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An exhaust fan apparatus for assessing the air leakage characteristics
of houses
AN EUIAUST FAN APPAIWJ'US FOR ASSESS TNG THE A I R LEAKAGE CHARACTERTSTICS 01: IlOllSES
11
Y
H.17. Orr and D . A . F i g l e y
ANALYZED
TNTRODUCTTON
Air leakage t h l r o ~ l g l ~ t h e cxtcrior envelope of a residence can constitute a s i g n i f i c a n t encrgy l a s s , and can conxribute t o discomfort and t o concealed
condensation problems. A i r tightness is t h u s a desirable characteristic and
i s in f a c r necessary f o r control o f ventilation and heat recovery s y s t e m s .
The a i r tightness of a building enclosure c a n h& a s s e s s e d b y e x h a u s t i n g a i r
from t h e structure and measuring t h e a i r f l o w r a t e and cosrcsponding pressure
difference across the e n c l o s u r e (1-51. Although IT i s n o t a measurement o f
t h e natural a i r lcakagc r a t e , t h e m e t h o d c h a r a c t e r i z e s t h e building i n r e g a r d
t o its over-all a i r leakage p r o p e r t i e s and p r o v i d e s a r e l a t i v e l y simple and direct means to asscss its conformance to desircd standards.
TIlc cxhaust f a n a p p a r a t u s w a s developed by researcllers at t h e Prairic
Regional S t a t i o n o f t h c D i v i s i o n OF B u i l d i n g Research i n Saskatoon f o r air
leakage testing o f s m a l l buildings. Tt was d e s i g n e d to bc easily portable
and s i m p l e to opcratc, allowing a two-man crcw to pressure t e s t a housc i n
a b o u t 45 minutes.
DESCRIPTION OF THE APPARATUS
Tllc prototype a p p a r a t u s c o n s i s t s of a replacement plywood door section w i t h a f a n l o c a t c d in t h e c e n t r e . A b e l l mouth nozzle is used t o measure t h e air flow rate and the door has an o u t d o o r pressure sensor c o n n e c t e d to it for measuring t h e n e t pressure d i f f c r c n c c e x c r t e d across t h c building envelope.
F i g u r e s 1 and 2 show drawings o f t h e d o o r section. The vcrtical p i a n o h i n g e a l l o w s it r o be p l a c c d i n e i t h e r 800 o r 900 mm w i d c doorlvays. The
h o r i z o n t a l h i n g e s a l l o w t h e a p p a r a t u s t o be folded to a s i z e t h a t can be handled by one p e r s o n and p l a c e d in the t r u n k o f a c a r .
Tl~e f a n c h o s e n f o r t h e pressurization a p p a r a t u s is a J o y Bfanufacturing Company Axivnne f a n , Model Alf-9-6.5-65n, r a t e d a t 5 4 3 L/s f r e e ai.r and
1625 Fa static pressure. It i s capable o f p r o d u c i n g a 50 Pa differential p r e s s u r e across a l l b u t t h e most l e a k y s i n g l e - f a m i l y house enclosures. It is very compact and light, w e i g h i n g l e s s t h a n 9 k g . Thc f a n o p e r a t e s from a
27 volt power s u p p l y a n d d r a w s a maximum c u r r e n t o f 50 amperes. A v a r i a b l c tsansforrncr ( V t i r i a c ) and rectifier circuit convert standard 110 v o l t AC power to t h e v o l r a g e r e q u i r e d b y t h e motor. T h i s s y s t e m allows t h e fan specd to be
A second pressurization unit i s now b e i n g c o n s t r u c t e d . This unit uses the
same f a n as the existing u n i t b u t the motor i s b e i n g rewound as a 110 v o l t DC
m o t o r w i t h a separately excited f i e l d . T h i s arrangement will allow t h e m o t o r spced to be controlled by a variable voltage SCK power s u p p l y , and w i l l
eliminate t h e . ,, need f o r t h e heavy t r a n s f o r m e r and "Variac'qin the p r e s e n t power
suppIy
.
An PISTIE l o n g r a d i u s f l o w n o z z l e (6) i s used t o measure t h e air f I o w rate
from t h e enclosure. The nozzle h a s a t h r o a t d i a r n e ~ e r o f 1 2 3 mrn and i s c o n n e c r e d to a d i f f u s e r s e c t i o n ~ d ~ i c h i s clamped t o t h e f a n i n l e t h o u s i n g .
To construct t h e fibreglass n o z z l e , a mold Iqas f i r s t cut from a
polystyrene b l a n k using a h e a t e d w i r e . A wax coar was t h e n applied a n d t h e
mold t u r n e d on a l a t h e t o smooth out s u r f a c e i m p e r f e c t i o n s and r e g a i n dimensional
t o l e r a n c e s . Fibreglass was t h e n applied to t h e mold. A similar technique was
u s e d r o f a b r i c a t e t h e d i f f u s e r s e c t i o n , with 5' expansion used to reduce shock
losses. The nozzle and d i f f u s e r s e c t i o n s a r e now being consrructed commercially u s i n g sprayed-on fibrcglass over a m e t a l mold
The n o z z l e is mountcd on t h e fan i n l e t so t h a t t h e air i s d r 3 m from the still room a i r . This ensures t h a t the air flow measlrrements a r e n o t i n f l u e n c e d
by wind e f f e c t s .
