Carbon dioxide measurement in open-classroom school with outside air-supply damper closed to conserve energy

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Carbon dioxide measurement in open-classroom school with outside

air-supply damper closed to conserve energy

2 0 1 8 1

CARBON DIOXIDE MEASUREMENT IN OPEN-CLASSROOM SGHOCL

WITH

OUTSIDE

AIR-SUPPLY DAMPER CLOSED TO CONSERVE ENERGY

G.T. Tamura

ANALYZED ABSTRACT

Carbon dioxide concentrations

in

the classrooms of

a small elementary school were measured one week in

February 1980

when

the o u t s i d e air-supply damper was

closed. They were found to be well below the maximum accepted occupationaL standard

of

0-50 per

cent.

The

leakage air through rhe closed damper and air i n f i l t r a t i o n through t h e building fabric provided about 4.7 L/s per person, equal to t h e _{minimum value given in the ASFIRAE} Ventilation Standmd 62-73.

The

cemparison of the annual energy consumption data before and a f t e r operating t h e ventilation system with the o u t s i d e dampers closed

during t h e heating season indicated a substantial saving

in

fuel o i l .

The buildup o f contaminants in t h e air

in

buildings is usually con- trolled by providing a continuous supply of lTfresh" a i r and exhausting an equal amount of " u s e d l b i r from t h e building. This supply of f r e s h air has to be heated in cold weather and thus contributes t o the heating load of the building. There i s an fncentive, therefore, to use only as much

fresh

a i r as is actually needed t o control the buildup of contaminants.

A recent study of a h i g h school in California (1) found t h a t reducing t h e ventilation

rare

from 6 . 3 L / s to 0 . 7 2 LJs per person resulted in no adverse changes

in

the level of

air

contaminants measured codour, ozone, colony

forming

particles, nitrogen oxides) except for carbon d i o x i d e , which increased but still remained below 0.20 per cent. In some buildings, ventilation systems a r e being operated with t h e o u t s i d e a i r supply dampers

completely closed during t h e w i n t e r , r e l y i n g on a i r leakage through the closed dampers and through the building envelope for outdoor a i r supply.

T h e Carleton Board of Education is operating some of t h e i r schools w i t h the fresh a i r dampers closed

in

cold weather. The Board was willing

to cooperate w i t h DBRINRC in measuring t h e conditions that prevailed in one of these schools. Tests were conducted in the Greely Public School (Figure 11, an open p l a n building located near Ottawa. The building is heated with an all air recirculating heating system with variable f r e s h air intake. Previous t o 1978, t h e outdoor air damper was kept at a

minimum open position

unless

the mixed a i r temperature o f t h e ventilation system rose above 2 0 ° C . The flaw rate of outside a i r a t t h e minimum open position was n o t known. During and after 1978, the control system was altered to close the outside air-supply dampers when the outside air

t e m p e r a t u ~ e dropped below 1.6"C and to operate normally above this temperature.

The purpose o f the t e s t was ta obtain concentration p r o f i l e s o f C02

i n the school generated by the occupants through r e s p i r a t i o n while t h e building was b e i n g operated with the outside air-supply dampers closed d u r i n g winter. The concentration of oxygen was not measured for i t s depletion because the requirement for the amount of fresh air necessary f o r proper physiological functions i s much less than t h a t required to limit t h e concentration of carbon dioxide. The measurements obtained in February o f 1980 are hereZn reported.

MEASUREMENTS

The f l o o r plan of The Greely Public School, constructed i n 1971, is shown in F i g u r e 2. It is a single storey building w i t h open-plan class- rooms serving s t u d e n t s from kindergarten to Grade 5 ranging in age from 5 to II years. The floor area, excluding the gymnasium and based on t h e inside dimension, is 1440 m2 and the volume

is

5,700 m3; those of the gymnasium are 217 m2 _{and 1,390 m3. The total population of the school i s} 11 a d u l t s and 162 students of which 2 3 are

in

the kindergarten c l a s s and a t t e n d the morning session only. The main teaching areas are the north and soush classrooms connected by a l i b r a r y reference room.

