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Maintaining acceptable air quality in office buildings through ventilation
Shaw, C. Y.
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NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=5ce65c15-0fe8-438f-9ea3-40300ab4325e https://publications-cnrc.canada.ca/fra/voir/objet/?id=5ce65c15-0fe8-438f-9ea3-40300ab4325eb y C.Y. S ha w
Poor air quality in office buildings can result in loss of productivity,
absenteeism and, in some cases, medical problems. The purpose of this
Update is to provide guidelines for property managers and engineers for
controlling indoor air quality using building ventilation systems.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 3
Th e m ost frequ en tly id en tified cau ses of p oor in d oor air qu ality th at are d irectly related to bu ild in g d esign s an d op eration , are air con -tam in an ts an d in ad equ ate ven tilation . For
office bu ild in gs, am on g th e m ajor air conta-mination sources are bu ild in g m aterials an d fu rn ish in gs, occu p an ts’ activities, an d office equ ip -m en t. In so-m e cases, air con ta-m in an ts gen er-ated ou td oors can be brou gh t in to a bu ild in g by th e ven tila-tion system . To m in im ize som e of th ese sou rces effectively an d econ om ically, d esign ers, bu ild in g ow n ers an d m an agers, an d bu ild in g en gin eers h ave been in creasin gly u sin g p reven tive m easu res, su ch as sp ecifyin g bu ild in g m aterials an d fu rn ish in gs w ith low em ission p oten tials, locatin g th e ou t-d oor-air in takes aw ay from kn ow n ou tt-d oor sou rces, an d u sin g sp ecial exh au st system s to rem ove localized con tam in ation sou rces.
How ever, m an y con tam in an ts, su ch as th ose gen erated by occu p an ts’ activities, h ave to be red u ced by bu ild in g ven tilation .
Ac c e pt a ble I ndoor Air Qua lit y
Th e Am erican Society of Heatin g, Refrigeratin g, an d Air-Con d ition in g En gin eers In c. (ASHRAE) Stan d ard 62-89 d efin es accep table in d oor air qu ality as “air in w h ich th ere are n o kn ow n con tam in an ts at h arm fu l con cen tration s as d eterm in ed by cogn izan t au th orities an d w ith w h ich a su b-stan tial m ajority (80% or m ore) of th e p eop le exp osed d o n ot exp ress d issatisfaction .” To ach ieve accep table in d oor air qu ality, th e Stan d ard recom m en d s tw o p roced u res: th e Ven tilation Rate Proced u re, sp ecifyin g th e qu ality an d qu an tity of ven tilation air for a sp ace, an d th e In d oor Air Qu ality
Proced u re, requ irin g th e con trol of all kn ow n con tam in an ts to som e sp ecified accep table levels w ith in a sp ace. Of th e tw o p roced u res, th e Ven tilation Rate Proced u re is m ost frequ en tly u sed .
Th e Ven tilation Rate Proced u re p rescribes ven tilation rates for variou s sp aces an d th e ou td oor air qu ality accep table for ven tilation . It also sp ecifies th at th e ven tilation system s sh ou ld be d esign ed an d in stalled to en su re th at th e ven tilation air is d elivered th rou gh -ou t th e occu p ied zon es an d th at th ey d o n ot cau se con d ition s th at con flict w ith ASHRAE Stan d ard 55-1992, Th erm al En viron m en tal
Maintaining Acceptable Air
Quality in Office Buildings
Through Ventilation
Figure 1.Typical high-rise office building in which the ventilation system plays a key role
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Con d ition s for Hu m an Occu p an cy. For office bu ild in gs in North Am erica, th ese requ irem en ts can u su ally be m et by su p p lyin g 10 L/ s of ven tilation air p er p erson in to a bu ild in g th rou gh ou td oor-air in takes th at are located aw ay from an y kn ow n ou td oor con tam i-n atioi-n sou rces, su ch as a p arkin g lot. Th e d istribu -tion of th e ap p rop riate am ou n t of ven tila-tion air to each occu p ied zon e can be reason ably ascertain ed by en su rin g th at th e CO2con
-cen tration in an y w orkstation is 1000 p p m or less.
