Getting radiator sizing right: A detailed guide for installers
Fitting a radiator is relatively straightforward – it’s choosing the right size that makes all the difference. A radiator that’s too small will never heat the intended space properly, while a radiator that’s too big is a waste of energy.
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Myson’s Room Heat Loss Calculator and Radiator Size Calculators make sizing simple.
How to choose the correct radiator size
A radiator’s heat output is measured in Watts, or historically, BTUs (British Thermal Units). A BTU output measures the thermal energy a system delivers per hour. To convert heat output from BTU/h to Watts, divide by 3.412. To convert Watts to BTU/h, multiply by 3.412.
In simple terms, the radiator needs to match the room’s heat loss per hour. Every room loses a certain amount of heat through walls, floors, windows, ceilings, and ventilation. The radiator’s heat output must replace that lost heat to maintain a comfortable indoor temperature.
A step-by-step guide to sizing a radiator
First, calculate the room’s total volume in cubic metres (m³). This figure gives the total air space that needs to be heated.
Next, find the base watts requirement. For properties from the 1990s-2000s, this calculation is typically room volume x 50. For newer properties from the 2010s and 2020s, this calculation is room volume x 40. This calculation gives the estimated heat output needed to maintain a comfortable temperature in that space.
Now, adjust for specific environmental conditions - insulation, wall construction, and room orientation can significantly affect how much heat escapes. Similarly, different rooms have different heating demands based on how they’re used. For example, a bathroom might require extra watts to maintain sufficient warmth after showers, while a bedroom can run slightly cooler without loss of comfort.
Myson’s Room Heat Loss Calculator makes refining this figure simple and considers a wide range of factors.
Finally, consider flow temperature. Radiator heat outputs are tested at a standard temperature differential (ΔT) – typically ΔT50°C (delta T50) in the UK. If the heating system runs at lower flow temperatures (such as a heat pump or eco boiler), the radiators will output less heat in reality, so emitters must be larger to deliver the required watts or BTUs. Apply the following correction factors to the preferred radiator to get the actual (corrected) heat output, using correction factors below:
ΔT (°C) | Typical system type | Correction factor |
|---|---|---|
50°C | Traditional gas boiler (@80/60) | x 1.0 |
30°C | Eco boiler (@55/45) | x 0.52 |
22.5°C | Heat pump system (@45/40) | x 0.35 |
If the final heat output figure falls between two sizes, it’s usually better to go slightly higher, as heat can always be regulated using a thermostatic radiator valve (TRV).
Guessing or ‘eyeballing’ a radiator size can result in poor thermal performance or wasted energy. Myson’s Radiator Size Calculators can help identify the exact size requirements.
Other factors to consider
Make sure the radiator is positioned where it can heat the room evenly and isn’t obscured by furniture. Most rooms will only need one radiator, as long as it’s properly sized to meet the room’s heat requirement. If the available wall space means one single radiator cannot deliver the required output, then it’s fine to use two – it’s the total output across radiators that matters.
With the right calculations and the right emitter, installers can easily deliver the heat output each room needs. Myson’s calculators and range of radiators make it easier to get sizing right from the start.
Explore Myson’s heating solutions or contact the team for further guidance.