Except f o r the hot water convector units in the vestibule entrances and an electrical resistance heater

in

one office, the school is served by two all a i r systems; one for the classrooms, library, offices and o t h e r rooms, and t h e o t h e r f o r t h e gymnasium. T h e h o t water for The main and reheat coils i s heated by a boiler fired with No. 2 fuel o i l . Because

of malfunction, The humidifier in the ventilation system was n o t in operation during t h e heating season. The washroom and other exhaust fans are t u n e d on at about 8:15 a.m. and switched o f f at about 4:30 p.m.

A i r

sampling

taps were installed

in

t h e branch d u c t of t h e return air grill of the south and n o r t h classrooms ( F i g u r e 33 and i n the m i i n r e t u r n and supply air ducts. These taps were connected by 0.60 cm plastic

tubing to t f ~ c sampling s c n n n c r o f tlie C02 recording ctluil~mcnt [T;i gurc 4 1

l o c a t e d i n thc s o u t h classroom. The c o n c e n t r a t i o n o f c a r l ~ o n dioxide a t each sampling t a p was measured with a nondispersive type i n f r a r e d gas

analyzer and recorded on a s t r i p chart recorder. The analyzer was zeroed with nitrogen gas and c a l i b r a t e d with a gas of known concentration

(0.396 p e r c e n t ) once each day. The C02 concentration was monitored f a r one week with the outside air-supply damper closed.

To

compare t h e carbon d i o x i d e concentration in t h e room and i n t h e branch return air duct, air samples were taken i n t h e s o u t h classroom at 0 . 9 1 , 1.5 and

2.1

_{m above} f l o o r level and 5.5 rn away from the return air grill. The rate o f outside a i r i n t a k e w a s calculated from air velocity readings in t h e supply air d u c t taken w i t h a h o t wire anemometer. The inside temperature and relative humidity were measured w i t h themohygrographs placed in t h e north and s o u t h classrooms.

RESULTS AND DISCUSSIONS

The C02 concentration profiles i n the building for a typical day w i t h t h e outside air-supply damper closed a r e shown

in

F i g u r e 5 . The o u t s i d e air-supply r a t e measured w i t h a h o t wire anemometer was 344 L J s _{or 2.0 L / s}

per person which was about 10 per cent of that with t h e outside damper wide open, The initial concentration at t h e start of the school day was

0.036 p e r cent. The C 0 2 concentration i n t h e main r e t u r n air duct, w i t h hranch a i r d u c t s connected t o t h e various occupied spaces, can be con- sidered

as

the average concentration in t h e school. The concentration in t h e branch r e t u r n a i r d u c t of t h e south classroom w i t h older students

(Grades 4 and 53 was generally higher t h a n t h a t of the

main

return air, whereas t h e concentration of t h e n o r t h classroom with younger students

fluctuated above and below t h e return a i r concentration. The concentration in t h e air-supply d u c t diluted b y t h e intake o f o u t d o o r air was less than that

of t h e return air. The e f f e c t of students leaving the school during recesscs and lunch p e r i o d s is evident from t h e d i p in the concentration curves caused by t h e r e d u c t i o n i n the i n d o o r generation of C02 and in- creased infiltration rate w i t h t r a f f i c through t h e entrance d o o r s . A l s o , the space volume effect is evident by t h e buildup of C02 d u r i n g t h e e a r l y p a ~ t of the day. The comparison of t h e C02 concentrations in t h e s o u t h classroom at 0 . 9 1 , 1 . 5 and 2.1 rn above floor level a n d in t h e branch return a i r duct to that room were found to be about t h e same. T h i s indicated a h i g h degree o f mixing in t h e room.

Table 1 gives the maximum and average concentrations of carbon d i o x i d e

recorded during t h e t e s t period. W i t h t h e outside air-supply damper closed,

t h e leakage flow through the dampcr varied from 270 to 370 L / s . The

maximum COP _{concentration of 0 . 1 2 per cent was recorded i n t h e south class-}

room and 0.096 p e r cent in the main return air duct. The corsesponding average values were 0.090 and 0.078 per cent respectively. These values

The dilution equation applicable to t h e changes in t l ~ c concentmtion

of t h e carbon dioxide generated i n an occupied space is:

C = carbon d i o x i d e concentration in an occupied space a t time, t, p e r c e n t concentration by volume of GO2 in air

C = carbon d i o x i d e concentration at t=O, percent concentratien by O

v a l m e of C02 in air

C = o u t s i d e carbon dioxide concentration, percent concentration by

S

volume of C 0 2 in air

G

= volume generation r a t e of carbon dioxide, m3/5

K = _{mixing factor (K=1 is p e r f e c t mixing and}

_K=O

_{is non-mixing)}

9

i

= volumetric flow o f infiltration a i r

,

m3/s

volumetric flow o f outside a i r through t h e ventilation sysxem,

'

= m 3 / s

t = time, see

V = volume of space, m 3

A t steady state, i . e . , when t =

If ths generation rate i s zero t h e n t h e decay o f carbon, d i o x i d e w i t h

time gives ,

where

9

i s t h e air change rate p e r unit t i m e .

