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Central Heating Radiator Sizing

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Radiators are often not correctly sized for the room size or intended use. It is important to correctly size radiators for the size of room to be heated and environmental conditions. Rooms with large windows, north facing [in northern hemisphere] and south facing [in southern hemisphere] take more energy to maintain at a comfortable temperature. Radiator sizing also needs to take account of ceiling height. As heat rises a larger radiator size is needed for higher ceilings.

Always check manufacturers data for heat output, stated in BTU/Watt values. Note bathroom towel rail type radiators are generally not suitable to heat a bathroom. Be sure to calculate room heating requirements when choosing a towel radiator.

Chrome towel radiator. Check heat output Watt/BTU if you want this radiator to also heat your bathroom.

The table below details the correct values for different types of rooms and locations. The values are based on 1m³. You can multiply the table values by the volume of your room [length x width x height] to see what you need. Manufacturers and Retailers can give details of Watts/BTU values for each radiator size and type.

Per m³ lounge, sitting room, study, games room, dressing room, dining room bedrooms, bathrooms and shower rooms common areas, halls, landings and kitchens
Approximate heat loss value Approximate heat loss value Approximate heat loss value
Room type Watts BTU Watts BTU Watts BTU
standard room, where non of the factors below apply 51 174 41 140 31 106
north facing room 59 200 47 161 36 122
very large windows or french windows 61 209 49 168 37 127
double glazed 46 157 37 126 28 95
very large double glazed or french windows 56 191 45 154 34 116
north facing with double glazing 54 183 43 147 33 111
north facing* with large windows or french windows 69 235 55 189 42 143
north facing room with very large double glazed windows or french windows 64 218 51 175 39 132

* North facing in northern hemisphere and south facing in southern hemisphere. This is because the exterior structure of the building is in permanent shade. This could also apply to buildings in close proximity to each other. It may also be the case that exterior walls are in permanent shade.

When adding a new radiator to an existing system it is important to check the radiator sizes of all existing radiators. The heat output capacity of your boiler needs to be checked to confirm the additional radiator[s] will work efficiently.


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