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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 [ (ti - tr) / ln( (ti - ta) / (tr - ta) ) 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)

ti = water temperature inlet (oC)

tr = water temperature outlet (oC)

ta = 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 70oC - and surrounding air temperature 20oC (difference 50oC)

Example - Heat Emission from Radiator

The heat emission from a radiator with nominal*) heat emission 1000 W with water inlet temperature ti = 70oC and outlet temperature tr = 50oC 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 ti = 80oC, outlet water temperature out tr = 60oC and surrounding air temperature ta = 20oC

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.

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

ti - actual water inlet temperature to the radiator (oC)

tr - actual outlet water temperature from the radiator (oC)

ta - surrounding air temperature in the room (oC)

n - constant describing the type of radiator

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.

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)

ti - actual water inlet temperature to the radiator (oC)

ta - 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 of standards:

• BS 3528 "Specification for convection type space heaters operating with steam or hot water" (withdrawn, replaced by BS EN442) - flow temperature 90oC, return temperature 70oC, air temperature 20oC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

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

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