doors, those which measure air flow

directly using nozzle or orifice meters, and those which use a calibration

formula to relate the air flow rate to the fan speed and the inside-outside

pressure difference. Direct flow measurement techniques are based on relations

between the air flow rate through a nozzle or orifice and the pressure drop

across such a constriction. Flow rate measurement with orifices and nozzles is a well documented technique [14-19], but the existence of these constrictions

1
decreases the flow capacity of a fan. In addition, orifice and nozzle meters

require ducting and flow straighteners to condition the flow before it passes

through the constriction, and this further decreases the flow capacity and makes

the device more bulky. For reasons of portability and expense, many designers

have developed "calibrated" blower doors in which the air flow rate is given by

a general function

q s f(u,A p ) ( 1 )

where

q = air flow rate, m3/ s w s fan speed,

Ap = inside-outside pressure difference, Pa.

The fan speed to is the number of revolutions per second of the fan blade and not

the blade tip speed. Calibrated blower doors will be the topic of discussion in

the remainder of this report, however the material on calibration also applies

to blower doors which employ direct air flow rate measurement.

3. Calibration Techniques and Formulations

3.1 Calibration Requirements

The technique of pressurization testing presents specific requirements on

the calibration of blower doors. When pressure testing a building, one induces

several inside-outside pressure differences generally ranging from 10 to 70 Pa

in increments of about 10 Pa [13]. One notes the fan speed required to induce

each pressure difference, and using calibration formula, calculates the air flow

rate at that pressure difference.

Blower doors must be able to induce a wide range of air flow rates to

enable testing of homes ranging from small and tight to large and leaky. At

each pressure difference, one must be able to induce and measure reliably a wide

range of air flow rates. One may determine a desirable range of air flow rates

for a blower door by considering a small, tight house ( 1 80 m3, 2.5 exchanges/hr

at 50 Pa) and a large, leaky house (600 m^, 15 exchanges/hr at 50 Pa). By

considering pressurization test data that would correspond to these limiting

cases, we calculated the maximum and minimum air flow rates for several pressure

differences. Table 1 shows the results of these calculations. At each pressure

difference the maximum flow rate is twenty times the minimum, with the maximum

flow rate at 75 Pa about 64 times the minimum flow rate at 12.5 Pa. This table

gives desirable air flow rate capacities for a blower door, however not all

doors are able to cover this range of flow rates.

2
Table 1 . Range of Blower Door Flow Rates

Inside-Outside

Pressure Difference

(Pa)

12.5

25.0

37.5

50.0

62.5

75.0

Small, Tight House*

Flow Rate

(m^/s)

0.051

0.080

0.104

0.125

0.145

0.163

Large, Leaky House*

Flow Rate

(m^/s)

1 .015

1.593

2.074

2.500

2.890

3.254

* The small, tight house has a volume of 1 80 m3 and a 50 Pa flow rate of 2.5

exchanges/hr. The large, leaky house has a volume of 600 and a 50 Pa

flow rate of 15 exchanges/hr. The flow exponent for both houses is assumed

to be 0.65. The data in this table is ficitious and is used only to

calculate a desirable range of air flow rates for blower doors.

In addition, blower door calibration formulas must deal with the effects of

air density on the air flow rate. Air density varies with air pressure,

temperature and relative humidity, and therefore blower door tests are conducted

at different air densities, depending on the ambient conditions. At constant

air pressure and relative humidity, air density changes by about 7$ for a 20°C

change in temperature. A change in air pressure corresponding to a 500 m change

in altitude changes the air density by about 6$. Changes in air temperature and

pressure can combine to cause 10$ variations in air density. Blower doors are generally calibrated at a single air density of about 1.2 kg/m^ (corresponding

to an air temperature of 20°C and atmospheric air pressure), and the question

exists of how to adjust the calibration to account for measurements at other air

densities. We know the air flow rate for a given density, fan speed and

pressure difference, and want to know the air flow rate for the same values of

fan speed and pressure difference but different air density. While fan laws

exist to deal with some analogous situations, these particular circumstances are not amenable to fan laws [14].

