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The heat emitted from a radiator or heating panel depends primarily on the temperature difference between the radiator and the surrounding air. The heat emission can be expressed as:

P = P₅₀ [ (t_i - t_r) / ln( (t_i - t_a) / (t_r - t_a) ) 1 / 49.32 ]ⁿ         (1)

where

P = the emitted heat (W, J/s)

P₅₀ = the emitted heat with an temperature difference 50^oC (W). In general radiators are dimensioned¹⁾ for a middle temperature of 70^oC and surrounding temperatures of 20^oC (a difference of 50^oC)

t_i = water temperature inn (^oC)

t_r = water temperature out (^oC)

t_a = surrounding air temperature (^oC)

n = constant describing the type of radiator (1.33 for standard panel radiators, 1.3 - 1.6 for convectors)

Example - Heat Emission from Radiator

The heat emission from a radiator with nominal heat emission 1000 W (water temperature inn t_i = 80^oC, water temperature out t_r = 60^oC, surrounding air temperature t_a = 20^oC) with water temperature inn t_i = 70^oC and temperature out t_r = 50^oC can be calculated as

P = (1000 W) [ ((70 ^oC) - (50 ^oC)) / ln( ((70 ^oC) - (20 ^oC)) / ((50 ^oC) - (20 ^oC))) 1 / 49.32 ]^1.33

    = 736 W

Radiator Heat Emission Calculator

Heat Emission and Water Flow

The calculator below can be used to calculate the actual heat emission and water flow from a radiator outside standard conditions - like increasing or decreasing the water inlet or outlet temperature or the surrounding room air temperature.

P₅₀ - nominal heat emission from radiator?- from the manufacturer (W)

t_i - actual water inlet temperature to the radiator (^oC)

t_r - actual outlet water temperature from the radiator (^oC)

t_a - surrounding air temperature in the room (^oC)

n - constant describing the type of radiator

Water Return Temperature and Flow

The calculator below can be used to calculate the water return temperature and the water volume flow through radiators based on actual heat emission and inlet water temperatures.

Oversized radiators are quite common since it is almost never possible to adapt standard radiators exactly to the required heat loss from a room. With the calculator below it is possible to study the consequence of out of standard heat emission when the radiator is oversized or when the outside temperature are higher than the design temperature, inlet or surrounding room temperature can be studied.

P - actual heat emission from radiator?- heat loss from the room covered by the radiator (W)

P₅₀ - nominal heat emission from radiator?- from the manufacturer (W)

t_i - actual water inlet temperature to the radiator (^oC)

t_a - air temperature in the room (^oC)

n - constant describing the type of radiator

When checking heat emission capacities of radiators, be aware that testing standards differs. Examples:

• BS 3528 "Specification for convection type space heaters operating with steam or hot water"? (withdrawn, replaced by BS EN442) - flow temperature 90^oC, return temperature 70^oC, air temperature 20^oC
• BS EN442 "Specification for radiators and convectors." - flow temperature 75^oC, return temperature 65^oC, air temperature 20^oC

Testing the same radiator with BS EN442 will reduce the heat output with approximately 11% compared to BS 3528.

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