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Glossary · Energy industry & energy transition

Heating load

Heating load heat demand heat pump building power flow temperature heat planning

The heating load is the thermal output that a heating system must deliver so that a building maintains its desired indoor temperature at a specified low outdoor temperature. It is stated in watts or kilowatts. It is therefore a power value for a design case and not an amount of energy over the year. An example: a single-family house has a heating load of 8 kilowatts. Under the design conditions, the heating system must then be able to deliver about 8 kilowatts of heat over a longer period so that all rooms stay warm. You can picture the heating load as the stream of water that has to flow into a leaky bucket to keep the level constant. The heating system must supply exactly as much heat as is lost through walls, windows, roof and air exchange.

This must be distinguished from the heat demand. It is stated in kilowatt-hours and describes how much heat is needed over a period, for example in one year or one heating season. The heating load shows how powerful the system must be. The heat demand shows how much heat is consumed in total. The two quantities are related but cannot simply be converted into one another. Two buildings with the same annual heat demand can have different heating loads, for example because of a different climate at the location, a different construction method or a different use. A building with a low annual demand still needs a noticeable output on very cold days.

Calculation according to DIN EN 12831

In Germany, the standard heating load is calculated according to DIN EN 12831-1 (edition September 2017), together with the national supplement DIN/TS 12831-1 (edition April 2020). The standard defines the heating load as the heat flow required to reach the standard indoor temperature under standard outdoor conditions. The calculation is carried out room by room. At its core it consists of two parts: the transmission heat losses through the components of the building envelope, including thermal bridges and surfaces in contact with the ground, and the ventilation heat losses through leaks and ventilation. Where required, a heat-up output is added, for example after a setback phase. Domestic hot water heating is not part of the standard heating load in the narrower sense. It is considered separately when selecting the heat generator.

How strongly the low outdoor temperature enters the result is determined by the standard outdoor temperature. It is not an arbitrarily chosen cold value but a statistical design value: the coldest two-day mean of the air temperature that is reached or undercut at a location on average about every two years. The basis of the climate data in the national supplement is the period 1995 to 2012. There, the two-day mean that was reached nine times in these 18 years is used. The values are stored in DIN/TS 12831-1 by postal code. Older versions of the standard gave values only for larger cities. For Aachen, for example, the result is a standard outdoor temperature of -8.6 °C, whereas an older calculation used -12 °C there. Averaged over all locations, the new values are about 1.7 kelvin higher than the old ones. This lowers the calculated heating load. For heavy, thermally sluggish buildings, the design temperature may additionally be raised somewhat. A house in a mild river lowland is therefore assessed differently from an identically built house at a cold, high-altitude location.

Legal context in Germany

The heating load calculation is not only a planning rule but also anchored in law. Since 29 July 2026, the Building Energy Act (Gebäudeenergiegesetz) has been called the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG). Section 60c of this act requires the following: if a heating system with water as the heat transfer medium is installed or set up in a building with at least six dwellings or other independent units of use, the heating system must be hydraulically balanced at commissioning. This hydraulic balancing comprises at least three steps: a room-by-room heating load calculation according to DIN EN 12831-1 in conjunction with DIN/TS 12831-1, a check and, where appropriate, optimisation of the heating surfaces with the aim of the lowest possible flow temperature, and adjustment of the controls. The following, among other things, must be recorded in writing: the heating load of the building, the set output of the heat generators and the design temperature. On request, the confirmation must be presented to tenants. For smaller buildings, this provision does not prescribe a heating load calculation. From a technical standpoint, however, it is also the basis of a sound design there.

Distinction from consumption, boiler output and connected load

The heating load cannot be read directly from a gas bill. Annual consumption also includes hot water, boiler losses, user behaviour, vacancies and the weather of the year in question. An approximation can be derived from adjusted consumption data together with weather data. Such an estimate is helpful for a plausibility check but does not replace a room-by-room calculation when the building data are available.

