Methods for estimating air change rates and sizing mechanical ventilation systems for houses

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Methods for estimating air change rates and sizing mechanical

ventilation systems for houses

Ser

m

National Research Conseil national

I*

Canada

Division of Division

des

Building Research recherches en Etiment

Methods for Estimating

Air

Change Rates

and

Sizing

Mechanical

Ventilation

Systems for

Houses

C.Y. Shaw

237

METHODS

ESTIMATING AIR,

RATES

A N D

SIZING

MECHANICAL VENTILATION SYSTEMS FOR HOUSES

ANALYZED

by

C.Y.

Shaw

Building

Division

Building Research

BRN

'ESN 0701-5232

Ottawa, November 1985

TAsLE OF CONTENTS

Page

Estimation of Air Change Rates Without Mechanical Ventilaron

-

-

-

-

-

-

-

-

-

SelectSng Mechanical Ventilation Systems

Discussion

Bef erences

Appendix A: Estimation of Neutral Pressure I e v e l for Houses with C&imneys or Exhaust Devices

Appendix B: Forced Ventilation Rate of an Exhaust-Only

ABSTRACT

This paper presents a s i m p l e method for estimattng the t o t a l air change

rate of a house with or without mechanical ventilation. The proposed method

can be used t o assess the effect: of a mechanical ventilation system on t o t a l

air change rates. It can also be included in existing simple computer

programs for estimating heating requirements for houses.

A calculation procedure is a l s o presented for sizing mechanical

ventilation systems for houses. This procedure can be used to estimate the forced ventilation rate required to achieve the desired t o t a l air change

rate.

Ce document p d s e n t e une d t h o d e d e calcul simple du renouvellement dtair t o t a l drune maison avec ou sans ventilation d c a n i q u e .

La

s u r les taux de renouvellement dlair total et elle peut &tre introduite dans

les programmes de calcul par ordinateur des besoins en chauffage des

maisons

.

Un calcul p e n n e t de dimensionner l'installation de ventilation

mgcanfque. C e t t e d t h o d e peut @ere u t i l l s E e pour calculer la ventilation

A = area of b u i l d i n g envelope, i . e . , area of e x t e r i o r wall above grade and c e i l i n g area of the t o p floor, m2

ach = t o t a l air change rate with mechanical ventllation, ac/h

C = flow coefficient

,

F = correction factor as d e f i n e d in Eq. (10)

g = acceleration of gravity, 9.807 m/s

h = neutral pressure level with chimneys or fans, m

I = a i r infiltration rate, ac/h

K = o v e r a l l f l o w coefficient, L/(s*P~")

M = a i r flow rate through the chimney and/or forced vencPlation rate

(see

91,

n = flow exponent

AP = uniform equivalent pressure difference across b u i l d i n g envelope, Pa q = a i r leakage rate measured by fan pressurization method, ac/h

Q = forced ventilation rate at d e s i g n indoor conditions* ac/h

RE

-

(calculated value

-

At = indoor to outdoor tenperature difference, K

T = absolute air temperature, K

V = building volume including basemeat, m3 U -= on site wind speed, m/s

p = a i r density, kg/m3 Subscripts

a = above neutral pressure Level

b = below neutral pressure l e v e l B = balanced ventilation system

D = design value

E = exhaust only system

i = indoor

lrg = larger value of

IS

t,

o = outdoor s = stack effect

sml = smaller value of I, and sQ = larger value of I, and Q w = wind effect

us

'

A

= mean

INTRODUCTION

The amount of ventilation a i r introduced into a house has a major

influence on its energy consumption, indoor air quality and moisture

problems. In the past, most houses were v e n t i l a t e d by a i r leakage through the house envelope. However, the demand f o r energy conservation i n recent

years has led to the construction of tighter houses where air leakage can no longer be r e l i e d on as the sole source of ventilation air. As a result, mechanical ventilation systems are required in these tight houses.

Because of the lack of g u i d e l i n e s for selecting and designing

mechanical ventilation systems, an increasing number of tightly constructed and mechanically ventilated houses have shown signs of problems with chimney backdrafting, excessive pollutant accumulation from building materials and

furnishings, and elevated humidity levels. Also, occupants sometimes have

turned off the mechanical ventilation system to eliminate noise and cold

drafts. A series of studies has, therefore, been undertaken on four

detached two-storey houses to derive the relationship of air change with

weather factors and the operation of h e a t i n g and mechanical ventilation systems. The results have been reported previously [1,2,3,4].