Standards exist for testing the performance of fans [15-16]. ASHRAE

standard 51-75 describes several physical arrangements for the fans and flow

rate measuring equipment [15]. In general, the fans are either mounted in a duct of a diameter similar to that of the fan or installed in a large plenum

(see fig. 1). In most cases an exhaust system is required to control the back

pressure against which the fan blows. The calibration of blower doors should be

carried out in a physical setting that closely approximates the manner in which

such devices are used in the field. When blower doors are used to test a house,

they are installed in a doorway. The fan, in effect, is mounted in a vertical

3
plane. Having a fan mounted in a duot will affect the characteristics of the

air flow through it, and therefore a calibration obtained with a fan in a duct

will not apply to the same fan used in a blower door. Thus, blower doors should

be calibrated with the device mounted in a large chamber into which the fan

blows air. Suoh a chamber allows the air to "settle" or reach an approximately

static condition. A simple schematic of such a chamber is shown in figure 1

.

The air flow rate measurement device must be appropriate to the magnitude of the

air flow rates and the air flow/pressure difference combinations in table 1

.

Several measurement techniques are discussed in the following section.

3.2 Air Flow Rate Measurement

There are several options for measuring the air flow rate out of the

calibration chamber including devices such as orifice plates, nozzles, and pitot

tube arrays. Eaoh alternative has advantages and disadvantages, and several

options are disoussed below, concluding with the flow rate measurement technique

used in the NBS calibration faoility. The use of orifice plates and nozzles

installed in pipes is a well documented procedure for measuring fluid flow rates

[17-19]. The references listed provide specifications for orifice design,

lengths of pipe upstream and downstream from the metering section, flow

straighteners and other aspects of design, installation and use. When the

appropriate specifications are followed, the uncertainty of the air flow rate

measurement is well determined. The basic problem with such meters is the

significant pressure drop caused by the pipe lengths, flow straighteners and the

orifice or nozzle. The pressure drop is generally too large to obtain the air

flow/ pressure difference combinations in table 1. Therefore, one requires a large capacity, variable exhaust system to control the back pressure. Such an exhaust system complicates and increases the cost of the calibration facility.

Also, several different sized orifices and nozzles are required to cover the

wide range of flow rates of interest.

To avoid the large pressure drops associated with orifices in pipes,

several researchers have used orifices mounted in the walls of plenums or settling chambers [12,13]. This technique is indeed much simpler than using

orifices in pipes, but it is not well documented, nor is there a statement of

uncertainty associated with the measurements. No discussion of this technique

is found in the three basic handbooks of orifice metering referred to earlier

[17-19]. A text on air flow measurement by Over and Pankhurst [20] does mention

the technique, but not in detail. The authors refer to reference 18 as a source

of discharge coefficients for such orifices, but Ower and Pankhurst appear to

have derived these coefficients from extrapolating data for orifices in large

pipes. A practical handbook of fan engineering [21] also provides discharge

coefficients for such a situation but does not provide the source of the values.

This lack of documentation of discharge coefficients is one problem with this

technique. The value of the discharge coefficient is probably not as significant a source of error as the characteristics of the flow impinging on

the orifice. The theory of orifice meters is based on conditions of fully

developed, turbulent flow upstream of the constriction. Ower and Pankhurst [20]

mention that these conditions may not be met for flow discharging from a chamber

through an orifice and refer to the need to eliminate flow disturbances such as drafts. The existence of such drafts and the lack of fully developed flow

impinging on the constriction may induce unknown measurement errors when using

this flow measurement technique. The indeterminant accuracy and the lack of

documentation of the technique lead us not to use it nor to recommend its use.