A similar expression applies for any other contaminants n o t absorbed by filters, structures and f u r n i s h i n g s o r those t h a t do not s e t t l e as

particulates.

The

air

change rates were calculated from the decay

curve

of carbon d i o x i d e of t h e main r e t u r n a i r duct recorded after t h e students l e f t t h e school at 3:00 p.m. The values given in Table I 1 were calculated from Eq. (39 assuming a value of mixing factor K = 1 (perfect mixing]. With t h e outdoor air-supply damper closed, the air change rates

(air

leakage through closed damper and building fabric) varied from 0 . 5 0 to 0 . 6 6 air change p e r hour corresponding to 795 to 1050 L l s (4.60 20 4.0 L / s per person], which was much higher than the outdoor air-supply r a t e , through a closed damper, o f 270 to 370

L/s.

The difference between the a i r change rates as determined from t h e C02 decay curve and the outdoor supply air rate through t h e closed damper can be attributed to air infiltration through t h e building f a b r i c induced by temperature difference and wind forces and the operation of the washroom and other exhaust systems. Assuming that the rates af a i r infiltration and outdoor air-supply are additive,

(outdoor air-supply r a t e and exhaust rate o f the ventilation system are

balanced), the air infiltration rates varied from 0.29 to 0 . 4 9 a i r changes per hour; t h e average value was 0.35.

The carbon dioxide concentrations recorded in the supply and return air d u c t s

OE

the ventilation system serving the gymnasium are shown in F i g u r e 6 . Measurements w e r e made f o r one day only with the outside a h - supply damper closed. The rate of leakage flow through the closed damper could no2 be measured. The air was supplied to the gymnasium from

overhead d i f f u s e r s and exhausted f r o m t h e return air grilles located just above f l o o r level. Maximum concentrations were below 0.10 p e r cent.

Figure 7 shows the outdoor and indoor humidity ratios with t h e damper closed for s i x consecutive days, during which time the indoor humidity r a t i o remained wcll above t h e outdoor humidity r a t i o at a l l times, appar-

ently caused by t h e moisture generated by rhe school occupants being absorbed by the building structure and furnishingsand released from them d u r i n g unoccupied periods. In c o n t r a s t , the C02 concentration had

decreased t o t h e _{outdoor l e v e l by midnight of each school day.} The indoor relative humidity varied from 20 to 3 0 per cent during the t e s t p e r i o d .

The oil consumptions expressed in MJ/rn2 and corrected to the avcragc

Ottawa degree days of: 4780°C and electricity consumption for the ca1end:lr years o f 1977, 1978 and 1979 were:

Fuel Oil Electricity

The change in t h e operation of the v e n t i l a t i o n system was made in I978 when t h e outside air-supply damper was s c h e d u l e d to c l o s e below an o u t - s i d e temperature o f l.GOc. Comparison of the 1977 and 1979 f i g u r e s indicates a 36 per c e n t reduction (27,400 I,) in o i l consumption w l t h t h e

outside air-supply damper closed during c o l d w e a t h e r . The electricity consumption f o r t h e t h r e e y e a r s remained about the same. No o t h e r cncrgy saving measures a r e known t o have been made s i n c e 1978 except f o r improving

t h e efficiency of t h e two boilers by about Z06 each in 1979.

