Heat Emission from Radiators and Heating Panels

Heat emission from a radiator or a heating panel depends primarily on the temperature difference between the hot surface and the surrounding air. The heat emission can be calculated

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

where

P = heat emission from radiator (W, J/s)

P₅₀ = heat emission from radiator with temperature difference 50 ^oC (W)

t_i = water temperature inlet (^oC)

t_r = water temperature outlet (^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)

Note that radiators are in general designed for middle panel temperature 70^oC - and surrounding air temperature 20^oC (difference 50^oC)

Example - Heat Emission from Radiator

The heat emission from a radiator with nominal^*) heat emission 1000 W with water inlet temperature t_i = 70^oC and outlet temperature t_r = 50^oC can be calculated

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

    = 736 W

^*) nominal when inlet water temperature t_i = 80^oC, outlet water temperature out t_r = 60^oC and surrounding air temperature t_a = 20^oC

Radiator Heat Emission Calculator

Heat Emission and Water Flow

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

Return Temperature Water 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 a standard radiator 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 a radiator is oversized.

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

• BS 3528 "Specification for convection type space heaters operating with steam or hot water" (withdrawn, replaced by BS EN442) - flow temperature 90^oC, return temperature 70^oC, air temperature 20^oC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 compared to BS 3528 reduces the heat output with approximately 11%.