 What size radiator do I need to heat my room?
A radiator is sized to provide the heat required for your room. This heat is usually referred to using the terms Watts or BTU. Note: 1 Watt = 3.412 BTU. E.g. I need 1000 watts/3412 BTU to heat my bathroom so what dimension of radiator will give me that output?.
The Watts or BTU you require is dependant on two main factors:
a) The temperature you wish the room to heat to e.g. 21ºC
b) How much heat the room loses through walls, windows etc. You are trying to calculate how much energy (Watts/BTU) you need to put into a room to reach a certain temperature given that X amount of this energy or heat will be lost through walls, windows etc.
a) Temperature Required for Room
The following are recommended temperatures required for rooms depending on their type:
- Kitchen/Hall/Toilet/Bedrooms 18ºC
- Living Areas & Bathrooms (with shower) 20-21ºC
b) Heat Loss of the Room
- To get a definitive calculation on this you need to take into account the area of all floors, walls, and windows. The U Values of all construction materials and other relevant information such as whether the building is on a sheltered or exposed site.
- The Heat Loss Calculator on this website takes into account most aspects and will give a fairly accurate calculation on the size of radiator you require.
- There is an older more general method which just multiplies the volume of the room by a factor of 40 to give you the number of watts required. There are similar calculations for BTUs. We recommend that you do not use such a general system. The old method of over-sizing your radiator “just to be sure” is an inefficient way of heating. Yes you can use a thermostatic valve, but you will also be paying for a larger radiator that you don’t need.
What effect do different heating systems have on the size of radiator I require?A number of years ago there really was only a few forms of heating- oil, gas, solid fuel. Each of these systems supplies water into your radiator at high temperatures e.g. 80-90ºC
When you were sold a radiator you were told for example that the radiator will give you 1000 watts/ 3412 BTU.
The assumption was made that the temperature of the water going into the radiator was say 80-90ºC. This is usually called Δt 60 ºC (Delta T 60 ºC) or Δt 50 ºC
(Delta T 50 ºC).Delta T = (Input Temp – Return Temp) / 2 (minus desired room temp)e.g. (90ºC + 70ºC) / 2 – (20 ºC) = 60 ºC so Δt 60 ºC (Delta T 60 ºC).
Alurad Mistral 450mm high x 400mm wide = 610watts/2081BTU at a Δt 60 ºC (Delta T 60 ºC).
However if you took this radiator home and installed it with e.g. a heat pump which supplies water at approx 45 ºC, then you would not receive the 610watts/2081BTU as the Delta T is much lower. This radiator would only provide you with 161 watts / 549 BTU. You can therefore see why you need to oversize radiators when using them with a heat pump. The table below illustrates this point.
| |
Standard Oil/Gas/Solid Fuel |
Condensing/Pellet |
Heat Pump 55ºC |
Heat Pump 45ºC |
| |
Delta T 60ºC |
Delta T 50ºC |
Delta T 30ºC |
Delta T 22.5ºC |
| Correction Factor |
|
0.778 |
0.387 |
0.264 |
|
Mistral 450mm high
x 400mm wide
|
610 watts/2081 BTU |
474 watts/1619 BTU |
236 watts/805 BTU |
161 watts/549 BTU |
It is very important when purchasing your radiator to know what heating system you will be using so that what you purchase is capable of giving you the outputs you require.
The Build Your Radiator of this website allows you to take this into account, asking you to choose a heating system from one of the above examples. We will then estimate what size radiator you need based on the heating system chosen.
You can also download here (PDF 23kb) the outputs of each Alurad model with the Delta T’s listed above.
What if I am using none of the heating systems options provided on your website?
If none of the above systems / Delta T’s apply to you, you can calculate the outputs of Alurad for your own Delta T by using the following downloadable table:
 Download Correction Factor Table 2.64 Mb
Otherwise please contact us with your heating system details and we will provide you with a quotation based on your individual system Delta T.
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Technical 