Where the outdoor air-supply r a t e cannot be measured with any precision, and the infiltration rate is not known, measurement

ef

C02 in a space occupied by non-smokers o f various age groups and activities can be used to adjust t h e outdoor air-supply damper to a suitable minimum p o s i t i o n and to demonstrate t h e need t o tighten up t h e building envelope in order to reduce air infiltration. More information, however, is required in r e l a t i n g c o n c e n t r a t i o n o f COT generated b y occupants to t h e odour l e v e l in a room. For spaces occupied by smokers, other air

contaminants, as well as C02, should be measured. It should be pointed

out t h a t C02 measurement r e l a t e s only to air contaminants generated by occupants and does n o t i n d i c a t e levels of a i r contaminants generated by b u i l d i n g components, furnishings and processes-

SUMMARY

CQ2 concentrations measured in _{the test school w h i l e t h e outside} air-supply dampe~ was closed indicated t h a t t h e maximum v a l u e o f C02 r e c o r d e d in t h e classroom during t h e test p e r i o d was 0.12 per cent and the corresponding maximum average value of 0.091 per cent. These a r e well

below the acceptable occupational limit of 0 . 5 0 per cent. The leakage flow through t h e closed damper and air infiltration through the building f a b r i c provided outdoor air to t h e occupants at a rate of about 4.7 L / s p e r

person, equal to t h e minimum value given in t h e ASHRAE Ventilation Standard 6 2 - 7 3 [ 2 ) for classrooms. The fuel consumption data indicated t h a t closing t h e damper during t h e heating season resulted i n a sub- stantial s a v i n g of fuel for t h i s school.

ACKNOWLEDGEMENT

The author is indebted to the Carleton Board of Education f o r

permission to carry out this study and to t h e staff of the Greely Public School for t h e i r assistance d u r i n g the tcst. He a l s o gratefully acknowledges t h e contribution o f L. Jones in discussions on thc energy consumption o f

t h e t e s t s c h o o l and for reviewing t h i s paper, the a s s i s t a n c e o f R.G. Evans and R. Adm in the f i e l d t e s t s , and of G . L . Johnson i n carrying out t r i a l computer calculation of h e a t i n g loads.

REFERENCES

1. J.V.

Berk, C.D.

Hollowell, C. Lin and 1 .

Turfel,

The Effect of

Reduced

Ventilation

on Indoor A i r Quality and Energy Use

in

Schools.

LBL-9382, June, 1979.

nnr-nn m l n

_{. - . * .}

a - 1 ~ OP- m a - .+ 4 U V U U U . - . r l d d P d 4 U U U U U Q O O O O L O t b b t r ) m t J - # m w m h k

Figure I Test school 9

(

NORTH C L A S S R O O M

)

+ SOUTH C L A S S R O O M

P"

-

1

F l p r e 2 Floor plan

A . . A

F i g u r e 3 South classroom

z S O U T H C L A S S K O O M 0. h Z - C Q w NORTH C L A S S R O O M

2

0 . 1 0 W C) 2 MAIM R E T U R N A I R 0 u 0 . 0 8 LL1 a - X a 0 . 0 6 - n z 0 0.04 m E < 0 0.02 a 7 A.M. 8 9 I0 11 12 13 1 4 1 5 1 6 17 18 19 0 . 16 TIME. h 0.14 r R

F i g u r e 5 C a ~ b o n dioxide concentration vs time, outdoor supply-air damper closed (730 cfm)

I I I I I I I I I I 1 - DATE - F E B R U A R Y 14. 1980 - 0.18 0. 14 PATE - MARCH 3 , 1980 M O N D A Y EF z 6.12

-

C Q E b a. l o W L) Z 0 u 0.08 R E T U R N A I R w 0 - X 0 - D b - a S U P P L Y A I R Z 0 . 0 4 m lx 4 U 0 . 0 2

a

7 A.M. 8 9 10 I 1 1 2 13 14 1 5 1 6 17 18 1 9

Figure 6 Carbon d i o x i d e concentration in gymnasiarn. outdoor supply-air damper c l o s e d

D.005

-

I I 1 UC 0 0.004

-

x S O U T H C L A S S R O O M

-

_m 3 H U M I D I T Y R A T I O + t-- - / - - A \ kLTr=*-f- f

'-

0 / I d fi\ _/ I

-

-bP--# I I 1 I B I S O U T H C L A S S R O O M TEMPERATURE f.-. J \ -

:

'"'

\ \ fl-\ 5 \ ---1 \ - I _{\ 1} \ / - b - d O 1 1 , I I -%J 1 _I \ W E D N E S D A Y T H U R S D A Y F R I P A Y S A T U R D A Y S U N D A Y M O N D A Y F E B R U A R Y 13-18. 1980

Figure 7 Indoor and outdoor humidity ratios and temperatures, outside supply-air damper closed.