Ve nt ila t ion St ra t e gie s
On ce a bu ild in g is occu p ied , ven tilation is rou tin ely u sed by bu ild in g en gin eers to m ain tain th e in d oor air qu ality level. Ven tilation is p articu larly effective in tw o situ ation s:
• to re-establish th e in d oor air qu ality level after a sch ed u led sh u td ow n of th e
(HVAC) system ;
• to sp eed u p th e rem oval of con tam in an ts in areas w h ere ren ovation s are takin g p lace.
To u se th e ven tilation strategy for th ese p u rp oses efficien tly an d econ om ically, it is n ot en ou gh to sim p ly in crease th e ven tilation rate. Th e bu ild in g en gin eer m u st kn ow w h en , by h ow m u ch an d for h ow lon g th e ven tilation rate m u st be in creased .
Tw o ven tilation op eration strategies are u sed to m ain tain accep table air qu ality in offices: a n orm al m od e an d an en h an ced m od e. Th e n orm al ven tilation m od e is th e op eratin g m od e u sed for day-to-day op era-tion s. The enhanced m od e is th e on e u sed to con trol th e in itial off-gassin g th at occu rs im m ed iately after th e in stallation of n ew m aterials th at em it ch em ical gases.
Normal ventilation mode
Th e ven tilation system in m ost office bu ild -in gs is sh u t d ow n d u r-in g u n occu p ied h ou rs as an energy-saving measure. The unoccupied p eriod for a regu lar w eekd ay lasts abou t 12 h ou rs, for a regu lar w eeken d abou t 60 h ou rs an d for a lon g w eeken d abou t 84 h ou rs.
Th e ven tilation strategy requ ired to d eal w ith th ese con d ition s com p rises tw o cycles: th e flu sh in g cycle, u sed before em p loyees arrive at w ork, an d th e occu -p an cy cycle, u sed w h ile th ey are at w ork.
The flushing cy cle
Th e con cen tration of con tam in an ts gen er-ated by bu ild in g m aterials, fu rn ish in gs an d equ ip m en t is gen erally m ain tain ed at an accep table level by th e HVAC system w h en it is op eratin g. On ce th e ven tilation system is tu rn ed off, h ow ever, th e con cen tration of off-gassed ch em icals in creases. Th e flu sh in g cycle is u sed to re-establish th e qu ality of th e in d oor air to an accep table level before th e em p loyees arrive in th e m orn in g. Th e ven tilation rate u sed d u rin g th e flu sh in g cycle m u st be greater th an th at u sed d u rin g th e occu p an cy cycle.
Th e ou td oor air requ ired for flu sh in g varies from bu ild in g to bu ild in g an d h as to be d eterm in ed exp erim en tally. Based on exp erien ce acqu ired in m an agin g p u blic bu ild in gs, tw o air ch an ges ap p ear to be an ap p rop riate startin g valu e. Th is valu e m u st be ad ju sted based on th e m easu red con cen -tration s of CO2(i.e., ≤1000 p p m ) an d / or
oth er key con tam in an ts, su ch as volatile organ ic com p ou n d s, an d th e reaction of th e occu p an ts.
As th e con tam in an t bu ild -u p is greater over a w eeken d an d greater still over a lon g w eeken d sh u td ow n , m ore air ch an ges are requ ired to red u ce th e con cen tration of con tam in an ts to th e sam e level ach ieved follow in g a w eekd ay sh u td ow n . Th u s, fou r an d five ch an ges of ou td oor air are recom -m en d ed as startin g valu es follow in g regu lar an d lon g w eeken d s, resp ectively.
Th e n u m ber of air ch an ges requ ired for a flu sh in g cycle m u st be tran slated in to a ru n n in g tim e for th e HVAC system . Du rin g sp rin g an d fall w h en it is feasible to u se free coolin g, th e requ ired n u m ber of air ch an ges can be p rovid ed in a sh orter tim e by u sin g a h igh er ven tilation rate. For exam p le, if th ree is th e n u m ber of air ch an ges requ ired for flu sh in g, th e op erator can ad ju st th e ven tilation system to d eliver
Figure 2.Achieving acceptable IAQ depends not only on the quantity of ventilation air but also on adequate distribution to individual workstations.