4
Several commercial flow measurement devices exist, such as a multi- point

pitot tube traverse station combined with a flow straightener, The flow

measurement is based on the difference between the total and static pressures of

the flow. Based on the magnitude of the flow rates of interest and the size of

the devices, the pressure differences (total minus static) which must be

measured are quite small, in some cases on the order of 1 Pa. It is difficult

and expensive to measure such small pressure differences accurately. Also,

pitot tubes are generally inaccurate for the small flow rates of interest here.

In addition, these pitot tube arrays should be calibrated, and this presents the

same problem we are trying to solve. Another commercially available device is

referred to as a laminar flow element which channels the air flow through a large number of narrow, parallel passages. These channels are sufficiently

narrow that the flow through them is laminar, and the magnitude of the air flow

rate is related straightforwardly to the pressure drop across the channels.

These devices are expensive and several would be required to cover the range of

flow rates of interest. In addition, the pressure drop through the devices is

extremely large. Other devices exist but none satisfactorily combine cost,

minimal pressure drop and accuracy.

The flow measurement technique used in the NBS calibration facility is a constant flow, tracer gas injection scheme [22]. Tracer gas (sulfur

hexafluoride, SFg) is injected at a constant and known rate into the air stream,

and the concentration is measured as far downstream from the injection point as possible (see fig. 2). Under conditions of good mixing of the tracer and the

air flow, the air flow rate can be determined from the SFg injection rate and

the measured concentration according to

q = i/c, (2)

where

i = SFg injection rate, m^/s

c = SFg concentration, ppm.

The SFg injection rate is measured and controlled with individually calibrated

rotameters. The accuracy of these devices is ±1$ of full scale and four

different meters are used, providing injection flow rates from 10 to 2500

ml/min. The SFg concentration downstream of the injection is measured with an infrared gas analyzer with a 1 .5 m path length operating at 10.7 m. The tracer

gas injection rate is adjusted such that the downstream concentration is 25 ppm.

The gas analyzer is calibrated with a 25 ppm gas mixture prepared by a gas

distributor to an accuracy of ±1$. This air flow rate measurement system has a minimal pressure drop through it. To obtain the desired air flow/pressure

difference combinations, we still must use constrictions anc a variable exhaust

fan in the exit duct.

A schematic of the NBS calibration chamber is shown in figure 2. Tne

chamber is used in the large environmental test facility at the Center for

Building Technology at NBS, which enables control of air temperature and

5
humidity, and therefore air density. The calibration chamber has a square base

with 3.66 m sides and is 1.83 m high. Air is drawn into the calibration chamber

from the larger environmental enclosure through the blower door being tested.

The air is exhausted to the outside of the large enclosure through a 4.88 m long

exit duct. The SFg is injected at the upstream end of the exit duct and the

concentration measured at the downstream end. The exhaust fan and constriction

locations are shown in figure 2. The details of data recording and preliminary

results are discussed later in this report.

3.3 Calibration Formulation

Calibrations of blower doors have generally been expressed as linear

relations between air flow rate and fan speed for each inside-outside pressure

difference [12,13]. These linear relations are the result of applying linear

regressions to the calibration data. While such a formula is straightforward to

use, it is not physically correct and no indication is given of how to account

for density effects. In addition, as will be shown later, such a linear

approximation breaks down at low fan speeds. A physically correct calibration

formulation has been presented previously which involves nondimensionalization

of the air flow rate q and the pressure difference Ap [4,11]. The

nondimensional air flow rate is expressed as

a q/wd^ t (3)

and the nondimensional pressure difference as

3 = Ap/p(cod) 2

, (4)

where

p = air density, kg/m^

d = fan diameter, m.

A fan's characteristics are described by a relation between a and 3 , expressed

in general as

a = g(3 )

.

