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Formulas to calculate the required power output of a radiator for a room of a specific size. Do they exist?

Arran Cameron
Are there any recognised formulas to calculate the required power output of a radiator for a room of a specific size that also takes into account variables such as extra height, large areas of glass, insulation of walls etc? Good quality formulas devised by people who know about thermodynamics as opposed to plumber's rules of thumb passed down from the 1950s.


Numerous online radiator power outlet calculators exist but the formula behind them is not disclosed and the results for the same input variables vary from website to website.
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    I'm sure that, given a bit of time, a physicist could give you an near-exact formula to calculate the size of radiator you would need to maintain the temperature of a given room.


    The trouble is, it would have many variables that you don't have the exact values for.  Plugging in wild guesses would give you a result no more accurate than the plumber's rule of thumb.


    Off the top of my head, you would need to know:

    The temperature of the water in the radiator (and do you want to account for the temperature drop from the inlet to the outlet?).

    The thermal conductivity of the steel and paint of the radiator.

    The surface area of the radiator.

    The specific heat capacity of the air in the room, including any water vapour in it.

    How well the air is circulated around the room - do you want to do computational fluid dynamics on that?

    The thermal conductivity of the walls, floor, ceiling, windows and doors.

    The temperatures on the other sides of those walls, floor, etc. (not necessarily the same as the outdoor temperature).

    How much cold air leaks in around doors and windows.

    What temperature that air is at.

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  • 0
    I'm sure that, given a bit of time, a physicist could give you an near-exact formula to calculate the size of radiator you would need to maintain the temperature of a given room.


    The trouble is, it would have many variables that you don't have the exact values for.  Plugging in wild guesses would give you a result no more accurate than the plumber's rule of thumb.


    Off the top of my head, you would need to know:

    The temperature of the water in the radiator (and do you want to account for the temperature drop from the inlet to the outlet?).

    The thermal conductivity of the steel and paint of the radiator.

    The surface area of the radiator.

    The specific heat capacity of the air in the room, including any water vapour in it.

    How well the air is circulated around the room - do you want to do computational fluid dynamics on that?

    The thermal conductivity of the walls, floor, ceiling, windows and doors.

    The temperatures on the other sides of those walls, floor, etc. (not necessarily the same as the outdoor temperature).

    How much cold air leaks in around doors and windows.

    What temperature that air is at.

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