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Current approaches for mechanical ventilation of houses
Haysom, J. C.; Reardon, J. T.
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https://nrc-publications.canada.ca/eng/view/object/?id=96acba7c-afd4-4ea1-94b0-1f8f3500c582 https://publications-cnrc.canada.ca/fra/voir/objet/?id=96acba7c-afd4-4ea1-94b0-1f8f3500c582b y J.C. Ha y som a nd J.T. Re a rd on
This Update, the second of tw o on mechanical ventilation systems for
houses, describes current approaches to design and installation. It also
examines the distribution problem and looks at some of the shortcomings of
current approaches and how they might be overcome.
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 . 1 5
Curre nt Approa c he s
Cu rren t ap p roach es to th e m ech an ical ven tilation of h ou ses are fou n d in th e CSA Stan d ard F326, “Resid en tial Mech an ical Ven tilation System s,” an d in th e Nation al Bu ild in g Cod e of Can ad a 1995.
CAN/CSA-F326
Th e CSA Stan d ard F326 w as first p u blish ed in 1989 an d revised in 1991. It is th e m ost com p reh en sive stan d ard available on th is su bject, an d attem p ts to ad d ress th e critical issu es in th e d esign an d in stallation of an id eal m ech an ical ven tilation system (see Con stru ction Tech n ology Up d ate No. 14 for a d iscu ssion of th ese issu es). Th e exten t to w h ich Stan d ard F326 h as been able to d o th is is lim ited by available tech n ology.
CAN/ CSA-F326 is gen erally w ritten in p erform an ce term s (th at is, it states w h at m u st be accom p lish ed rath er th an h ow to accom p lish it) an d th erefore p rovid es a great d eal of flexibility w ith regard to th e system con figu ration n eed ed for com p lian ce.
Th is stan d ard ad d resses th e p ertin en t issu es as follow s:
Control
Th e greatest sh ortfall of a system th at com p lies w ith Stan d ard F326 (h ereafter referred to as F326), relative to an id eal system , is in th e area of con trol. Becau se available tech n ology for d em an d -con trolled
ven tilation system s d oes n ot ap p roxim ate th e id eal very closely, F326 requ ires on ly th at th e system h ave an on / off sw itch . Oth er form s of con trol, su ch as d eh u m id i-stats, are p erm itted , bu t th e on / off sw itch m u st be able to over-rid e th em .
Ca p a city
F326 requ ires th e ven tilation system to h ave a total cap acity equ al to th e su m of th e in d ivid u al room ven tilation requ ire-m en ts sh ow n in Table 1. For ire-m ost h ou ses, th is w ill ad d u p to abou t 0.3 air ch an ges
Current Approaches for
Mechanical Ventilation
of Houses
Room Capacity, L/s
Master bed room 10 Oth er bed room s 5
Livin g room 5
Din in g room 5
Fam ily room 5
Recreation room 5
Basem en t 10 Oth er h abitable room s 5
Kitch en 5
Bath room or
w ater closet room 5
Lau n d ry 5
Utility room 5
2
p er h ou r (ach ). Th e stan d ard also requ ires th at th e system be cap able of ru n n in g at a rate of 50% of th is am ou n t. In oth er w ord s, it m u st h ave at least tw o d elivery m od es — fu ll cap acity an d h alf cap acity.
Distrib ution
F326 requ ires th at th e system be cap able n ot on ly of p rovid in g th e total in d oor/ ou td oor air exch an ge rate (d eterm in ed from Table 1) bu t also of ach ievin g th is rate in each room . Alth ou gh th e stan d ard d oes n ot sp ecify h ow th is is to be accom p lish ed , a d u ct system is gen erally requ ired .