T h i s paper presents (i) a brief summary of the results, (ii) a

calculation procedure for estimating t o t a l air change rates with or without mechahical ventilation, and (iif) a method for selecting a mechanical

ventilation system that will provide a specified air change rate without causing a r i s k of chimney backdrafting.

ESTIMATION OF AIR CHANGE U T E S WITHOUT MECHANICAL VENTILATION In this paper, the term 'air infiltration rate' as defined by

ASHRAE [ 5 ] , means the uncontrolled inward air leakage through the envelope

of the house caused by weather factors. The t t o t a l a i r change rate' means the t o t a l outdoor air supply rate due to the combined action of a i r

infiltration and mechanical ventilation. The 'forced ventilation rate'

means the nominal air flow rate through a mechanical ventilation system at

the indoor design conditions.

Air infiltration rates depend on the s i z e s and distribution of leakage openings in the envelope (i.e., a i r leakage characteristics), a d the

pressure differences across these openings. As these two parameters cannot be explicitly evaluated, approximations have to be made in developing models

for estimating a i r infitration rates. In a series of studies [1,2,31 i t w a s

assumed that:

(i) the air leakage characteristics of a house could be approximated by two

characteristic values, C and n, according to the relationship:

N O C H I M N E Y OR F A N W I T H A 8 cm E X H A U 5 T V E N T

0 . 8 W l T H A \2.1 c m OPEN C H I M N E Y

AIR LEAKAGE RATE

BY F A N TEST 0 . b - LINEAR ff GlON L 0 . 5 - 0. 4 - VEN1lLAiiol.r

-

WlTH CHIMNEY ROUGH ENVELOPE = X

h ' h

N E U T R A L PRESSURE PLANE H E I G H T A B O V E G R O U N D . m

Figure 1. Supply and exhauat a i i flaw rates versus height above ground l e v e l for a two-8 tore y house

The relationships between this pressure difference (and hence the air

infiltration rate) and the drfving forces have been obtained experimentally [2,3]. They are presented below.

Temperature-Induced Alr Infiltration

Flgure 1 shows the response of the a i r inflows of a two-storey house t o the operation of a chimney and/or an exhaust fan at At=28 K. Twr, different types of chimney were s t u d i e d : a 12.7 cm open chimney of a conventional gas

furnace and a 8 cm wall mounted exhaust vent of a medium efficiency induced draft: gas furnace. It also shows the a i r outflbws through the house

envelope and t h e chimney or the exhaust fan. The a i r E l o w rate through the chimney was 19 L/S and that through the vent was 10 Lls at At-28 K. In addition, Fig- 1 s h o w s the change i n the chimney f l o w of the open chimney with the air flow rate of the exhaust fan. For the neutral pressure l e v e l below the ceiling of the top storey, the a i r Inflow through the house

envelope w a s measured using the tracer gas decay method. Above the ceiling,

it w a s equal to t h e a i r f l o w rate of the exhaust fan. The air flow rates

of a pair of total pressure averaging tubes. The neutral pressure level below the c e i l i n g w a s determined from the pressure differences measured at

several elevations along the exterior walls under calm wind conditions and

that above the ceiling was estimated from the pressure difference across the

envelope ac h f from the equation:

where: h and h' are the neutral pressure l e v e l of the house with and without chimneys or exhaust fans respectively.

As

there. This is because the pressure difference across the ceiling is either

negative or zero. Once the neutral pressure l e v e l i s above the c e i l i n g , the exfiltration through t h e house envelope ceases and the amount of a i r inflow

( a i r change rate) is equal t o the a i r exhaust rate of the exhaust devices. The results i n d i c a t e that as the neutral pressure level is r a i s e d by a chimney or a fan, the a i r inflow increases at three different rates. In the linear region where h i s at or below the c e i l i n g , the a i r infiltration rate

increases l i n e a r l y w i t h h. In the transition region where h is slightly above t h e ceiling and is, therefore, influenced by both t h e exhaust fan and stack effect, a sharp increase in a i r flow occurs due to the increased

pressure difference across the c e i l i n g . And, in the power law region where the pressure difference depends only on the exhaust fan, the a i r inflow varies with the n th power of AP.

Method I, Based on Overall Flow Coefficients

Figure 2 shows t h e measured a i r infiltration rates o f a two-storey house with and without a 12.7 cm open chimney [31. For this particular house, the air infiltration rates have been found to be expressed by the equations:

I = 0.32 C

(A/v)(A~)'

In the above equations, the flow coefficient, C , defines the leakage openings in t h e entire house envelope rather than those experiencing a i r

infiltration only. For the same house, the neutral pressure level is higher

with a chlmney than without. There are therefore more leakage openings

experiencing air infiltration with a chimney than without, even though the

values of C with and without a chimney are about the same. Because of t h i s , Eqs. ( 3 ) and (3a) d i f f e r by a constant. This suggests that for

generalization, an additional parameter should be included to account for

EQ. 3a- u W I T H CHIMNEY z

.