Figu re 3 sh ow s th e d aily CO2con cen
tra-tion p rofiles m easu red on variou s floors of a 22-storey office bu ild in g w ith a con stan t am ou n t of ou td oor air bein g su p p lied by th e HVAC system . It sh ow s a rap id bu ild -u p of CO2startin g arou n d 7:30 a.m ., a fairly h igh con cen tration d u rin g th e d ay, an d th en a stead y d ecrease startin g arou n d 4 p .m . Th e grap h su ggests th at CO2 gen erated by
occu p an ts d istribu ted th rou gh ou t th e bu ild in g w ill be reason ably w ell m ixed in th e in d oor air very qu ickly. It is feasible to u se th e CO2con cen tration s to con trol th e
ven tilation system . It is also exp ected th at m ost office bu ild in gs w ith p rop erly d esign ed an d com m ission ed all-air con stan t-volu m e HVAC system s w ou ld p rovid e a sim ilar degree of mixing between occupant-generated CO2an d th e in d oor air, an d w ou ld therefore
be suitable for using CO2ventilation d em an d con trollers.
Enhanced Ventilation Mode
Th e en h an ced ven tilation m od e sh ou ld be u sed on ly w h en p art of a bu ild in g is bein g ren ovated . It h as tw o ap p lication s: th e con trol of off-gassin g an d th e con trol of con tam in an t m igration .
Control of off-ga ssing
Som e n ew ly in stalled m aterials release con tam in an ts at a h igh rate, bu t th e rate d ecreases w ith tim e. Th e d ecay p rocess reach es a stead y rate of off-gassin g w ith in d ays or w eeks of th e in stallation of th e m aterial, d ep en d in g on th e typ e of m aterial. Du rin g th is p eriod , ad d ition al ven tilation is n eed ed to d ilu te th e large qu an tity of con tam in an ts bein g released in to th e air.
Figu re 4 sh ow s th e con cen tration p rofile (w ith backgrou n d con cen tration s su btracted ) m easu red in an office follow in g th e
in stallation of a n ew carp et.
It is ad visable d u rin g th e ren ovation p eriod to in crease th e ven tilation rate to th e fu ll cap acity of th e bu ild in g’s HVAC system to d ilu te th e con tam in an ts. It is also ad visable to suspend th e p ractice of sh u ttin g d ow n th e ven tilation system in th e even in g an d on w eeken d s.
Control of conta m ina nt m igra tion
Although most of the off-gassing products can be effectively rem oved from th eir p lace of origin (e.g., a ren ovated sp ace) by op eratin g th e HVAC system in an en h an ced ven tilation m od e, som e of th em m ay m igrate th rou gh th e bu ild in g. In n ew ly con stru cted bu ild in gs, th eir m igration m igh t n ot be th ree air ch an ges p er h ou r for on e h ou r,
in stead of on e air ch an ge p er h ou r for th ree h ou rs. Based on th e ven tilation rate (i.e., air ch an ges p er h ou r), th e startin g tim e of th e HVAC system can be d eterm in ed .
Du rin g th e h eatin g season an d on w arm su m m er d ays, th e am ou n t of ou tsid e air th at can be brou gh t in d u rin g th e flu sh in g cycle d ep en d s on th e h eatin g or coolin g cap acity of th e HVAC system .
The occup a ncy cy cle
Du rin g th e h ou rs of n orm al occu p an cy, th e ven tilation rate m u st be ad equ ate to m ain tain th e m in im u m ou td oor air su p p ly of 10 L/ s/ p erson . As occu p an ts p rod u ce CO2, it is expected that the CO2concentrations in office
bu ild in gs w ou ld be p rop ortion al to th e number of occupants. One way to ensure that a bu ild in g receives th e ap p rop riate am ou n t of ven tilation is to u se a CO2 ven tilation
d em an d con troller th at au tom atically ad ju sts th e total ou td oor air su p p ly rate, accord in g to th e n u m ber of occu p an ts in th e bu ild in g.
Tw o basic requ irem en ts m u st be m et for a CO2ven tilation d em an d con troller to w ork p rop erly:
• th e CO2con cen tration sh ou ld be relatively u n iform th rou gh ou t th e bu ild in g, an d • for th e in stallation of th e con troller, a
location m u st be fou n d w h ere th e con -cen tration of CO2 is rep resen tative of con cen tration s th rou gh ou t th e bu ild in g.