S ound le v e l
Cu rren tly, F326 d oes n ot set a m axim u m sou n d ou tp u t for fan s sin ce m eth od s of test-in g an d accep table sou n d levels h ad n ot been fu lly agreed u p on w h en th e stan d ard w as last revised . It is exp ected th at th is issu e w ill be ad d ressed in a fu tu re ed ition . Inte rfe re nce w ith othe r sy ste m s
For d w ellin gs w ith ven ted com bu stion ap p lian ces, F326 requ ires th at th e ven tila-tion system , w h en op eratin g at fu ll cap acity an d at th e sam e tim e as a cloth es d ryer an d an y oth er exh au st d evices w ith a cap acity of 75 L/ s or greater, n ot d ep ressu rize th e d w ellin g m ore th an 5 Pa. Th is lim it (of 5 Pa) can be exceed ed if th e com bu stion ap p lian ce h as been rated for a h igh er level of d ep su rization . Th e p oten tial for su ch d ep res-su rization can be d eterm in ed by calcu lation , w h en th e airtigh tn ess ch aracteristics of th e d w ellin g are kn ow n , or by a test th at actu ally m easu res d ep ressu rization . Inte rfe re nce w ith the b uild ing e nv e lop e F326 lim its th e am ou n t of ou td oor air th at can be brou gh t in relative to th e am ou n t of air th at is extracted , in recogn ition of th e fact th at an y excess of in take flow over exh au st flow m u st be m ad e u p by ou tw ard leakage th rou gh th e en velop e, w h ich can lead to in terstitial con d en sation . Wh en th e ven tilation system is op eratin g at fu ll cap acity, th e am ou n t of ou td oor air brou gh t in th rou gh in take com p on en ts (e.g., su p p ly fan s) m u st n ot exceed th e am ou n t of in d oor air extracted th rou gh exh au st com p on en ts (e.g., exh au st fan s) by m ore th an 0.12 L/ s/ m2 of th e bu ild in g en velop e’s in n er su rface. Th is valu e is based on th e h istorical ap p roach to ven tilatin g h ou ses in w h ich an ou td oor air in take d u ct is con n ected to th e retu rn air p len u m of a forced -air fu rn ace;
th e valu e rep resen ts th e flow of air likely to be p rod u ced by su ch a system . Th is ap p roach h as been w id ely u sed in Can ad a w ith ou t sign ifican t p roblem s, even th ou gh it can lead to p ressu rization of th e h ou se.
As w ell as lim itin g th e flow of air, F326 also p laces a lim it of 10 Pa on th e p ositive p ressu re th at can be brou gh t abou t by th e d iscrep an cy betw een th e ven tilation system in take an d exh au st flow s. Th is requ irem en t is n ecessary becau se in a very tigh t h ou se, even if th e flow d iscrep an cy criterion d escribed above is satisfied , th ere is still th e p ossibility of creatin g a h igh p ositive p ressu re in th e h ou se, w h ich can lead to very h igh flow at in d ivid u al leaks, cau sin g local in terstitial con d en sation p roblem s.
1995 National Building Code
Th e 1995 NBC requ ires all h ou ses to h ave m ech an ical ven tilation system s. System s th at com p ly w ith F326 satisfy th is requ ire-m en t. How ever, F326 is a coire-m p reh en sive stan d ard an d is w ritten in p erform an ce term s sp ecifically for train ed m ech an ical system s d esign ers. Th e NBC th erefore p ro-vid es p rescrip tively d escribed altern atives, w h ich are based on F326 an d w h ich can be u sed by th ose w h o are n ot exp erts.
Th ese p rescrip tive altern atives ad d ress th e issu es id en tified in th e d escrip tion of th e id eal system as follow s:
Control
Th e 1995 NBC p arallels F326 in th at it requ ires on ly an on / off sw itch cap able of over-rid in g an y au tom atic con trols. Ca p a city
Th e NBC also p arallels F326 in th is area, an d u ses th e sam e table of in d ivid u al room airflow requ irem en ts (th at is, th e total cap acity of th e system m u st equ al th e su m of th e room ven tilation requ irem en ts sh ow n in Table 1). It also requ ires th e system to be cap able of op eratin g at a rate equ al to th e fu ll cap acity as w ell as at 50% of th is cap acity.
Distrib ution
Th ere are basically tw o p rescrip tively d escribed ap p roach es in th e 1995 NBC — on e th at can be u sed in h ou ses w ith forced -air h eatin g system s an d on e th at can be u sed in th ose w ith ou t su ch system s (see Figu res 1 an d 2 for th ese con figu ration s).
tem p eratu re before it reach es th e fu rn ace h eat exch an ger an d th e livin g areas of th e h ou se. (Very cold air p assin g over th e h eat exchanger can cause premature deterioration.)