1

p t

&

Q-

-.

A t . I N S I OE-OUTS I DE TEMPERATURE D IFFERENCE, K

Figure 2. Air infiltration rates with and. without chimney as determined by tracer gas decay method (31

s a m e house as w e l l as in different houses. Since the portion of the house. envelope experiencing air infiltration and the pressure difference across i t are proportional to the neutral pressure level, Eqs. (3) and (3a) can be m o d i f i e d to:

I;

c

f(at/e)

-

( U V )

It can be concluded t h a t t h e simplest expression for f(h/H) and f(hr/H) would be (h/H) and (hf/H) respectively. S u b s t i t u t i n g the two variables into Eq. ( 4 ) and evaluating the two constants a and a' from the measured a i r

infiltration values (Fig. 2), Eq. ( 4 ) becomes: 1

: = 0.5

c

where: I = air i n f i l t r a t i o n rate caused by stack effect alone, ac/h,

2

,

n = flow exponent,

At = indoor t o outdoor temperature difference, K, 0.5 = dimensional constant

,

The values of C, n, h t and h can be evaluated as follows. For existing houses, C and n can be measured d i r e c t l y using the fan pressurization

method. h' and h can be determined by measuring the pressure d i f f e r e n c e across the exterior wall at several elevations under no w i n d conditions. For houses under design, C and n can only be estimated from t h e measurements of similar houses. An examination of the results of 160 standard houses and

40 low energy houses, mostly with the chimney s e a l e d , indicated t h a t C varied from 0.014 to 0.36 ~ / ( s - r n ~ - ~ a ~ ) and n varied from 0.6 to 0.72 [6,71. Assumfng a constant n of 0 . 6 5 15,8], the suggested flow coefficients for houses are listed in Table 1. h' i s usually assumed t o be one half the distance between the ground level and the t o p of the envelope and h can b e estimated using the method given in Appendix A.

Method 11, Based on a Reference Air Infiltration Rate

In recent years, advances have been made in tracer gas apparatus and

measuring techdques. It is no longer impractical to request such a test,

when needed. If the a i r i n f i l t r a t i o n rate at one At is known, the values

for other At's can be evaluated from the equation:

where: 1: and h* are the air infiltration rate and neutral pressure l e v e l corresponding t o At*.

Figure 3 shows :I a s a function of C at At*=20 K for houses with and

without chimneys [ 2 , 3 , 9 ] . The linear relationship between I]: and C suggests that Eqs. (ha) and (5) will give similar results. Therefore, the t w o

equations are valid for most houses. However, if a house has been

retrofitted to improve airtightness, Eq. (4a) w i l l no longer be v a l i d and

Eq. (5) should k used. This is because 1; not only depends on the size of t h e leakage openings but a l s o on t h e i r location relative to the neutral pressure l e v e l , whereas C depends only on the size of the leakage o p e d n g s . Therefore, i f the openings near the neutral pressure level have been sealed, a reduction in C but little or no reduction in :1 will result. Thus,

Wind-Induced Air Infiltration

To estimate wind-induced air i n f i l t r a t i o n in additlon to wind speeds, one has t o consider the effects of wind direction and wind shielding. These effects may be approximately accounted for by considering a house to be either exposed to, or shielded from, wind depending on t h e wind direction and the surroundings (21. In a developed residential area, houses are normally surrounded by other houses. These houses would l i k e l y be shielded from wind, unless they have a windward w a l l d i r e c t l y facing an open area such as a long roadway or parkland,

Figure 4 shows the measured air infiltration rates as a function of wind speed for both exposed and shielded conditions [ Z ] . The empirical relationships between the t w o have been found to b& [21:

Shielded

where: I,= the wind induced a i r infiltration rate in ae/h,

W = the on site wind speed in m/s, measured at about 20 m above the ground.