(5)

The data we collected, and the values of a and 3 derived from other calibration

formulas [4,11], fit well to an equation of the form

6
a = A

1 exp(A2B), ( 6 )

where A<| and A2 are constants. By substituting eq (3) and (4) into eq (6) one obtains the following expression for q,

q _ exp (

A

2 Ap/ pw 2

), <7)

where A^ and A

2 are constants obtained by absorbing the constant fan diameter d

into A

1 and A2

. Thus, for Ap=0 a linear relationship between q and w is

appropriate. For nonzero pressure differences, the exponential factor causes a deviation from a straight line, particularly at low fan speeds. This deviation

from linearity and its significance will be discussed in the next section.

This nondimensional calibration formulation leads to a straightforward

correction for air density, and explains the inappropriateness of fan laws for

such a correction [14]. Fan laws are obtained by holding 8, and therefore a,

constant. A constant value of these nondimensional quantities implies certain

relations between the physical quantities from which they are constituted. For

our situation in which Ap and w are constant, but the density p is variable, one

sees that a and 8 change and therefore the fan laws are not appropriate.

4. Test Results

This section describes preliminary results obtained in the NBS blower door

calibration facility. Only one door, referred to as Blower Door A, was tested

in the counter-clockwise direction of fan rotation. Blower Door A consists of a 0.46 m diameter axial fan coupled by a belt drive system to an electrically

reversible, variable speed constant- torque DC motor.

4.1 Experimental Procedure

The most important factor in the tracer gas flow rate measurement technique

is the mixing of the tracer gas with the air in the exit duct. The tracer gas

must be well-mixed for eq (2) to apply. Good mixing is obtained by injecting

SFg at several vertically coplanar points immediately upstream of the exhaust

fan shown in figure 2. Under this injection scheme the downstream SFg

concentration is constant in time and location in the duct within about -0.25

ppm of the 25 ppm concentration setting. In addition to good mixing, this flow

rate measurement technique assumes that the SFg concentration in the air flowing

through the blower door into the calibration chamber is 0 ppm. This assumption

is checked after every other flow rate measurement by turning off the SFg

injection and making sure the SFg concentration in the exit duct returns to 0

ppm. If the SFg concentration in the outgoing flow returns to 0 ppm, then there

is no SFg in the air flowing through the blower door into the calibration

chamber.

7
The desired air flow/ pressure difference combinations are obtained by using

the exhaust fan and by installing constrictions in the exit duot. A high speed

setting on the exhaust fan corresponds to testing large, leaky homes, i.e. low

resistance to air flow. Constrictions in the exit duct correspond to smaller,

tighter homes with a higher resistance to air flow. The measured ranges of fan

speed and air flow rate for each pressure difference are given in table 2. The

air flow rates range from about 0.05 to 1.10 m^/s. This is not as large as the

maximum range of interest given

capacity of the exhaust fan used

in table 1 , due primarily to the limited flow

•

Table 2. Range of NBS Calibration

Inside-Outside

Pressure Difference

(Pa)

Fan Speed

(s-1)

Air Flow

(mVs)

12.5 7.50-23.33 0.050-0.725

25.0 10.83-30.00 0.075-0.950

37.5 12.50-35.83 0.075-1 .050

50.0 14.17-38.33 0.075-1 .050

62.5 15.83-38.33 0.075-0.975

For each calibration point, the fan speed , pressure difference across the

chamber wall and the air density are also measured. The fan speed for this

blower door is measured in the field with a digital tachometer employing a magnetic transducer and a toothed gear interrupter which was found to agree

within 0.02 s"^ with a digital strobosoope/tachometer with an accuracy of i0.02

s

-1

. The pressure difference across the calibration chamber wall was measured

in four locations with magnehelic pressure gauges individually calibrated

against an inclined manometer. The four measured pressure differences agree

within the range of uncertainty of the pressure gauge calibration, - 0,6 Pa.

The air density was determined by measuring the air temperature, relative

humidity and barometric pressure, and calculating the density using the

appropriate formulas. Based on the uncertainties in the measurements of

temperature, humidity and pressure, the air density determination has an uncertainty of - 0.02 kg/nw. We then calculated a and Busing eq (3) and (4).

