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Heat flow in buildings
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Heat flow in buildings

Heat is a form of energy which flows from a hotter to a colder source. Solar radiance is the most important factor in determining heat flow; in summer heat is likely to flow into a building whilst during the winter the flow is reversed. Northern Europe has long winters and short summers whereas the length of the seasons is reversed in southern Europe.

The level of comfort inside a building depends upon the season, time of day and whether the room is being heated or cooled; it also depends upon the level of activity such as working during the day or sleeping at night. In the home the largest amount of energy is used for space heating and it is important to understand the ways in which heat can flow in order to reduce our energy consumption and use energy more efficiently.

Heat transfer

Heat energy can be transferred from a hot to a colder object in three ways

  • Conduction where the heat is physically transferred across a like a wall or window
  • Convection in which the heat is circulated away from the heat source by moving air
  • Radiation in which heat is transferred by heat waves, the sun being the ultimate radiation source

These heat transfer processes are all dependent upon the difference in temperature between the hot and cold source; the greater the temperature difference the faster the heat transfer.


IIlustration of heat transfer processes
The process is of course reversible; in summer the heat flow is inward because it is warmer outside than inside whilst during the winter the flow is outward when it is warmer inside than outside. The objective of good passive architecture is to reduce both sets of heat flow arising from radiation (see Chapter 5).

The role of insulation is to reduce the rate of at which heat flows by conduction. The opening and closing of windows will alter the ventilation. If there are no ventilation vents the windows will need to be open sufficiently to give good air quality but not too far to prevent unnecessary heat loss.



 

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