Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Heat Emission from Radiators and Heating Panels

The heat emission from a radiator or a heating panel depends on the temperature difference between the radiator and the surrounding air.

Heat emission from radiator or heat panel

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)


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

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.

- heat loss from the room covered by the radiator (W)

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%.

Related Topics

  • Heating Systems

    Design of heating systems - capacities and design of boilers, pipelines, heat exchangers, expansion systems and more.

Related Documents


Search is the most efficient way to navigate the Engineering ToolBox.

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

Translate this Page

Translate this page to Your Own Language .

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .


This page can be cited as

  • The Engineering ToolBox (2003). Heat Emission from Radiators and Heating Panels. [online] Available at: [Accessed Day Month Year].

Modify the access date according your visit.

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Unit Converter