Radiator Sizing

Summary: Tips and information on how to calculate the size of radiator required to heat a room including a BTU calculator to help you.

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What Temperature Do You Want Your Radiators To Heat Your Room To?

The size of radiator required for a room depends on two factors.

Firstly, the temperature that you want it to be able to maintain which is a relatively straightforward task and you can use the table below as a guide.

Room Ideal Temperature (deg C)
Lounge 21
Dining Room 21
Kitchen 16
Bedrooms 15
Bathroom 23
Stairs 18

How Much Heat From The Radiators Is Going To Escape From My Rooms?

The second consideration to take into account is the the heat loss from your room. The calculations for this are quite complex since they depend upon the size of windows, numbers of doors and, in particular, the construction materials used in the building. Too big and the system will overshoot its temperature and be less economical to run, to small and it won't reach its desired temperature.

Some of the poorer installers get round this complex step by putting in radiators that are too big, and then fitting thermostatic radiator valves to every radiator to cut the heat down. This calculation produces a heat loss figure in watts, of how much heat you need to warm that room up to the design (and desired) temperature from -3 deg C in one hour.

Calculate The Size Of Your Radiators Using A BTU Calculator

A quick and easy way to calculate the size of the radiator required for any room in your house is by measuring the room in cubic feet and then entering the information into a BTU (British Thermal Units) calculator or using the table below to estimate the total BTU's required to correctly heat your room.

Room Multiplication Factor
Lounges and dining rooms Multiply cubic feet by 5
Bedrooms Multiply cubic feet by 4
Common areas and kitchens Multiply cubic feet by 3
For rooms facing north Add 15%
For French windows Add 20%
For double glazing Deduct 10%

This will give you the output of any radiator in BTUs (British Thermal Units). Adding the total for all the rooms in your home will give you the approximate demand in BTUs for the whole house. Add 20% to the total for a hot water circulating tank and 10% for general losses. This will give you the boiler size you need for your house. Obviously these calculations are approximate and while giving you an excellent idea of the sizes you will require, it would be best to get this checked by the installer.

It is very unlikely that any radiator will match the exact heat required, so select the first size of radiator above the heat requirement. With rooms greater than 6 meters (18ft) in any one direction, it is worth considering distributing a number of radiators to minimise the thermal gradient within the room.

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