TABLE 1. Suggested Values for Flow Coefficient and Exponent

F ~ O W F1QW

Coefficient C, exponent, Number of Construction Types b t i n g ~ / ( s - r n ~ = ~ a ~ ) n houses

Tight Bungalow Average h o s e Standard Tight 0.08 S p l i t

Level*

54

Low Energy Split Level* Average 0.07

hose 0.12

right 0.03

Two-storey Average 0.15 0.65 15

Loose 0.27

AT.20 K - W 4 3.5 mls NO CHIMNEY W lTH CHIMMY REF. 2, 2-STOREY 0 REF. 10, BUNGALOWS

REF. 2, SPLIT LEVEL

-

Figure 3. Belationship between flow coefficient and air infiltration rate

for AT = 20 K

5 mls 1- 0 0. P 0.8 0.7 LL 0. b

= SWLLER VALUE OF

Is

0.5

I,r9 = U R G E R VAlUE OF

4

Iw

0.4 am 0.10 0.20 0 . 0 0.40 0.50 0.60 a80 1 . 0 ~ * S r n ~ / ' ~ r g % m l Figure 5. F versus

-

Ira

Likewise, if a refere~lce a i r infiltration rate is known, the following equations can be w e d to calculate the values f o r other wind speeds:

Shielded Combined Wind- and Temperature-Induced Air Infiltration

As the pressure d i f f e ~ n c e s due to individual effects are additive, the a i r Infiltration rate caused by the combined action of wind and temperature can be approdmated by an equation similar to Eq. (2):

wheke the paraeter F has been included t o account far t h e Interaction between the pressure differences caused by stack effect and w i d . This is necessary because their effects on air infiltration are not always

independent

[lo].

substituting E q . (8) into Eq. (7) we have:

Using Eqs. (4a) or ( 5 ) and ( 6 ) or (6a) for estimating Is and respectively, and he measured values for

L,,

F factor, as shown in F i g . 5, could be e x p r e s s e d by the equation: for

= 0.8 s m l r g ) O 1 far 0.1 G {Isal/llrg) < 1.0

where: Isml = smaller value of

5

L,

A similar expression ( w i t h o u t the correction f a c t o r , F ) has been proposed by Sherman and Grimsrud [ 5 , 1 1 ] . A s indicated by Eq.

( l o ) ,

only if one of the two weather factors is mch greater than the other and becomes the dominant driving force.

ESTIMATION OF A I R CHANGE RATES WITH HECWICAL VENTILATION

Mechanfcal Ventilation Systems

Figures 6 and 7 show the four mechanical ventilation systems s t u d i e d ( 4 1 . They can be c l a s s i f i e d as either 'balanced' systems or

exhaust-only systems, A balanced system c o n s i s t s of a supply f a n , which draws the outdoor a i r Into the house, and an exhaust f a n , which exhausts an equal amount of indoor a i r to the outdoors. In a c t u a l installations, the s u p p l y and the exhaust flow rates are rarely equal because no attempt is made t o adjust the supply a i r rate to account f o r varying outdoor air d e n s i t y . If the two flow rates are equal under summer conditions, the supply air flow can be as much as 20% greater than t h e exhaust air flow under winter conditions ( F i g . 6a).

An exhaust-only system consists of an exhaust fan. The air supply

flows through the leakage openings in the envelope as a result of the pressure difference created mainly by the mechanical exhaust.

Balanced Systems I and I1

Figures 6a and 6 b show the effect of ambient temperature and wind speed on total air change rates for the t w o balanced system each w i t h a nominal forced v e n t i l a t f o n rate of 0.5 ac/h [4]. As shown, the t o t a l a i r change rate increases somewhat as the ambient temperature decreased (Fig, 6a), but is relatively insensitive t o wind speed ( F i g . 6b).

The house ventilation a i r comes from two sources: a i r infiltration, I ,

and mechanical ventilation, Q. Since the operation of a balanced v e n t i l a t i o n system appears t o have little effect on the house p r e s s u r e

SUPPLY

p-

"

2

%

1

---

---

--

,

,

,

,

, {

to. OUTDOOR A I R TEMPERATURE. "C

W. WIND SEED, kmlh Q t BALANCED SYSTEMS

'

-

I

-

EXTERlOR WALL, Pa

Figure 6. Tatal air change rate and pressure difference profile of balanced systems [ 4 ]