Based on the uncertainties in q, Ap, to, and p, a and B are determined within

about ±6?

.

8
4.2 Presentation of Results

Figure 3 is a plot of a versus 6 for counter-clockwise fan rotation for

Blower Door A as determined in the NBS calibration facility. Fitting a curve of

the form of eq (6) to the data yields

a = 0.368 exp(-1 .72 B ). ( 8 )

The coefficient of determination r2 has a value of 0.9928, and the standard

error of the estimate of a is roughly ±4$ of its mean value.

Equation (8) can be expressed as a relation between flow rate q and fan

speed w, pressure difference Ap and air density p as

q

= co(3 *52 X 10“2

) exp(-8 .23 Ap/pcu2

)

.

(9)

As discussed earlier, eq (9) is not a linear relation between flow rate and fan

speed, however the deviation from linearity is small except at low fan speeds.

Figure 4 shows several lines relating flow rate to fsn speed derived from eq

(9), each for a different pressure difference. The solid portion of each line

corresponds to the fan speed range for which calibration data was actually

collected. The dashed portions extend beyond the measured range. The measured

range of fan speed for each value of pressure difference are given in table 2.

The dotted lines extending to the lower left in figure 4 are straight line fits

to the calibration data. The straight line fits are indistinguishable from the

actual calibration lines except at low fan speeds where the deviation can be

significant. Past calibration formulations have employed straight lines,

corresponding to these dotted lines [12,13]. We see that the use of such

straight lines will lead to erroneously low estimates of air flow rates at low

fan speeds. Table 3 shows air flow rates at Ap = 50 Pa for a range of fan

speeds derived from eq (9) and from a straight line fit to the calibration data.

The errors are large for the lower fan speeds, which implies that if straight

line fits are used, then tight or small houses will be reported as tighter than

they actually are.

9
Table 3* Comparison of 50 Pa Flows from Equation (9) and a Straight

Line Fit to the Calibration Data

Fan Speed Air Flow Rate $ Error of

(s”1)

Eq (9)

(m^/s)

Straight Line

Straight Line

13.33 0.068 0.031 -54$

14.17 0.090 0.065 -28$

15.00 0.115 0.099 -14$

15.83 0.142 0.134 - 6$

16.67 0.171 0.169 - 1$

17.50 0.201 0.203 + 1$

18.33 0.233 0.238 + 2$

Table 4 shows the results of applying both eq (9) and straight line

calibration formulas to a sample of pressurization data for a small, tight

house. The fan speed is given for each pressure difference, along with the

corresponding air flow rates as calculated using eq (9) and a straight line fit

to the calibration data. The air flow rates determined using the straight line

sire all lower than the flow rates calculated from eq (9). Curves are fit to the

data, as is often done in analyzing pressurization test results from the field,

and two common building tightness measures are calculated, the flow rates at 4

and 50 Pa. The 4 Pa air flow rate from the straight line fit is 24$ lower than

the 4 Pa rate from eq (9). The 50 Pa flow rate from the straight lines is 10$

lower. Thus, we see the potential for error when using straight line

calibration formulas to analyze blower door data.

1 0
Table 4. Comparison of Pressurization Test Results Using Equation (9)

and Straight Line Fits to Calibration Data

Pressure

Difference

Fan Speed

(s-1)

Air Flow Rate

(m^/s)

