calculate heater

Radiators and radiator calculate the heating body exchange

calculate heater

Radiators are the interface between heating and to be heated ambient air. Is the size and output of the radiator to the room to be heated not appropriate energy is wasted or the room is not sufficiently heated. the necessary heat dissipation for the space should be calculated before buying new radiators. The insulation plays a major role: The heat value of the house should be included in the calculations. If the windows penetrate drafts, repair or replacement of windows is highly recommended.

In this article, we provide the basis for the most accurate calculation radiators. A professional heat demand calculation can not replace them. However, home improvement and building owners, it offers a good frame of reference.

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calculate heater

Calculate Heating: Site

  • Radiators calculate a simple example
  • Radiators standards and their use
  • Surcharges for certain structural conditions
  • Example of calculation of a heater in a building with insulating windows
  • Determination of the heating value of the old radiator

Radiators calculate a simple example

The calculation of the required heat output of radiators for new buildings is a very complicated formulary and requires a high level of expertise. But here it should go only to the exchange already mounted radiator. These calculations can be performed by lay the basis of relatively simple formulas.

Wanted is always a suitable radiator for the required power in watts (W) per square meter (sqm). 

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Radiators standards and their use

Basic condition for all calculations of radiators is the knowledge of the following DIN standards:

  • DIN 4707 alt: Flow temperature 90 ° C, reflux 70 ° C,
    Room temperature 20 ° C, Shortcut: 90/70/20

  • DIN 4707 new: Flow temperature 70 ° C, reflux 55 ° C,
    Room temperature 20 ° C, Shortcut: 70/55/20

  • EN 442: Flow temperature 90 ° C, reflux 70 ° C,
    Room temperature 20 ° C, Shortcut: 75/65/20


The DIN 4707 in its old version is suitable only still for comparison purposes. Meanwhile be except on very old heating systems, radiators only in accordance with DIN newly built 4707th This is also called low-temperature heating.

The EN 442, are calculated using the built in other European countries radiators, can not be transferred directly to the new version of DIN 4707th Rule of thumb for the conversion is here: EN 442 = 1.2 x heating capacity according to DIN 4707th

Surcharges for certain structural conditions

new on the basic heat demand, according to DIN 4707 about 100 watts / m, following service charges must be added for certain structural conditions: 

  • 20 W / m²:  Award without thermal insulation glazing
  • 10 W / m²:  Extra charge for a detached house
  • 10 W / m²:  Extra charge for blankets over 2.60 m

Example of calculation of a heater in a building with insulating windows

Now we can venture out to the calculation:

Given a room with 20 m² in an insulated, semi-detached building with insulated windows and high ceilings normal:

20x100 W = 2000 W

The same calculation for the free-standing building with high ceilings and single glazing falls considerably less favorable:

20x100 W + 20x20 W / m + 20x10 W / m + 20x10 W / m = 2800 W

Note, that the only compromise is a rule of thumb in this calculation, of course. An experienced installer charged significantly more refined methods and measurements the heat demand much more accurate and more energy efficient. As a frame of this bill is all but sufficient.

Determination of the heating value of the old radiator

calculate heater

But there is another way: In the old cast iron radiators, the approximate previously built thermal value can also be based on the dimensions and the design estimate. A prerequisite for this calculation is that a pleasant room temperature could be achieved with the existing plant.

Important parameters are here, in addition to the dimensions of the radiator, known as the hub distance between the supply and return connection pipe and the number of radiator fins (see the drawing)

following table applies to the calculation based on the dimensions of the old radiator.

Note: The heat output is registered in "Watts per rib"


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Suppose for our sample calculation that a "rib" the old cast-heater over a boss spacing of 50 cm has, 16 cm deep and 60 cm high.

In this case can be found in the table, a heating power of 63 watts per "rib" New DIN 4707th We now expect that our radiators has 25 ribs, the following calculation:

25 x 63 1575 watts per radiator

Based on this value, a new radiator can now be purchased - provided that the old power of the heater was to heat the room adequately. Ask the expert usually cost less than the incorporation of false radiator: If in doubt, always applies.