Heating load

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In structural engineering, heat load is the heat supply necessary to maintain a certain room temperature; it is specified in watts . The heating load depends on the location of the building , the construction of the heat-transferring building surrounding areas and the intended purpose of the individual rooms. The need for thermal protection measures and the design of the heating system are based on this . The determination of the heating load is standardized in EN 12831 .

For existing buildings, the heating load can be determined more precisely and more easily using statistical methods than is possible with building physics methods.

Heating load according to EN 12831

Logo of the German Institute for Standardization DIN EN 12831
Area Heating systems in buildings
title Procedure for calculating the standard heating load
Brief description: Calculation of the standard heating load
Latest edition 2017-09

EN 12831 specifies a calculation method for determining the heat input that is required under standard design conditions in order to ensure that the required standard internal temperature is achieved. These guidelines, which are aimed primarily at planners, builders and operators of heat supply systems , place the method for calculating the standard heating load on a uniform basis across Europe.

The standard describes the procedure for calculating the standard heating load:

  • on a room or zone basis for the purpose of designing the heating surfaces
  • based on the entire heating system for the design of the heat generator

The value parameters and factors required for calculating the standard heating load are stored in so-called national annexes to EN 12831 (e.g. DIN EN 12831 supplement 1). In Annex D of EN 12831 all factors that can be determined at national level are listed and standard values ​​are given for all cases in which no national values ​​are available.

It has been shown that heaters according to EN 12831 are too large. For this reason, a new edition of the national supplement was published on July 1, 2008, which reduces the results to the values ​​of the old DIN 4701. If no national annex to this standard is available, the values ​​can be taken from Annex D of EN 12831.

EN 12831 defines the procedure for calculating the standard heat losses and the standard heating load for standard cases under design conditions. The following buildings are standard cases:

  • Buildings with a limited room height (not more than 5 m)
  • Buildings that can be assumed to be heated to a steady state under standard conditions

Description of the procedure

The heating load of a building is determined by the sum of all transmission and ventilation heat losses plus a re-heating of individual rooms, each based on a space internal temperature and a standard outdoor temperature .

The standard heating load of a room corresponds to a temperature reduction factor (for normally heated rooms = 1), multiplied by the sum of transmission heat loss and ventilation heat loss :

The transmission heat loss of a room is determined by the sum of all the surrounding areas , multiplied by the associated corrected U-values and multiplied by the difference between the indoor and standard outdoor temperatures :

The ventilation loss in a room is determined by the volume flow , multiplied by the specific heat capacity and density of the air and by the difference between the inside and the standard outside temperature :

where in the so-called simplified method the room volume  V and an air exchange rate  n (minimum 0.5) are used. The ventilation loss is thus:


A number of important effects are not taken into account. These include a. Internal and solar gains (energy input from the sun, people, devices), balancing storage effect of components and usage influences, e.g. B. Simultaneous standard design temperature (usually at night) and setback mode, mean statistical absence of some residents in large residential units and only partially standard-compliant heating and ventilation of all rooms. Oversizing cannot therefore be ruled out. There is still a need for research on this, so that reduction factors or milder standard design temperatures could possibly be introduced in the future.

Determination of heat load according to energy consumption in the existing building

Statistical heating load determination according to Jagnow / Wolff

Example of statistical heating load determination

If the energy consumption of a building is known over a longer period of time, its heating load can easily be determined. The average power for each month in kilowatts is calculated from the consumption of individual months .

The process is independent of climate fluctuations. The heating limit and the hot water power requirement of a building can also be determined. The result is influenced by usage habits, which, however, are statistically averaged in larger buildings with several users.

The average power is plotted against the average temperature of the month in a diagram . The result is a point cloud through which an approximate straight line can be drawn. The value to be read from this so-called regression line at the standard design temperature (e.g. −10 ° C) is the standard heating load. The linear regression method can also be applied analytically . The corresponding values ​​can also be daily mean temperatures of individual days, whereby the daily mean temperatures should be below 10 degrees and should show greater fluctuations. Errors can also arise from solar gains.

It should be noted that the standard heating load is based on the heating requirement , but the heating energy requirement is considered here . The statistically determined value is, depending on the degree of utilization of the heat generator, higher than the standard heating load.

Determination of heat load according to Weiersmüller

This method is proposed by the Swiss Federal Office of Energy and delivers good results for residential buildings with boiler outputs <100 kW. Weiersmüller suggests calculating the heating load from annual consumption and an estimated annual heating time .


A house has a consumption of 90,000 kWh / year for heating and hot water.
For the Swiss Central Plateau , this time is around 3000 hours.
The required boiler output is: 90,000 kWh / 3,000 h = 30 kW.
(Conversion: 1 liter of heating oil corresponds to approx. 1 m³ of gas corresponds to approx. 10 kWh → heating oil equivalent )

In Germany, the number of boiler full load hours is used instead of the annual heating time. Depending on the energy standard (old building, refurbished old building, new building) and location, boiler manufacturers quote between 1,600 and 2,100 full load hours for residential buildings.

Consequences of incorrect heating load determination

Investigations according to Wolff / Jagnow in Germany showed that the boilers were dimensioned 1.8 times larger than necessary. In addition to higher acquisition costs, this results in considerable efficiency losses and thus higher costs in operation. To name are among others:

  • the circulation pumps are too big and use too much energy.
  • modern condensing boilers are not operated at the optimal operating point.
  • of power companies a basic price is often calculated, which is calculated according to the heating load or after the performance of the heat generator. If the heating load is incorrectly specified or the dimensions are incorrect, the user pays for the power not required.

In contrast to a thermal insulation calculation according to DIN 4108 or DIN 4701, solar and internal gains are not taken into account when calculating the heating load. The “worst case” is assumed. Particularly in the case of highly insulated buildings in the low-energy or passive house standard, the activated storage mass of which has an additional dampening effect on temperature fluctuations, dimensioning the heating load according to EN 12831 often leads to oversized and underutilized heating technology.


  • Ernst-Rudolf Schramek, Hermann Recknagel (Hrsg.): Pocket book for heating + air conditioning 07/08. Volume 73, Oldenbourg Industrieverlag, Munich 2007, ISBN 978-3-8356-3104-5 .
  • Erich Draxler, Norbert Kleeber: Manual for calculating the heating load in buildings. With reference to ÖNORM EN 12831 and ÖNORM, Volume 7500, Verlag Austrian Standards plus GmbH, 2007, ISBN 978-3-8540-2096-7 .
  • Karl Volger, Erhard Laasch: House technology. Basics - Planning - Execution, 11th fully updated edition, BG Teubner, Stuttgart 2005, ISBN 978-3-663-10289-2 .
  • Hermann Rietschel, Hubertus Protz, Wilhelm Raiss: heating and air conditioning. Second volume, methods and documents for calculation, fifteenth edition, Springer Verlag, Berlin / Heidelberg 1970.
  • Hermann Rietschel, Wilhelm Raiss, Fritz Roedler: Textbook of heating and ventilation technology. 14th improved edition, Springer Verlag, Berlin 1960.

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