Figure 8 shows a co~qarison between the calculated and measured air change

rates. It indicates that Ep. (11) coasistently overestimates the air change

rate by as much as 25%. The reasons for t h i s are not c o m p l e t e l y known* Bowever, Fig. 6c shows that the operation of the balanced system d i d lower the neutral pressure level slightly, which, in curn, reduced the air

infiltration into the house. Therefore, the air i n f % l t r a t i c m rate with mechanical ventilation was slightly less than that: without. An attempt was

then w d e t o combine the two flows u s h g an expression similar t o Eq. ( 9 ) . Since Fig. 6a, which shows the estimated increase in the supply a i r flow as

a function of to, suggests that by applying a temperature correctton on Q to

account for varying outdoor air density, the dependence of total air change rates on c, can be reduced. Furthermore, because t h e forced ventilation rate should be much greater than the air infiltration rate, F is close to unity. Thus, Eq. (9) becomes:

where achl is the t o t a l a i r change rate caused by the combined a c t l o o af a i r

inffltration and balanced meehanlcal ventilation, and Q is the forced

ventilation rate at Ti= To. It will be shown that Eq. (12) will give a

- -- --- - - --- o I-a CAN EXHAUST-ONLY SYSTEMS l 1 l ~ l j l I I l I l [ I -

-

m

F i g u r e 7. T o t a l air change rate versus temperature difference for exhaust-only systems ( 4 1

E x h a u s t ~ c r l y Systems '111 and

IV

Figures 7a and 7b show the relationship between house a i r change rates and forced ventilation rates for the two exhaust-only systems [4]. The results indicate that t h e total a i r change rate was greater than the corresponding forced ventilation rate when Q = 0.25 acfh, but w a s

approximately equal when Q = 0.5 ac/h. This i n d i c a t e s t h a t as the forced ventilation rate increases, the neutral pressure l e v e l moves upwards and the area of building envelope experiencing air exfiltration decreases.

As the effect of an exhaust fan on a i r change rates i s similar t o that of a chimney the same linear relationship e x i s t s between a i r change rate and neutral pressure l e v e l , An expression of the form of Eqs. (ha) and ( 9 )

should, therefore, be v a l i d for t h e exhaust only system. Furthermore, since the estimated house a i r change rate should never be smaller than Q , I in Eq. (9) must be r e p l a c e d by Is

,

(t

Figure 8, Comparison between the measured air change rate and rate calculated by Eq. (11)

equal to the forced ventilation rate. Thus,

I/n + <ln)n

achg = P

(Is.

for

where achg is the t o t a l air: change rate with exhaust-ly systems.

SIZING P I E C U N I W VJ3NTILATION SYSTEMS

The forced ventilation rate of a mechanical ventilation system should

be determined on the basis of t h e required ventilation rate and the

air

infiltration rate. However, in practice air i n f i l t r a t i o n is rarely

considered because 5 t is difficult to estfmate, As a resulr, the t o t a l air change rate under the combination of mechanical ventilation and a i r

infiltration often exceeda the required ventilation rate, causing an unnecessary increase in energy consumption. The energy consequence is not serious under m i l d weather conditions because both temperature differences and a i r infiltration rates are small. This is not the case under winter conditions. For a house w i t h an average air i n f f l t r a t i o n rate of 0.26 aclh in winter, Fig. 4a shows that the total air change rate ~ 5 t h a balanced

system can exceed the forced ventilation rate of 0 . 5 aclh by as much as 40%

On one hand, to satisfy the ventilation requirement, a mechanical ventilation s y s t e m should always be capable of delivering the design ventilation rate. On the other hand, to conserve energy, it should be

operated under a reduced flow during the winter months to take advantage of

the increased air infiltration. Because of the frequent opening of windows

in non-air conditioned houses, the need for mechanical ventilation is much

less during m i l d weather conditions than during the winter months. Thus mechanical ventilation s y s t e m s could be equipped with a flow controller such as a two-speed fan, and operated continuously with a reduced capacity

suitable for the winter months. A manual switch, an outdoor temperature

controller or an indoor humidistat can be used to increase the flow to the design value, as required. The forced ventilation rate for tdnter operation

can be determined on the basis of the deslgn ventilation r a t e and the mean a i r f n f i l t r a t i o r t rate for the winter months. These t w o parameters are estimated below.

Design Ventilation Rate

For houses, ASHRAE Standard 6 2 1981, "Ventilation for Acceptable Indoor Air Q u a l i t y " , c a l l s for 5

L/s

room, aa additional 50 L / s intermfttent air supply for kitchens and a 25 L / s intermittent a i r supply for each bathroom [121- For a t y p i c a l 3 or 4

bedroom house, this roughly translates into an outdoor a i r supply rate of

0.5 aclh. Thus, the design a i r change rate, achD, can be based on the

ASHME Standard: 0.5 ac/h is a reasonable value for t y p i c a l houses.