The use of heating ducts to distribute outdoor air. Th e ou td oor air is th en d istribu ted th rou gh ou t th e h ou se by th e regu lar h eat-d istribu tion (d u ct) system .
The supplementary fans. Th ese fan s (e.g., kitch en an d bath room exh au st fan s) make up the discrepancy between the capacity of the principal exhaust fan and the minimum requ ired cap acity of th e system .
Th e m ain featu res of th e con figu ration for h ou ses w ithout forced -air h eatin g sys-tem s are:
The principal exhaust fan. Th is fan p erform s th e sam e fu n ction as th e fan in a h ou se w ith forced -air h eatin g an d is sized an d con trolled in th e sam e w ays. How ever, its con trol is w ired to th e su p p ly fan rath er th an to a fu rn ace circu lation fan .
The supply fan. Th is fan h as th e sam e cap acity as th e p rin cip al exh au st fan an d op erates w h en ever th e p rin cip al exh au st fan op erates, d raw in g ou td oor air in th rou gh th e ou td oor air d u ct. Th e m ain featu res of th e con figu ration
for a h ou se w ith forced -air h eatin g are:
The principal exhaust fan. Th is fan is th e h eart of th e system . It d raw s air from th rou gh ou t th e d w ellin g. Alth ou gh th e occu p an ts an d th eir activities d eterm in e its op eration , th e fan sh ou ld be cap able of op eratin g con tin u ou sly. At th is tim e, h ow -ever, th ere is n o stan d ard m eth od of testin g an d d esign atin g fan s for con tin u ou s u se. Th erefore, su ch a d esign ation is n ot yet m an d atory.
Th e p rin cip al exh au st fan is exp ected to p rovid e a relatively low level of ven tila-tion — 50% of th e m in im u m requ ired cap acity of th e system (su ch th at it can ru n con tin u ou sly w ith ou t excessive n oise an d w ith ou t excessive en ergy p en alty). Its con -trol is p laced in a cen tral location , su ch as a livin g room , fam ily room or h allw ay, an d it is w ired d irectly to th e fu rn ace circu la-tion fan so th at w h en ever th e p rin cip al exh au st fan is tu rn ed on , th e circu lation fan is also activated .
The outdoor air duct. Wh en th e fu rn ace circu lation fan is tu rn ed on , it d raw s ou t-d oor air th rou gh th e ou tt-d oor air t-d u ct. Th is air is th en m ixed w ith retu rn air in th e retu rn air p len u m in ord er to in crease its Figure 1.Ventilation system configuration for a house with a forced-air heating system
Bathroom exhaust fan intended for intermittent use
Kitchen exhaust fan intended for intermittent use Outdoor air duct Manual control Normal heating ducts Manual or automatic control, interlocked to furnace fan Return air plenum Principal exhaust
fan suitable for continuous use
Manual control
Figure 2.Ventilation system configuration for a house without a forced-air heating system
Bathroom exhaust fan intended for intermittent use Manual or automatic control, interlocked to air handler Kitchen exhaust fan intended for intermittent use Outdoor air duct Manual control Supply fan Ventilation ducts Principal exhaust
fan suitable for continuous use High outlets Manual control Duct heater Thermostatic control
The outdoor air duct. Wh en th e su p p ly fan is tu rn ed on , ou td oor air is d raw n in th rou gh th is d u ct. Sin ce, in th is case, th ere is n o retu rn air to tem p er th e ou td oor air, an oth er m ean s m u st be u sed to w arm u p th is air before it is circu lated to th e livin g areas.
The duct heater. Th is h eater w arm s u p th e in com in g ou td oor air before it reach es th e su p p ly fan .
The use of ventilation ducts to distribute outdoor air. Th e ou td oor air from th e su p p ly fan is d istribu ted th rou gh a ru d im en -tary system of sm all d u cts in stalled for th is p u rp ose. Th ese d u cts m u st go to each bed -room an d to an y storey w ith ou t a bed -room . Un like h eatin g d u cts, th ey can be of com -bu stible m aterials.