* Error of

Straight Line

(Pa) Eq (9) Straight Line

12.5 7.33 0.053 0.043 -19*

25.0 10.50 0.078 0.066 -15*

37.5 13.17 0.105 0.093 -11*

50.0 15.33 0.126 0.113 -10*

62.5 17.33 0.146 0.135 -8*

75.0 19.00 0.161 0.148 -8*

Curve Fits

uation (9) Q = 0.0104

.raight Line Q = 0.0070

Error of Straight Line

Ap0 -637

ap0.712

Q(4) = 0.025

Q( 4) = 0.019

Q(4) : -24*

Q( 50) = 0.126

Q( 50) = 0.113

Q( 50 ) : -10*

4.3 Comparison to Other Calibrations and Discussion

Figures 5 and 6 show calibration data for Blower Door B, which was built

by the same manufacturer as the door tested in the NBS facility. The data on

Blower Door B were obtained by the manufacturer by measuring air flow rates with

an orifice plate in the wall of a settling chamber as described earlier. The

individual "bunches" of points correspond to the different orifices used in the

air flow rate measurements. There are fewer data points and more scatter than

in the NBS calibrations shown in figure 3* The manufacturer fit the counterclockwise fan rotation calibration data to the following expression

Q = 234 - 355vftp + 178 w. • ( 10 )

Q is air flow in m^/hr. Besides being physically incorrect, eq (10) predicts a nonzero air flow rate for zero Ap and w. Fitting the NBS calibration data to a curve of the same form yields

Q = -1 - 265 /Ap + 149

1 1

( 11 )
While eq (11) still leads to errors at low fan speeds, it does prediot

essentially zero air flow for Ap and u equal to zero. While eq (9) is

physioally oorreot and more aoourate than the linear formulations in eq (10) and

(11), it is also more complex and questions have been raised ooncerning misuse

by field personnel [23]. Equation (11) could be used as a substitute without

muoh loss of aoouraoy, but only for a limited range of fan speed.

Figure 7 shows a comparison of the ourve fit to our calibration of Blower

Door A and the measurements made on Blower Door A by the manufacturer (oounterolookwise fan rotation). The manufacturer's calibration data for Blower Door B are also inoluded in the figure. The manufacturer tested Blower Door A using

only a single orifice, and therefore all the points are dose together. These

data are about 25$ greater than the ourve fit to our calibration of the same

door. The manufacturer obtained more data on Blower Door B, and these data are above our ourve fit to Blower Door A, about 0.02 higher on the a scale. This

difference corresponds to about 50$ of a for high values of 3 and 6$ for low

values. There are a few manufacturer's points, with a equal to about 0.45

»

whioh lie muoh farther from our line. These points are very different from a ourve fit to the other manufacturer's points, and this difference is obvious in

the nondimensional presentation of the data. The reason for the otherwise

oonsistant difference between the manufacturer's calibration of Blower Door B

and our calibration of Blower Door A is not clear. The difference could be due

to imperceptible differences in the doors.

5. Conclusions

This report has presented the problem of calibrating blower doors and the

techniques used in the NBS calibration facility. The NBS facility was used to

calibrate one blower door and a physically appropriate calibration formulation was applied to the data. Previous calibrations have used linear relations

between air flow rate and fan speed which can lead to errors in flow

determination at low fan speeds. It is pointed out that suoh errors can lead to

tight or small houses being reported as more airtight than they aotually are if

the straight line fits are used. Such straight line calibrations can be used if

the range of fan speed is appropriately limited. In addition, our calibration

formulation allows for straightforward density corrections. Other blower doors,

including those which employ direct air flow rate measurement, may be tested in

the facility in the future.

1 2
6. Acknowledgments

The author would like to express his appreciation to Phil Engers, Jay Murphy and

Steve Schweinfurth for their assistance in constructing the calibration facility

and in conducting the tests.

13
7. References

[1] Harrje, D.T. , Dutt, G.S. , Beyea, J. , "Locating and Eliminating Obscure but

Major Heat Losses in Residential Housing, ASHRAE Transactions . Vol.85(I),

1979.

[2] Harrje, D.T., Gadsby, K. , Linteris, G. , "Sampling for Air Exchange: Rates

in a Variety of Buildings," ASHRAE Transactions . Vol. 88(1), 1978.

[3] Kronvall, J. , "Testing of Homes for Air Leakage Using a Pressure Method,"

ASHRAE Transactions . Vol. 84(1), 1978.

[4] Persily, A.K. , "Understanding Air Infiltration in Homes," Report No. 129,

Center for Energy and Environmental Studies, Princeton University, 1982.