Mean Air Infiltration U t e For Winter Months

The a i r infiltration rate of a house varies with weather conditions. However, for temperature differences greater than 20 K and wind speeds ranging from 3.5 to 10 m/s ( t y p i c a l w i n t e r conditions for many regions of

Canada) the average air infiltration rate can be estimated by t h e equatlon [21:

Forced Ventilation Elate For Continuous Operation

Balanced Ventilatfon Systems

Based on air quality considerations, it is advisable t o estimate the forced ventilation rate without taking varying outdoor a i r density into

account. This will give a slightly greater forced ventilation rate than

with air density correction. Substituting achD and

\,

Exhaust Only Systems

The exhaust f l o w rate can be estimated from Eq. ( ~ 2 ) derived in Appendix B:

YI

300

-

W BY FAN TEST C a Il= - 3 200 - 0 -I IJ- &

-

NEUTRAL PRESSURE PLANE H E I G H T A B O V E GROUND. m

Figure 9. Effect of exhaust: fan on the venting performance of an open chimey

lrais equation indicates that Q is indaterminate if (achD/fwe) is greater than (Hlh'). lhis condition occurs when the forced ventilation rate is sufficient to raise the neutral pressure l e v e l above the ceiling l e v e l of

the top storey. Under t h i s condition, the design forced ventilation rate is equal t o the d e s i g n air change rate, i.e., Q = achD.

SELECTING MECHANICAL VENTIEATION SYSTEHS

For houses where there might be sources of radon or other contadnants

in the building structure, a balanced system is recommended. Otherwise, the

choice between a balanced system and an exhaustanly system should be determined on such factors as a i r leakage characteristics, type of heating systems and cost. In general, if the operation of an exhaust-only system will not cause chimney backdraft it should be used instead of a more expensive balanced system.

Figure 9 shows the effect of an exhaust fan on the venting capacity of a 12.7 cm open chimuey in a tightly built two-storey house (see also

F i g . 1). The chimney f l o w rates w e r e measured with the chimney draft f u l l y established before the exhaust fan w a s turned on. To detect the s p i l l a g e of flue gases, the concentration of CO was monitored in the basement near the furnace+ As shown, the chimney flow decreased continuously as the air flow rate through the exhaust fan increased. The chimney backflow occurred when

the neutral pressure level reached 35 m above the ground l e v e l . The

corresponding pressure difference across the envelope was 35 Pa and the a i r exhaust rate was 310 Lfs. The presence of CU w a s detected when the chimney f l o w had been reduced to a b w t 14 L / s . Tbis suggests that s p i l l a g e of the flue gas occurs well before the chirrmey flow is zero.

It should he pointed out that the cxhaus t fan was turned oa after the chimney d r a f t was fully e s t a b l i s h e d . If it was operating before the furnace

w a s on, the chimney would fill with the c o l d outdoor air and lose i t s draft. Under this condition, the chimney would continue t o back flow even though the exhaust fan flow drops to a value w e l l below what t a needed to initiate the back flow.

For housee w i t h chfnmeys, the stack effect reinforced by the a i r f l o w

through the chimey is usually lmch stronger than the wind effect during the winter months [31. The backflow w i l l not likely occur as long as the

neutral pressure level is below the ceiling l e v e l of the top storey. The c e i l i n g l e v e l , therefore, would be a reasonable choice as the limit for

sstisfactary chimuey startup.

With the limit for satisfactory chimep startup established, the interaction between an exhaust-only system and a chimep can be determined

using an air f l o w vs neutral pressure level graph similar t o Fig. 1. Such a graph can be constructed for any housee If

,

:

I

that :1 and h t are the a i r infiltration rate and the neutral pressure l e v e l of the house without chimneys or exhaust fans. For existing houses, these

parameters can be measured. For houses under construction, the values of

h' and n are normally assumzd to be H i 2 and 0.65 respectively. The value of 1: can be obtained from Fig. 3. To construct such a graph a reference point A representing I$ is plotted on a graph with a i r f l o w rate as the

ordinate and neutral pressure level as the abscissa. The air infiltration

l i n e is p l o t t e d next by drawing a straight line through A and the origLn, and extending it t o EI. Since at h r a i r infiltration rate is equal t o a t r e d i l t r a t i o n rate, the emrfiltration line should a l s o go through Point A. To

f i n d another point on the exfiltration line, a value for h between h' and H is selected and the air exhaust rate correspondgng to b 5s calculated f r o m the equation b a l m . Tfiis equation w a s obtained from Eq, (A8) by replacing

qs

The e d i l t r a t i o n rate corresponding t o h (Point 8) is then obtained by subtracting Q from the air infiltration rate. The exfiltration line is obtained by connecting a stralght line through A and B. And a liae

representing 'infiltration

-

house has a chfmuey the neutral pressure level correspcmdfng t o the chimney can be calculated' from Eq. (16). Finally, a horizontal line representing achD is p l o t t e d

on

if

line

infiltration line below H, the chimney will start to vent when the furnace starts. On .the other hand, if t h e achD l i n e intercepts the infiltration line above H, chimney backdraft w i l l occur,