The supplementary fans.Th ese fan s h ave th e sam e sizes an d fu n ction s as th ose in a h ou se w ith a forced -air h eatin g system . S ound le v e l
Th e 1995 NBC requ ires all fan s con stitu t-in g p art of th e ven tilation system to h ave sou n d ratin gs n ot exceed in g 2 son es (53 d ecibels) w ith th e excep tion of kitch en exh au st fan s, w h ich are p erm itted to h ave sou n d ratin gs of 3.5 son es (60 d ecibels). Inte rfe re nce w ith othe r sy ste m s
Th e p ortion of th e system th at is su itable for con tin u ou s u se in clu d es th e p rin cip al exh au st fan alon g w ith eith er th e fu rn ace circu lation fan or th e su p p ly fan (in h ou ses w ith ou t forced -air h eatin g). Wh en op eratin g in th is m od e, th e system is balan ced sin ce th e volu m e of air extracted by th e p rin cip al exh au st fan is ap p roxim ately equ al to th at brou gh t in by th e oth er fan s.
Th e p ortion of th e system th at op erates in frequ en tly, for sh ort p eriod s of tim e, (kn ow n as th e “h igh rate or ep isod ic” p or-tion ) in clu d es th e su p p lem en tal exh au st fan s, w h ich op erate w h en th e p rin cip al exh au st fan is n ot able to p rovid e an ad e-qu ate rate of air ch an ge. Wh en op eratin g in th is m od e, th e system is u n balan ced , alth ou gh it d oes n ot u su ally d ep ressu rize th e h ou se sign ifican tly u n less oth er exh au st d evices (e.g., a cloth es d ryer) are u sed at th e sam e tim e. How ever, if th ere are oth er large exh au st d evices (e.g., a stove-top barbecu e) in th e h ou se, h igh levels of d ep ressu rization can occu r if th ese d evices are op erated on th eir ow n . For th is reason ,
in h ou ses w ith sp illage-su scep tible com bu stion ap p lian ces, an y su ch large exh au st d evices (i.e., greater th an 75 L/ s exh au st cap acity) m u st be p rovid ed w ith m ake-u p air.
In th e p ast, th e NBC an d oth er cod es an d stan d ard s ten d ed to rely on th e p assive su p p ly of m ake-u p air th rou gh op en in gs p rovid ed for th is p u rp ose. Th is is n o lon ger felt to be a reliable ap p roach for a sim p le, p rescrip tively d escribed system becau se it d oes n ot h ave sop h isticated d ep ressu rization con trols (su ch as th ose p rovid ed in F326). Accord in g to th e 1995 NBC, m ake-u p air m u st be p rovid ed by a su p p ly fan th at is au tom atically activated w h en ever th e exh au st d evice requ irin g m ake-u p air is activated . If sp illage-su scep tible com bu stion equ ip m en t is n ot u sed , m ake-u p air d oes n ot h ave to be p rovid ed .
Even at th e relatively low level of d ep ressu rization likely to occu r w h en th e ven tilation system is op erated at its “h igh rate or ep isod ic” level, an op en firep lace op eratin g in its “d ie-d ow n ,” or sm old erin g, stage can sp ill p rod u cts of com bu stion in to th e h ou se. In th e absen ce of m ore sop h isti-cated con trols to p reven t su ch levels of d ep ressu rization (su ch as th ose p rovid ed in F326), th e on ly available safegu ard is th e in stallation of a carbon m on oxid e d etector in an y room th at h as an op en solid -fu el-bu rn in g d evice. Wh ere th is is n ot a viable op tion , th e p rescrip tively d escribed alter-n atives m u st be abaalter-n d oalter-n ed ialter-n favou r of a system th at com p lies w ith F326.
Inte rfe re nce w ith the b uild ing e nv e lop e Becau se th ese con figu ration s are eith er p ressu re-n eu tral (w h en on ly th e p rin cip al exh au st fan is op eratin g), or m ild ly d ep res-su rizin g (w h en oth er fan s are op eratin g), th ey are n ot likely to p ressu rize th e h ou se an d th erefore n ot likely to in crease th e p oten tial for in terstitial con d en sation in th e bu ild in g en velop e.