[5] Liddament, M. , Allen, C. , "The Validation and Comparison of Mathematical

Models of Air Infiltration," Technical Note AIC 11, Air Infiltration

Centre, Bracknell, UK, 1983.

[6] Kronvall, J. , "Correlating Pressurization and Infiltration Rate Data - Tests of an Heuristio Model," Lund Institute of Technology, Division of

Building Technology, Lund, Sweden, 1980.

[ 7 ] Sherman, M.H. , Grimsrud, D.T., "Infiltration-Pressurization Correlation:

Simplified Physical Modeling," ASHRAE Transactions . Vol. 86(11), 1980.

[8] Shaw, C.Y., "A Correlation Between Air Infiltration and Air Tightness for

Houses in a Developed Residential Area," ASHRAE Transactions . Vol. 87(11),

1981

.

[9] Warren, P.R., Webb, B.C., "The Relationship Between Tracer Gas and

Pressurization Techniques in Dwellings," First Symposium of the Air

Infiltration Centre, Windsor, England, 1980.

[10] Persily, A.K., Grot, R.A., "Air Infiltration and Building Tightness

Measurements in Passive Solar Residences," Solar Engineering - 1983 f The

American Society of Mechanical Engineers, New York, 1983.

[11] Persily, A.K., "Repeatability and Accuracy of Pressurization Testing,"

Proceedings of ASHRAE/DOE Conference on Thermal Performance of the

Exterior Envelope of Buildings II, Las Vegas, December 1982.

[12] Gadsby, K.J. , Linteris, G.T. , Dutt, G.S., Harrje, D.T., "The Blower Door,"

Report No. 124, Center for Energy and Environmental Studies, Princeton

University, 1981.

[13] ASTM E 779-81, "Standard Practice for Measuring Air Leakage by the FanPressurization Method," The American Society for Testing and Materials,

Philadelphia, 1981.

[14] ASHRAE Handbook. Equipment . The American Society of Heating, Refrigerating

and Air Conditioning Engineers, Atlanta, 1983.
[15] ASHRAE 51-75, AMCA 210-74, "Laboratory Methods of Testing Fans for

Rating," Air Moving and Conditioning Association, Ino. and American

Society of Heating, Refrigerating and Air Conditioning Engineers, New

York, 1975.

[16] BS 848, "Fans for General Purposes. Part 1, Methods of Testing

Performance," British Standards Institution, London, 1980.

[17] Zlald. Ms-ters., Iteln Theory jandL AroHoaklPIl, Report of the ASME Research

Commitee on Fluid Meters, 5th ed., The American Society for Mechanical

Engineers, New York, 1959.

[18] Orifice Metering of Natural Gas . Report No. 3, American Gas Association,

Arlington, VA, 1978.

[19] BS 1042, "Methods of Measurement of Fluid Flow in Closed Conduits,"

British Standards Institution, London, 1981.

[20] Ower, E. , Pankhurst, R.C., The Measurement of Air Flow . 5th ed. , Pergamon

Press, Elmsford, NY, 1977.

[21] Madison, R.D., ed. , Fan Engineering . 5th ed. , Buffalo Forge Co., Buffalo,

NY, 1948.

[22] Svensson, A., "Methods for Measurement of Airflow Rates in Ventilation

Systems," Bulletin M83:11, The National Swedish Institute for Building

Research, Stockholm, 1983.

[23] Gadsby, K.J., Princeton University, personal communication.

15

Air

flow

measurement

Figure 1

Simple

Schematic

of

General

Calibration

Chamber
4.88

m

SF6 concentration measurement

Exit duct square,

0.56 m sides

Constriction (optional)

Exhaust fan

1—sf6

injection

Chamber height

E

CD

1.83 m CD

A

00

f

|oo| J

Blower

door

ENVIRONMENTAL CHAMBER:

Controllable temperature and humidity

Figure 2 Schematic of NBS Calibration Faoility
( e pm/b)MOU HIV

SS3HNOISN3VMIQ ‘

Figure

3

Dimensionless

Flow

Rate

vs

Dimensionless

Pressure

Difference,

CounterClockwise

Fan

Rotation,

NBS

Calibration

of

Blower

Door

A
lO

CO

o

CO

ir>

CM

o

CM

ID

o

I CO

o

LO

LU

0.

