Building heat loss calculator
Heating a room means replacing what it loses. Fabric loss goes through the walls; ventilation loss goes out with the air.
W/(m2 K)
Heat loss at design conditions
800W
- In BTU per hour
- 2730
- Fabric loss
- 420
- Ventilation loss
- 380
- Radiator or emitter size with 15% margin
- 920
- Heat loss per square metre
- 40.0 W/m2
- Room volume
- 48.00
- Rough annual heating energy
- 2304 kWh
Show the working
- Floor area = 20
- Ceiling height = 2.4
- External wall, roof and glazing area = 35
- Average U-value = 0.5
- Air changes per hour = 1
- Inside temperature = 21
- Design outside temperature = -3
- dt = 21 - -3 = 24
- Room volume = 20 * 2.4 = 48
- Fabric loss = 35 * 0.5 * 24 = 420
- Ventilation loss = 0.33 * 1 * 48 * 24 = 380.16
- Heat loss at design conditions = 420 + 380.16 = 800.16
- In BTU per hour = 800.16 * 3.412141633 = 2730.26
- Heat loss per square metre = 20 > 0 ? 800.16 / 20 : 0 = 40.008
- Radiator or emitter size with 15% margin = 800.16 * 1.15 = 920.184
- Rough annual heating energy = 800.16 * 24 * 200 / 1000 * 0.6 = 2304.46
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- fabric loss = area x U-value x temperature difference
- ventilation loss = 0.33 x air changes per hour x volume x temperature difference
- the 0.33 is the volumetric heat capacity of air in Wh per cubic metre per kelvin
A proper heat loss calculation treats each element separately with its own U-value. This is a whole-room approximation.
Radiators are rated at a specific temperature difference, usually delta T 50. At a lower flow temperature they emit far less.