T he Dist ribut ion Proble m
In th e tw o con figu ration s d escribed above, th e d istribu tion of ou td oor air to th e room s or sp aces w h ere it is n eed ed is relatively sim p le in h ou ses w ith forced -air h eatin g system s bu t som ew h at m ore p roblem atic in h ou ses w ith oth er typ es of h eatin g system s.
In th e con figu ration sh ow n in Figu re 2, d u cts m u st be ad d ed to a h ou se th at w ou ld oth erw ise n ot n eed th em . For th is reason , IRC research ers are in vestigatin g altern ative m eth od s of en su rin g p rop er d istribu tion of ou td oor air in h ou ses w ith ou t forced -air h eatin g system s. To d ate, th e fin d in gs can be su m m arized as follow s:
• Du cted system s can p rovid e ad equ ate d istribu tion of ou td oor air.
• Cen tral exh au st fan s, su ch as th ose in kitch en s an d bath room s, u sed w ith ou t som e oth er m ean s of d istribu tion , d o n ot p rovid e ad equ ate d istribu tion of ou td oor air to bed room s.
• Distribu ted exh au st system s th at h ave p ick-u p s in th e u p p er storey room s can p rovid e even d istribu tion of ou td oor air. An exam p le of th is typ e of system con -sists of an attic-m ou n ted exh au st fan w ith m an ifold ed in takes bran ch in g to all bed room s. Th is system cou ld be com -bin ed w ith a cen tral p assive in let ven t to red u ce th e d ep ressu rization of th e h ou se an d in crease total in d oor/ ou td oor air exch an ge.
• Th e com bin ation of a cen tral exh au st fan w ith d istribu ted p assive air in lets d oes n ot ach ieve an even d istribu tion of air sin ce th e in lets on th e u p p er level becom e ou tlets in cold w eath er as a resu lt of stack effect. Un d er th ese cir-cu m stan ces, low er storey room s receive too m u ch ou td oor air, an d u p p er storey room s too little. Th is ap p roach m igh t be viable in m ild er clim ates in Can ad a. • Closed bed room d oors w ith typ ically
sized d oor u n d ercu ts d o n ot h in d er th e ad equ ate d istribu tion of air p rovid ed by forced -air h eatin g system s w h en th e fu r-n ace circu latior-n far-n is op eratir-n g.
H e a t Re c ove ry
Becau se th e cold air en terin g th e h ou se m u st be h eated , in d oor/ ou td oor air exch an ge, w h eth er p rovid ed by leakage or by m ech an ical ven tilation , brin gs w ith it an en ergy an d a cost p en alty. In th e p ast, w h en th is air exch an ge took p lace w ith ou t th e w h irrin g of fan s, p eop le ten d ed to ign ore th e en ergy cost or sim p ly attribu te it to th e n eed for th e h ou se to “breath e.” Bu t on ce th ere is a greater aw aren ess of th e air exch an ge takin g p lace (by m ean s of a
m ech an ical ven tilation system ) an d of th e p ossibilities th e system offers for exercisin g som e con trol over th e ven tilation of th e h ou se, th ere is a ten d en cy to view th e en ergy cost w ith som e alarm an d to seek som e m ean s of red u cin g it.
On e w ay to d o th is is sim p ly to tu rn off th e ven tilation system , or to u se it less; h ow ever, th is can resu lt in p oor in d oor air qu ality, m ou ld on in terior bu ild in g su rfaces an d in terstitial con d en sation in th e bu ild in g en velop e. An oth er ap p roach is to in corp o-rate a m ean s of recoverin g h eat from ou tgoin g in d oor air an d tran sferrin g it to th e in com in g ou td oor air. Th e h eat recov-ery ven tilator is th e m ost com m on ly u sed equ ip m en t w ith th is cap ability. (Figu re 3 sh ow s h ow it can be u sed to satisfy th e requ irem en ts of th e NBC.) How ever, ven tilation system s w ith h eat recovery cap ability in evitably cost m ore th an th ose w ith ou t. Th erefore, th is extra cost n eed s to be w eigh ed again st th e cost of th e en ergy saved .