C0

z

<

ll

(S/ 8 W) MOHd HIV

Figure

4

Air

Flow

Rate

vs

Fan

Speed

at

Several

Pressure

Differences
CM

O

3

co

CO

LU

Q.

<w

LU

O

CO LU

z

^

LU LU ^

LL

Q ~

Ll

Q

QQ. LU

cc3

CO

CO

LU

CC

CL

LO

• CO C\J t- O • • • O O o o o

( epm/b) m

O

ld HIV

SS31IMOISN3IMC]

Figure

5

Dimemsionless

Flow

Rate

vs

Dimensionless

Pressure

Difference,

CounterClockwise

Fan

Rotation,

Manufacturer’s

Calibration

of

Blower

Door

B
CM

O

CO

O

CD

o

o

CM

a o

o

CM

*D

3

CD

CD

Lil

O

CD

Z

HI

Q.

<1w

LU

O

z

LU

OC

LU

LL

UQ

QQ^

^ LU

cc

D

CD

CD

LU

CC

CL

(sPm/b) MO"ld HIV

SS31NOISN3IA1IQ

ra

Figure

6

Dimensionless

Flow

Rate

vs

Dimensionless

Pressure

Difference,

Clockwise

Fan

Rotation,

Manufacturer's

Calibration

of

Blower

Door
a

co

CO

LU

a

<

LU

o

CO LU

Z GC

LU LU ^

LL

Q ~

LL

O

QQ. LU

0C

ZD

CO

CO

LU

OC

CL

( e pm/b) MCTId dIV

SS3HNOISN3IAIIQ

Figure

7

Comparison

of

Calibrations

of

Blower

Doors

A

and B
NBS-114A (rev. 2 - 80 ) U.S. DEPT. OF COMM.

4.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR 2 REPORT NO. NBSIR 84-2849

. Performing Organ. Report No. 3.

TITLE AND SUBTITLE

Publication Date

April 1984

Air Flow Calibration of Building Pressurization Devices

5. AUTHOR(S)

Andrew K. Persily

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

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

10. SUPPLEMENTARY NOTES

Research conducted while author held a National Research Council Postdoctoral

Research Associateship at the National Bureau of Standards.

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

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) Whole building pressurization devices, or blower doors, have been used to quantify

building air- tightness and to determine compliance with air tightness standards. Using

pressurization testing in air-tightness standards requires knowledge of the accuracy of

the air flow rate measurement techniques employed by blower doors. The quantitative

accuracy of existing air flow calibrations are not known and have been questioned. The

blower doors considered in this report employ calibration formula relating the air

flow rate through the door to the fan speed and the pressure difference across the

door. Such fan speed calibrations must be done accurately over wide a range of fan

speed/pressure difference combinations and in a physical setting that closely

approximates the manner in which the blower doors are used in the field.

In order to obtain an accurate and well-documented calibration of pressurization

devices, a facility was designed and constructed at the U.S. National Bureau of

Standards. The calibration facility discussed in this work was built to accurately

determine the flow rate through the fan as a function of fan speed, air density

and pressure difference across the fan. This reports describes the calibration facility

and the rationale for its particular design. Results from the calibration of one blower door are presented. The effect of the form of the calibration equation on the

accuracy of the air flow rate determination is also discussed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Air flow measurement; air tightness evaluation; blower doors; building tightness;

calibration; fan pressurization; pressure testing.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

Qxj Uni imited

| | For Official Distribution. Do Not Release to NTIS

~| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 30

15. Price 20402.

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