Two such graphs are shown on F i g . 10, one for a tight house and the other an average house. The values of

LE

respectfvely. The values of h m and n are a s s m d t o be 2.7 m and 0.65

respectively. The results indicate that an exhaust-only system works

satisfactorily in an 'averaget house with an open chimney but it w f l l cause

chimney backdraft in a tight house unless an outdoor air supply is provided

L a the envelope. On the other hand, the operation of an exhaust fan will significantly reduce the amount of inside a i r leaving the house through the walls and ceiling. If a house has no chimney (egg., an electrically heated house) an e h u s t - o n l y system w f l l , also be satisfactory and will reduce the

-

-

1

-

-

-

-

5

*

I

NEUTRAL P R E S S U R E PLANE H E l G H T A B O V E GROUND, m

Figure 10. Air flow versus neutral pressure l e v e l diagram f o r selecting ventflation systems

Equation 9 gives the air infzltration rate due to the combined action of stack effect and wind,

It

0. 8 I I _{1 I} LINE O F AGREEMENT 0 . 7 - 0 . 6 - 0 . 5 - 0 . 4

-

-

-

-

Figure 11. Gmparison of calculated and measured air infiltration r a t e s for houses T1 and T2

1. 0 O

m- . o o MEAN --

-0.4 a S T D . DEV. = 0.1535

Figure 12. Relative error versus measured a i r infiltration rates for houses T1 and T2

systematic difference between the c a l c u l a t e d and measured a i r tnfiltration rates, the data have been transformedto the form,

RE = (calculated value

-

/

Figure 12 shows the relative error, EtE, as a function af the measured a i r infiltration rate. The mean value of RE i s 0.037 and the standard deviation is 0.153. A student 't' test (t value is 0.24) indicates that the difference berween the mean and the ideal man of 0 is nut statistically significant, In other words, there is no systematic difference hetween the calculated values given by Eq. 9 and the measured air infiltration rates for the two houses.

S i d i a r comparisons have been made in Figs. 13 and 14 for the two

balanced systems and the two exhaustwnly system. Figure 13 shows that the agreement' between the calculated and measured air change rates is withtn f20% of the line of agreement for the balanced system. The agreement for the exhaust only -system Xs 215% {Fig, 14). Again there is no systematic difference between the calculat.ed and the measured air change rates.

A s i m p l e calculation procedure has been proposed f o r estimating the air change rate of a house u i t h or without mechantcal ventilation. This

procedure can be s ~ r i e e d as follows.

I) Prepare input data, C, a, A, V, h a , h, Tt, To, W and Q where C and n

can be measured directly or estimated from Table I ; A and V can be measured d i r e c t l y or estimated from the blue prints, hl,

h

also be measured or estimated using the merhad given in Appendices A and B. To, Ti and W can be o b t a i n e d from meteorological records for the

area.

(ii) Calculate the

air

Wfth Chimney

0 . 7 - S Y S T E M I o S Y S T E M

JI

,=

-

-

Figure 13. Comparison of calculated and measured a i r change rates for systems I and I1

S Y S T E M III

o S Y S T E M

IP

Figure 14. Comparison of calculated and measured a i r change rates for systems I11 and IV

(iii) Calculate the a i r inffltration rate due to wind alone from the equations: Exposed Shielded Exposed S h i e l d e d

<iv) Calculate the air infiltration rate due t o t h e combined action of

stack effect and wind from the equation:

where:

(v) Calculate the a i r change rate due to the combined action of a i r infiltration and mechanical ventilation from the equations:

and Exhaus t-Only Systems:

+ ) Far h C 1

achE = F (

ISp

=

Q

The proposed method has been checked against measured afr change

rates. The agreement, ia general, was within 25% of t h e measured values. In addition, the foll&ng,two equations are proposed for sizing mechanical ventilation systems for houses.

Balaneed Systems: Q

=

-

Q =

Ls

({

-

1")

G,

REFERENCES

I. S a w , C.Y. and Tamura, G.T., "Mark XI Energy Besearch Project:

Airtightness and Air l n f l l t r a t i o n Measurements", Dfvision of Building Research, HRCC, BR Note 162, 1980.

