4 . 1 I NTRO DUC T I O N : CALC ULAT I N G H EAT L O S S ES BY COMPUTER
Previous c hapters have buil t up a picture of what the o c cupant s of a hous e require for thermal comfort , and how Tasmania ' s c l imate affec t s the d emands made on heating systems . The f inal factor affec t ing heat ing requirements is the hous e itsel f . The eff ec t s o f insulation
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and d if f erent cons truc t ion mat er ial s on the heat ing r equirements of two hous ing d ivis ion houses can b e s een f rom the information presented in Chap t er Five . Tho s e resul ts are the result s o f detailed computations us ing the TEMPAL computer p ro gramme developed by Alan Coldicutt at the Univers i ty of Melbourne . In this chapter , we will show how these resul t s have been d erived , and eva luate the effects o f two variables - ho us e s ize and design - tha t are not adequately accounted for in the
computed resul ts .
When part of a house is mainta ined at a constant temperature , heat is lost by the proces ses of conduc t ion , through the bui lding enve lope
(walls , windows , f loor and roof ) and convec t ion , due to loss of warm air through natural vent ilation . By the same processes , s ome of the heat pa s s es from the heated section of the house to the unhea ted sec t ion . Part o f the heat ing load caused by these heat lo sses will b e supplied by body heat of the occupants and heat gains from appl iances and (during dayl ight hours) from solar radiat ion .
Figure 4 . 1 shows typ ical va lues of heat gains and losses occurring in an uninsulated house over a Hobart mid-winter ' s day . Thes e value s ind icate the relative impor tance of the various paths o f heat lo s s .
Conduction and convec t ion heat losses , and solar radiat ion heat gain , vary qui te markedly from hou s e to house . For a given location and t ime , the radiat ion heat gain d epends primarily on the p o s i t ion , size
Heat lo s s to 12 kWh
Heat lo s s through ceiling and roof
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38 kWh
So lar heat ga in through Conducted heat lo s s through windows
)L{ /'
hea t gains 17 loss through walls _____ "_" ____l
1
kW 1 2 kWh To tal inf iltration '- heat lossHeat lo s s through floor
F i g u re
4 . 1 :
Computed heat gains and losses ( exclud ing heater input) for an uninsul ated weath erb o ard hous e , heated f rom 7 a . m . to 1 1 p . m . , on a Hobart winter ' s day . The n e t heat loss o f 9 0 kWh i s close to the average daily heat l o s s of this house in midwinter (June and July) .and shad ing o f windows . Convected heat losses depend on exposure to wind , and the use o f measures ( such as weather-stripping) to control vent ilat ion . The maj or heat lo sses , by conduc t ion , depend on the
thermal resis tance o f the build ing envelop e , wh ich can b e greatly inc reased by the us e of insul ation .
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When condit ions vary over t ime , heat storage in the building s tructure becomes impo rtant . Some o f th e hea t gained from solar radiation or heating appl iances s erves to heat the building s tructure , rather than
the a ir in the house . At night , or when the heating is turned off , the stored heat may be r eleased , help ing to maintain the indoor temperatur e . As a resul t , heavywe ight b uildings , with high thermal s torage , d o no t exper ienc e the same daily extremes o f temp erature and heating load as lightweigh t buil d ings . This resul t s in improved year-round comfo r t , though annual hea t ing requirements are relat ively una ffec ted (Coldicutt
et a Z . , 1 9 78 ) . Heavyweight build ing materials also work in favour o f
the economics of hea t pump s , s ince t h e lower peak heating loads can b e suppl i ed b y smaller , les s expens ive units .
The thermal p erfo rmance of a numb er o f houses has b een predic ted by a computer programme ( TEMPAL) which uses values of a number o f thermal parame ters to produce a numer ical model of each house . The maj or aspects of this model , and the important thermal paramet ers , are
discussed in sect ion 4 . 2 . With the resources availabl e , it was po s s ib le to treat only a few out o f the vas t range of pos s ible Tasmanian ho uses . S ect ion 4 . 3 explains how the cho ices were made . Section 4 . 4 evaluates the effects o f hous e design on heat ing , and the ac curacy o f the assump tion tha t heat ing load is proport ional to area .
4 . 2 .
MO D E L L I N G THE RMAL P E RFO RMAN C ECon d uc t e d Hea t Lo s s es
The mos t us eful measure for calculating s teady-s tate conduc ted heat lo sses is th e U--vaZue . This is the amount of hea t energy per second tha t passes through a square metre o f the building envelope when a 1°C differenc e in (air) temp erature i s maintained acro s s i t . The U-value is the inverse of th e thermal resis tance ( R) , and the heat loss p er square me tre of the b uild ing envelope is calculated by the equation :
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