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Heater Size Calculation

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heater calculation diagram

To calculate the heater size required for your building, use the following formula :-

V x dT x K x B = Heater Size in BTU

  • V = Volume in cubic metres (width x length x height of building in metres)
    (or calculate in cubic feet then divide by 35)
    use the average roof height between eaves and roof apex


  • dT = difference between outdoor and required indoor temperature in degrees C
    normally use a figure of 20. i.e. heating from minus five degrees outdoors to plus fifteen degrees in workshop


  • K = coefficient determined by the building construction
    - K = 3.5 for simple non-insulated building in wood or corrugated metal
    - K = 2.5 for poorly insulated building, single brick layer, simple windows, simple roof
    - K = 1.5 for moderately insulated building, double brick layer, few windows, closed roof
    - K = 0.8 for well insulated , advanced construction building
    select the value that best describes your building


  • B = 4 (multiplication factor to arrive at BTUs)
    always multiply by this figure


  • Divide your answer in BTUs by 3333 to convert to KW (approximate conversion)



Approximate conversion between imperial and metric units
10,000 BTU = 3KW
10KW = 33,000 BTU

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