2. Shaw, C.Y., "A Correlation Between Air I n f i l t r a t i o n and Urtightness for Houses in a Developed Residential Area", ASH= Transactions,

V o l . 87, Part 2, 1981.

3. Shaw, C.Y. and Brown,

W.C.,

3rd AIC Cwference, bndon, U.K., 1982.

4. Shaw, C.P., "The Effect of Mechanical Ventilatfan on t h e Air Leakage Characteristic of a Two-Storey Detached Houee", Dtvfsion of Building Research, WRCC, BR Mote 204, 1983.

5, ASILRdE Handbook, " F u n d ~ n t a l s " , 1981,

6. Beach, R.K., "Relative Tightness of New Housing in the Ottawa Area", Division of Building Research, WBCC, BR Note 169, 1979.

7. Dumont, R.S,, Orr, H.W. and Ffgley, D.A., "Airtightness ~asuremeuts of Detached b u s e s in the Saskaroon Area", M v i s i a d of Building Research,

NICCC, BE Note 178, 1981.

8. Tamura, G.T., "The Calculation of House Infiltration Rates", ASHRAE Transactions, Vol. 85, Part '1, 1979.

9, b n l e y , P.J., Helmeste,

R.H.

10, S i d e n , F. W,

,

11. Sherman,

M.H.

D.T.,

12. ASHIRAE Standard 62 1981, "Ventilation for Acceptable Indoor Air Quality", 1981.

13. Shaw, C.Y. and Kim, A., "Performance of Passive Ventilation Systems in a Two-Storey House", Proceedings, 5th AIC Conference, Reno, Nevada, USA, 1984,

APPENDIX A

ESTIMATION OF NEUTRAL PRXSSURE LEVEL FOR HOUSES WITH CHIMNEYS OR EXHAUST DEVICES

a p;

-

NmTRAL PRESWRE LEVEL WITH CfllMHEY OR FAN

2 n d FLOOR NEUTRAL fXESSURE LEVEL

WITHOUT C H I W E Y OR FAN I r t FLOOR -- - - - I I///// - 6 - 4 - 2 0 2 4 6 A P . P R E S S U R E D l # F E R E H C E , Pa K b t K , ' C ' A

-

Figure A l . Distribution of leakage openings and pressure difference profile caused by temperature difference

If a house has a chimney or an exhaustanly ventilation system, the neutral pressure l e v e l , as shown in Fig. A l , w i l l s h i f t from h t t o h

.

known,

mS8 [1.3]:

%

Ebi

(All

Substituting the following relationships (from Fig. A l ) into Eq. ( A l l : APb = (h / h t ) APL

,

AP, = [ ( B

-

-

h

Q

F i g u r e A2,

-

-

hl

t,

In the above equation, the terms Ka and

K,,

characteristic of the house without either chimney or exhaust device. They

can be estfmated f r o m the mass balance equations [ 1 3 ] :

and

C*A = Kb

+

where C is the f l o w coefficient without chimneys ox exhaust f a n . Since:

AP; = [ ( H

-

a.

Kb = K,[(H

-

I " ( P ~ / P , )

Eguation (A6) can be solved for h/h' and H/h' as follows. (i) Existing Houses

The values of C, APL, h f and n can be measured and K, can be estimated

from E q s . (Ah) and (AS). The only unknown on the left hand s i d e of E q . ( A 6 1

Appendix 8). The value of M for a chlmney can be estimated using the method

given in the ASHRAE Handbook 114.J.

With all the variables on the left hand side of Eq. (A6) known, the value of (h/hl) can be obtained by graphical methods. Figure A2 shows a t y p i c a l plot of ( h f h t ) vs M / [ K ~ ( A ~ ) ] " for n = 0.65 and various values of H/hB. Similar p l o t s can be constructed f a r other values of

n.

(ii) b u s e s Dnder Design

For houses under design, the values of n and h' are usually assumed t o

be 0.65 and E l 2 respectively. The fbllowing approxinsitirrns would also

a p p l y

and

Replacing M with Q (i.e., converting the unit of M from L / s t o ac/h), Eq. (A6) becomes:

APPENDIX B

FORCED VENTILATION

RATE

The design exhaust a i r flow rate for winter operation can be estimated from Eq. (A6):

To s o l v e for M, one has to know (h/hl) and K.(AP~)". These two terms can be estimated as follows,

Consider a house with an average air i n f i l t r a t i o n rate of

,

,

i

noting that

Substftuting Eqs. ( B l ) and ( A 9 ) into Eq. ( A 6 ) and replacing M with Q ( i . e . , converting the unit of M f r o m L/s to ac/h), we have