3
Figure 3.Typical CO2concentration profiles in occupied zones on six
different floors of a 22-storey building
300 400 500 600 700 800 900 1000 0 4 8 12 16 20 24 2F 5F 8F 11F 18F 20F Time of Day, [h] CO 2 Concentration, [ppm]
n oticeable, sin ce th e off-gassin g p rod u cts are gen erated th rou gh ou t th e bu ild in g. In existin g bu ild in gs, th e in itial offgassin g is u su -ally localized . Off-gassin g p rod u cts are typ ically gen -erated w h ere an old carp et is bein g rem oved , a n ew carp et laid , n ew fu rn itu re m oved in , or w alls p ain ted . Even a m in u te am ou n t of off-gassin g p rod u cts m igratin g th rou gh th e bu ild in g can be d etected by th e occu p an ts. Th erefore, p recau tion ary m easu res sh ou ld be taken to keep th ese con tam in an ts from m igratin g from th eir p lace of origin to oth er occu p ied p arts of th e bu ild in g.
To better u n d erstan d th is p rocess of m igration of off-gassin g p rod u cts th rou gh a bu ild in g, com p u ter p rogram s h ave been u sed to p red ict h ow a con tam in an t sp read s from th e sou rce to th e su rrou n d in g areas u n d er variou s w eath er con d ition s. Th ese p rogram s also assess th e effectiven ess of variou s ven tilation strategies for rem ovin g con tam in an ts — for exam p le, ven tin g th rou gh an op en w in d ow or a vertical sh aft. Th e resu lts in d icate th at th e m ost effective m igration con trol can be ach ieved by seal-in g off th e retu rn -air grilles on th e affected floor, w h ile keep in g th e su p p ly air com in g in to th e affected floor an d rem ovin g th e air th rou gh a sm oke sh aft, a w in d ow op en in g or a stair sh aft th at op en s d irectly to th e ou tsid e. Th ese m ean s of con trol are effec-tive, p rovid ed th at th e p ressu res in d u ced by th e stack effect an d w in d d o n ot w ork again st rem oval of th e con tam in an ts.
For exam p le, in w in ter, con tam in an ts on low er floors w ill be p u sh ed to u p p er levels becau se of th e stack effect. Th e stack effect can be u sed to ad van tage by ven tin g th e con tam in an ts th rou gh a vertical sh aft d irectly op en to th e ou tsid e, e.g., a stair sh aft. How ever, u n d er w in d y con d ition s, ven tin g th e con tam in an ts th rou gh a leew ard w in d ow w ou ld be m ore effective.
Conc lusion
By follow in g th e recom m en d ation s in th is Up d ate, it is p ossible to u se th e HVAC sys-tem to m ain tain accep table air qu ality in an office bu ild in g.
Re fe re nc e s
1. Vacu lik, F. an d Sh aw, C.Y. 1995. Man agin g In d oor Air Qu ality Th rou gh th e Use of HVAC System s, In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a. NRCC 38546.
2. ASHRAE 1989. ASHRAE Stan d ard 62-1989, Ven tilation for Accep table In d oor Air Qu ality, ASHRAE, Atlan ta, Ga. 3. Reard on , J.T., Sh aw, C.Y. an d Vacu lik, F.
1994. Air Ch an ge Rates an d Carbon Dioxid e Con cen tration s in a High -Rise Office Bu ild in g, ASHRAE Tran saction s, 100(2).
4. Plett, E.G., Vacu lik, F. an d Sh aw, C.Y. 1992. Con trollin g In d oor Air Qu ality: Ven tilation En gin eerin g Gu id e, Pu blic Works an d Govern m en t Services Can ad a an d Nation al Research Cou n cil of Can ad a.
5. Said , M.N.A., Sh aw, C.Y., Plett, E.G. an d Vacu lik, F. 1995. Com p u ter Sim u lation of Ven tilation Strategies for Main tain in g an Accep table In d oor Air Qu ality in Office Bu ild in gs. ASHRAE Tran saction s, 101(1).
Dr. C.Y. S ha w is a sen ior research officer in th e
Bu ild in g Perform an ce Laboratory of N RC’s In stitu te for Research in Con stru ction .
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
© 1997
Nation al Research Cou n cil of Can ad a Jan u ary 1997
ISSN 1206-1220 Figure 4.TVOC (total volatile organic compounds)
concentrations in an office space with background concentrations subtracted, immediately following re-carpeting of the area (graph courtesy of Dr. G. Kerr, InAir Environmental Ltd.)
20
0 5 10 15
Number of days after area recarpeted 0 2 4 6 8 10 TVOC concentration, mg/m 3 25 Exponential fit line
12 14
