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close this book National design handbook prototype on passive solar heating and natural cooling of buildings
close this folder VII. Detail design
View the document A. General
View the document B. Solar access, shading and window protection
View the document C. Control of conductive heat flow
View the document D. Evaluation of internal heat loads
View the document E. Cross-ventilation and air flow
View the document F. Glass-mass relationship
View the document G. Air infiltration

G. Air infiltration

Air infiltration accounts for a very large proportion of the heat losses in most buildings that are not pressurized by air- conditioning. Infiltration Is generally more significant in winter than summer due to the large difference between inside and outside temperatures. If the building has tight construction at windows and doors then infiltration rates would be expected to be around 0.5 air changes per hour. This may be further reduced by carefully sealing openings in walls and around doors.


Figure 99. A 5-star/Ballinger graph showing glass-mass relationships for Sydney (carpet on concrete floor)


Figure 100. A 5-star/Ballinger graph showing glass-mass relationships for Sydney (tile on concrete door)

Chimneys can provide a very large infiltration loss as illustrated earlier. The top of a chimney is designed to be in an area of low pressure to assist its designed task of removing smoke and hazardous gases from the fireplace. Consequently it provides a continual exhaust and source of winter heat loss when not in use. Chimneys tend to be a greater source of heat loss than do cracks around windows and doors and so a damper should be provided to close off a chimney that is not in use. For a known set of temperature conditions it is possible to calculate the effect of a chimney on the infiltration of a building. This was discussed earlier in the section on cross-ventilation and air flow.

Permanent ventilation should not be used in houses, except where required by regulations. Instead it is better to use exhaust fans with automatic shutters. Kitchens and bathrooms are best ventilated by opening doors or windows, when required. In winter, ventilation should occur during the warm part of the day so that during the cold nights the house can be closed up. Summer ventilation is a different problem. If it is very hot outside it may be desirable to close up a house during the day and open it up at night. If it is not too hot, such as is the case on most summer days in Sydney, then windows are left open at all times. Fixed vents are not then required. A large, slow-turning exhaust fan in the centre of the house, may be the best solution for summer cooling. During still nights it would draw cool air through the house and thus cool down the structure ready for the next hot day.

The heat storage capacity of the air (volumetric specific heat) varies according to the humidity but is generally taken to be 1200 Jm³.degC and so if the air in a space of say 86m³ is changed four times an hour due to infiltration then the energy required to compensate for this over an evening of say six hours can be calculated as shown below. At 1989 electricity costs in Sydney, this amounts to 89C/evening just to compensate for cold draughts. This is just the ventilation losses and does not include the cost of energy lost through the building fabric. The cost in colder places such as the Southern Tablelands of New South Wales and Tasmania would be even greater.

Hv

= 1200 x DT x (N x V) x t /3600

 

= .33 x 15 x 4 x 86 x 6

 

Where

Dt

= temperature difference between inside and outside

N

= number of air changes per hour

t

= hours of heating per evening to compensate for draughts

V

= volume of space m³

A number of locally made seals and gaskets suitable for use on doors and windows are illustrated in figure 101.


Figure 101. Examples of locally-made seals and gaskets to reduce air infiltration around doors