Nozzle pressure measurements a r e made using an inclined manometer with a f u l l - s c a l e d e f l e c t i o n o f 2000 pascals, requircd to measure t h e nozzle pressure a t f u l l florrr.
F a r calibration, t h e fan-nozzle system was connected to a circular a i ~
d u c t in t h e laboratory. A 10 p o i n t p i t o t tube traverse was used to o b t a i n t h e pressure v s a i r f l o w r a t e calibration curve for the n o z z l e . T h i s c u r v e is presented i n F i g u r e 3 . The f a n n o z z l e system is capable o f p r o v i d i n g measured
a i r flow r a t e s up t o 0 . 5 0 m3/s. Details o f t h e n o z z l e c a l i b r a t i o n a r c g i v e n i n
Appendix 1.
T h e b u i l d i n g envelopc p r e s s u r e d i f f e r e n c e i s measured u s i n g a digital manometer with a f u l l - s c a l e d e f l e c t i o n of 100 Pa on t h e lowest range.
To stabilize the pressure r e a d i n g s under wind conditions, t h e o u t d o o r pressure s e n s o r is f j t z c d with a wind b a f f l e (71.
TEST PROCEDURE
The f a n pressurization t e s t procedure involves mounting t h e pressurization a p p a r a t u s in a doorway. Sevcral techniques have already been investigated f o r clamping t h ~ door panel into t h c doorway and devclapmenr in t h i s area is
c o n t i n u i n g . A t present, Gripmate* clamps are used t o secure t h e d o o r panel i n
t h e d o o r opening. The mounting p l u g s are p r e s s e d i n t o holes i n t h e door panel perimeter and tlre clamps a r e applied t o t h e d o o ~ casing. This system provides
a q u i c k and easy method o f securing t h e p a n e l i n p l a c e . The d o o r section is t h e n t a p e d to t h e d o o r frame t o ensure an a i r tight s e a l .
The o u t d o o r Frcssure sensor is c o n n e c t e d via a f l e x i b l e t u b e to t h e connection in t h e door p a n e l , Care m u s t be t a k e n to l o c a t e the o u t d o o r
pressure sensor i n a c l e a r a r e a , away from t h e house s o a i r turbulence due to wind w i l l b e minimized. A second t u b e connects t h e d i g i r a l manometer t o t h e i n s i d e c o n n e c t i o n on thc d o o r p a n e l . The second pressure t a p of the
manometer is l e f t open to t h e house pressure.
Two t e s t s a r c conducted on each house. The f i r s t i s done with t h e chimney and o t h e r v e n t s open, gi-\r.ing a t e s t t h a t may represent t h e house i n actual operation. I l e sccond i s performed with t h e chimney and o t h e r vents blocked,
allow in^
t h e a i r leakage component t h r o u g h t h c ceiling, walls,windows a n d doors t o be e v a l u a t e d . D u r i n g t h e t e s t s , 311 o u f s i d c doors a n d
windows a r e c I o s e d and all insidc doors are open to a l l o w f r e e a i r flow within t h e buildins enclosure.
The t e s t i n v o l v e s depressurizing the housc t o d i f f e r e n t levels h y
v a r y i n g t h e f a n speed w i t h the V a r i a c . At each pressure level, t h e air flow
r a t e i s measured. llouse pressures were v a r i e d between O a n d 100 Pa in t e n
equal increments whenever possible. This r a n g e o f pressures i s s u f f i c i e n t t o e s t a b l i s h a n a i r floiq versus envelope p r e s s u r e differcnce curve for t h e structure.
Several additional measurements a r c r e q u i r c d in o r d e r t o evaIuate t h e test d a t a . It is n e c e s s a r y to know the t o t a l house volramc and t h e cxposcd s u r f a c e area ahnve g r o u n d to determine the relative air tightness of t h e
building. Thcse measurements are taken at t h e a i r harrier boundar>*.
PRESENTATION OF RESULTS
During t h e summer o f 1979, t h e f a n pressurization a p p a r a t u s was
installed in a number of wood frame, single-family r e s i d e n c e s in S a s k a t o o n . The wide variation in thc air flow r a t e s , shown in Figures 4 and 5, indicates t h e e x t r e m e s i n a i r t i g h t n e s s t h a t have been found i n residential
c o n s t r u c t i o n to d a t e .
I n o r d e r to relate t h e f a n pressurization results f o r different s i z c d s t r u c t u r e s , t h e a i r flow parameter i s expressed as a s p e c i f i c a i r flow:
S p e c i f i c a i r flow = air flow ratc/exposed s u r f a c e a r e a
where t h e exposed cnvelope area i s t a k e n a s t h e xis b a r r i e r s u r f a c e a r e a abovc t h e ground l e v e l .
