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The typical home owner would like the inside of their house to be around 72 degrees on the inside in the winter. This is called the Winter Inside Design Temperature. However, because it is cold outside, heat travels through the building envelope. the walls, windows and ceilings to the outside. This heat is lost by conduction. Also, cold winter air leaks into the house and warm air leaks out. This is called infiltration.
There is a continuous movement of heat from the inside to the outside, which is measured in units called BTUs( british thermal units). The speed of the movement of ehat is called the heat loss and is measured in BTUH, which means BTUs per hour.
If it is 72 degrees inside the house and 52 degrees outside then the 20 degree temp. differential will cause a certain number of BTUs to leave the house each hour, let's say that the number is 9,768BTUH. The heat loss of this house at 52 degrees is 9,768 BTUH. This means that your heating system needs to produce 9,768 BTUHs each hour to keep the house at 72 degrees, when it is 52 degrees outside.
If it is even colder outside, then the house will lose more heat each hour, the heat loss will be higher. When selecting a heating system, at what outside temperature do you need to know the haet loss? Well, this of course depends on where you live, how cold your winers are. The temperature to use as an outside temp. is called the Winter Outside Design Temp.. This is the temp., say 10 degrees for instance, at which only 2 and a half% of the time is colder that 10 degrees. The heat loss of the house when calculated with an outside temp. of the Winter Outside Design Temp. is called the Design Heat Loss. Because the heat loss at any temp. other than the design temp, is not really a relevant number, we usually just say heat loss, rather than design heat loss.
So, to recap, the heat loss of the house is the number of BTUs lost each hour when the house is at the inside design temp inside and the outside is at the winter outside design temp.
FACTORS AFFECTING HEAT LOSS:
1. Temperature difference
2. Area of the building envelope
3. Thermal Resistence
4. Tightness
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