Figure 3.Ventilation system configuration with a heat recovery ventilator for a house with a forced-air heating system
Bathroom exhaust fan intended for intermittent use Manual control Manual or automatic control for HRV interlocked to furnace fan Kitchen exhaust
fan intended for intermittent use Heat recovery ventilator Manual control Normal heating ducts Return air plenum Principal exhaust fan Supply fan
Th is w eigh in g of costs h as been d on e in th e Mod el Nation al En ergy Cod e for Hou ses, w h ich in clu d es region ally sen si-tive requ irem en ts for all p arts of Can ad a. On e su ch requ irem en t d eals w ith w h eth er or n ot h eat recovery m u st be in corp orated in to m ech an ical ven tilation system s. In d ecid in g w h eth er or n ot h eat recovery sh ou ld be requ ired in a p articu lar region , th e com m ittee th at d evelop ed th is cod e em p loyed exten sive life-cycle cost an alyses based on region al en ergy an d con stru ction costs. Th e resu lts can be su m m ed u p as follow s:
• for gas-h eated h ou ses, h eat recovery is requ ired on ly in th e cold est region s; • for oil- an d electrically h eated h ou ses,
h eat recovery is requ ired alm ost every-w h ere in Can ad a.
T he Fut ure of M e c ha nic a l Ve nt ila t ion of H ouse s
In tim e, m ech an ical ven tilation system s w ill likely ap p roach th e id eal (see
Con stru ction Tech n ology Up d ate No. 14), as d em an d -con trolled ven tilation becom es m ore p ractical an d econ om ical as a resu lt of research an d d evelop m en t. At th e sam e tim e, it is p ossible th at th e am ou n t of in d oor/ ou td oor air exch an ge requ ired w ill d ecrease. Th is cou ld com e abou t th rou gh th e in trod u ction of lim its on p ollu tan t em ission s from bu ild in g m aterials an d fu r-n ish ir-n gs. How ever, ou r ability to id er-n tify p ollu tan ts of con cern an d to set safe an d p ractical lim its on em ission s is still very lim ited .
Fin ally, th ere is grow in g evid en ce th at h ou ses bein g bu ilt tod ay are even tigh ter th an th ose tested in th e 1989 su rvey. In ligh t of th is evid en ce, th e con tin u ed viability of th e p rescrip tively d escribed solu tion s in th e NBC for h ou ses th at in corp orate sp illage-su scep tible com bu stion ap p lian ces w ill h ave to be re-exam in ed .
Re fe re nc e s
1. ASHRAE 62-1989, Ven tilation for
Accep table In d oor Air Qu ality. Am erican Society of Heatin g, Refrigeratin g an d Air-Con d ition in g En gin eers, Atlan ta, GA. 2. Stan d ard CAN/ CSA-F326-M91,
Resid en tial Mech an ical Ven tilation System s. Can ad ian Stan d ard s Association , Etobicoke, ON.
3. Nation al Bu ild in g Cod e of Can ad a, 1995. Can ad ian Com m ission on Bu ild in g an d Fire Cod es, Nation al Research Cou n cil of Can ad a, Ottaw a.
4. 1989 Su rvey of Airtigh tn ess of New, Merch an t Bu ild er Hou ses. Haysom , J.C., Reard on , J.T., an d R. Mon sou r. In d oor Air ’90: Th e Fifth In tern ation al Con feren ce on In d oor Air Qu ality an d Clim ate, v. 4, Toron to, 1990.
5. Resid en tial Air System Design . Heatin g Refrigeratin g an d Air-Con d ition in g In stitu te of Can ad a (HRAI), Islin gton , ON, 1986.
6. Com p lyin g w ith Resid en tial Ven tilation Requ irem en ts in th e 1995 Nation al Bu ild in g Cod e. Can ad a Mortgage an d Hou sin g Corp oration , Ottaw a, 1996. 7. Airtigh tn ess an d En ergy Efficien cy of
New Con ven tion al an d R-2000 Hou sin g in Can ad a, 1997. Can ad a Cen tre for Min eral an d En ergy Tech n ology, Natu ral Resou rces Can ad a, Ottaw a.
Mr. John Ha y somis a sen ior tech n ical ad visor with th e Cod es an d Evalu ation Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
Dr. J.T. Re a rd onis a research officer with th e In d oor En viron m en t Program of th e N ation al Research Cou n cil’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
© 1998
Nation al Research Cou n cil of Can ad a May 1998