A few examples o f t h e house characteristic curves p l o t t e d from
pressurization d a t a a r e g i v e n i n F i g u r c 4 . It i s a l s o possible to plot t h e pressurization r e s z ~ l t s on a logarithmic s c a l e , as sIlown in Figure 5 .
t h e specific a i r flow and cnvclope pressure, and allows extrapolation of t h e pressurization d a t a with g r e a t e r accuracy.
.4 computer program was w r i t t e n i n HP Basic t o analyze t h e fan
p r e s s ~ l r i z a t i o n d a t a . T h i s program HPRES c a l c u l a t e s t h e c o n s t a n t s f o r t h e equation f o r t h e a i r flow t h r o u g h t h e house as
2 Q = s p e c i f i c air flow ( L / s m )
C = n o z z l e coefficient ( ~ / s m ~ ~ a ~ )
AP = house pressure (Pa) n = exponent
O t h e r information provided b y t h t s program includes t h e correlation coefficient f o r the d a t a and t h e a i s change r a t e s a t 50 Pa h o u s e pressure.
A d e v i c e f o r testing t h e a i r tightness o f h o u s e s has been developed. A fan is used to exhaust a i r from t h e h o u s e and t h e a i r flow r a t e is measured.
The air flow rate and correspond in^ pressure d i f f e r e n c e across t h e building envelope can then be used t o evaluate the relative air tightness of a house
enclosure. ?he f a n pressurization t e s t equipment i s partable, inexpensive, and s i m p l e to operate. With a two-person crew, a complete air t i g h t n e s s t e s t
on a house can be completed i n approximately 45 minutes.
REFERENCES
1. Kumar, R . , Ireson, A . D . , and Orr, H.W. 4n Automated Air Infiltration Measuring System U s i n g SF6 T r a c e r Gas i n Constant Concentration and Decay
blethods. To bc published, ASHRAE Transactions, 1 9 7 9 .
2 . RIomsterberg, A . K . , and Harrje, D . T . , Evaluating Air Infiltration Energy
Losses. ASHRAE J o u r n a l , May 1979, p. 25-32.
3 . Beach, R . K . Nethod o f T e s t for Determining the Rclative Air T i g h t n e s s of a Building Envelope by the Pressure Difference Eicthad. Private
Communication, DBR/NRC, blarch 1 9 7 9 .
4. Kronvall, J . T e s t i n g a f Houses f o r A i r Leakage U s i n g n I'ressure Method.
A S H M E Transactions, C b l . 8 4 , Part 1 , 1978, Papcr No. 2473.
5 . Tamura, G.T. blensurement of Air Leakage Characteristics of !louse
Enclosure. ESIERAE Transactions, Vol. 81, P a r t 2 , 1975, Paper No. 2339. h . ASHRAE Fundamentals Volume, 1977, p . 13-18.
7 Marshall, R . D . Thc bfeasurement o f Wind Loads on a F u l l Scale Mobile llome. N E S I R 77-1289, 1977.
- t Y
m y
E 1 - 3 < < C3 It -- X UO R l F l C E A P . Pa F I G U R E 3 N O Z Z L E C A L I B R A T I O N C U R V E 0 10 20 30 4 0 50 60 70 80 9 0 H O U S E P R E S S U R E , P a F l G U R E 4 S P E C I F 1 C A I R F L O W V 5 M O U S E P R E S S U R E F O R THREE S A S K A T C H E W A N R E S I D E N C E S W I T H C H I M N E Y S A N D V E N T S C L O S E D
H O U S E P R E S S U R E . P a
S P E C I F I C A I R F L O W V S H O U S E P R E S 5 U R E F O R T H R E E S A S K A T C M F \ V A N R E S I D E N C E S V!ITH C H I M N E Y A N D
APPENDIX T
CALIBRATION
CORRELATION
FOR LONG RADIUS FhOlV NOZZLEThe a i r volumc flow rate as measured, uskng a 10 p o i n t p i t o t t u b e
traverse, and t h e corresponding n o z z l e pressure d i f f e r e n c e f o r each flow rate ( F i g u r e 3 ) were analyzed using a log-log linear regression.
The resulting expression i s of the form
log
Q
= a + b l o g A P CA.1)where Q = a i r f l o w r a t e (rnS/s)
A P = nozzle pressure (Pa)
a = abscissa intercept
b = slope of the line
Taking the anti-log o f equation A . 1 results in
The values of t h e constants a and b were determined from t h e
linear regression as a = 0.01473 and b = 0 . 5 0 9 2 . F o r t h i s expression, the
correlation coefficient i s 0.9999.
The ai.r flow through a n o z z l e is given by Bernoulli's theorem as
where
ZAP Q = ACJ
A = nozzle c r o s s sectional area a t the
pressure t a p s (m2)
C = nozzle discharge c o e f f i c i e n t
p = density o f f l u i d (kg/m3]
Substituting t h e physical parameters for t h i s nozzle r e s u l t s in t h e following equation,
An expression f o r t h e discharge coefficient C can be determined by
combining equations 6.2 and A . 4 as
Equation A . 5 can be used to calculate C as a function o f the Reynolds Number a t t h e t h r o a t . This relationship is shown in F i g u r e A . 1 .
D I S C H A R G E C O E F F I C I E N J F O R T H E L O N G R A D I U S NOZZLE
