To enable you to understand the heat requirements of a space you need to understand how heat is lost from a space and how quickly this happens. This is to enable you to calculate the heat output you require and how quickly you need to heat a space to maintain the desired or design temperature.
Quick Rule of Thumb Route
Step One
Calculate the area of the space to be heated in m2
Measure and record the external or internal length in metres.
Measure and record the external or internal width in metres.
Example: Length 10m x Width 5m = 50m2
Step Two
Calculate the heat requirement of the space by using rules of thumb for different spaces.
Below are guides to heat requirements:
Living Room 60W/m2 21 Deg C
Bathroom 70W/m2 21 Deg C
Kitchen 60W/m2 16 Deg C
Bedroom 60W/m2 18 Deg C
Hall and Landing 60W/m2 16 Deg C
This is based on solid brick or block dwellings with cavity thermal insulation and double glazed windows.
Example:
Length 10m x Width 5m
= 50m2 x 60W/m2
= 3,000W
= 3 kW
This example can also be used to calculate the output required for a new boiler installation
Specific Heat Loss Calculation Route
To calculate the specific heat loss in a space you have to consider the temperatures of the outside space and the desired inside space. There is an accepted temperature criteria, which is – 4 Deg C minimum outside temperature and the desired temperatures as shown above in the Rule of Thumb step two.
To carry out the specific heat loss calculation you measure the space dimensions internally i.e. height, width and length in metres. Calculate the area of the windows in m2 and subtract this from the wall area and measure and calculate in m2 the ceiling and floor areas.
With the above carried out we can start to calculate the loss of heat through the material of the space e.g. Walls, floor, windows and roof or ceiling. In addition, calculate the heat requirements of the infiltration ventilation, from areas of the space that allow air into the space from outside from under doors or through windows with no draft proofing, etc…
This we use for air change rates within the space.
The way heat loss is calculated is by measuring the area and multiplying it by the known U value for the material. This is the thermal resistance of the material multiplied by the thickness of the material. There are tables available giving U values of materials e.g. an unfilled cavity wall has a U value of 1.6.
These values are calculated from the K value of the material multiplied by the thickness of the material.
Calculation example:
Space Temp 21/Outside -4/Air Changes 3/Differential Temp 25
Room: Living Dimensions x U Values = specific heat loss
Air 10 x 5 = 50 x 3 = 150m3 x 0.33 = 50
Glass 1 x 2 = 2m2 x 3.0 = 9.5
Walls (external) 10 + 5 x 3 = 45-2=43m2 x 0.45 = 19
Floor 10 x 5 = 50m2 x 0.10 = 5
Roof/Ceiling 10 x 5 = 50m2 x 0.54 = 27
Specific heat loss total = 110 x 25
Total Heat Loss = 2,750W HLoss Total
If we round up the result to 3,000w or 3kW you can see that we have lost 250W from our original rule of thumb calculation. This will not always be the case and if we had more window area, more outside walls or more air changes we would have more heat loss. In addition, it should be remembered that the rule of thumb is an approximation.
This can now be applied to each space and to radiator sizing calculations and schedules.
Historically a margin was always added to the calculation when radiator sizing of 11%. So, if we take our 3kW and add 11% it becomes 3.33kW say 3.5kW.