The output of the existing boiler is not a reliable indicator either. Many older oil and gas boilers were selected with large safety margins or with a view to fast hot water preparation. A 24-kilowatt unit in a single-family house therefore does not mean that the house has a heating load of 24 kilowatts. The actual value is often considerably lower. The electrical connected load of a heat pump is also something different. It describes the power that the unit draws from the electricity grid. With an instantaneous coefficient of performance (COP) of 3, 1 kilowatt of electricity yields about 3 kilowatts of heat. If the outdoor temperature falls or the required flow temperature rises, the coefficient of performance drops. For air-to-water heat pumps, the available heating output then usually decreases as well. The electricity consumption thus moves closer to the delivered thermal output.

Significance for heat pumps

With heat pumps, an incorrectly determined heating load has a more direct effect than with boilers. An oversized unit costs more, switches on and off more often in frequent part-load operation and therefore usually operates less efficiently. An undersized unit does not reach the set temperature on cold days or has to call on an electric immersion heater more often. The immersion heater converts electricity into heat almost one to one. It is therefore expensive to run and burdens the grid precisely during the cold hours. Where the heat pump alone is no longer sufficient, the planner specifies a bivalence point. Below this outdoor temperature, a second heat generator helps out.

The heating load alone does not determine the quality of the system. Equally important is the temperature at which the heat is distributed in the house. Two houses with the same heating load place very different demands if one has large heating surfaces or underfloor heating and the other needs small radiators with a high flow temperature. The room-by-room heating load shows which radiators are too small. Often it is enough to replace individual radiators so that the whole system can manage with a lower temperature. An insulation measure therefore works twice over: it reduces the required output and enables lower system temperatures. A high heating load does not rule out a heat pump. Whether it makes technical and economic sense also depends on the heat source, heating surfaces, hot water preparation and mode of operation.

Heating load in the electricity grid and heat planning

With the electrification of heat, the combined heating load of many buildings becomes important for the electricity grid. What matters is not only the annual amount of electricity but the timing of peaks. During cold spells, many buildings need a lot of heat at the same time. Distribution system operators therefore pay attention to simultaneous electrical power consumption. The sum of the individual heating loads is an upper limit that is practically never reached at the same time. This difference is described by the simultaneity factor. Buffer tanks, underfloor heating, the thermal storage mass of the building and smart controls can shift electricity consumption by a few hours. They do not change the heating load of the building.

In municipal heat planning, annual heat demands are usually mapped. For the design of a heating network, however, the peak output on cold days also counts. Pipe cross-sections, generators, storage tanks and transfer stations are sized according to the loads occurring simultaneously. In addition, the transport capacity of a network depends on the temperature spread between flow and return. High return temperatures in poorly balanced buildings reduce the spread. More water must then be circulated to transmit the same output. Refurbishment, hydraulic balancing and storage can reduce the required generator and network capacity.

Typical misinterpretations

  • The heating load is not a maximum consumption. Consumption is energy over a period of time, whereas the heating load is a power value under specified conditions. For annual costs, the amount of energy counts. For the size of the heat pump, radiators, transfer station or grid connection, the power counts.

  • The standard heating load does not occur constantly. It describes a deliberately unfavourable design case, in the German standard without any credited solar and internal heat gains. Most hours of the year are well below it, and heating systems mostly run at part load. Good planning takes both into account: the rare design point and the frequent everyday operation.

  • A flat-rate value in watts per square metre does not replace a calculation. Rules of thumb are suitable as a first orientation but do not reflect differences in insulation, window areas, airtightness and local climate.

  • Refurbishments do not only save kilowatt-hours. They also lower the heating load and thus enable smaller heat generators, lower flow temperatures, less frequent use of immersion heaters and a lower burden on local grids.

The heating load is thus the central power quantity of heat supply. It shows what thermal output a building needs under specified conditions and links building services with the electricity grid, heating network, refurbishment and investment decisions. Anyone who looks only at annual consumption knows the amount of energy, but not the demands on the technology in the hours when heat becomes scarce, expensive or decisive for the grid.

Sources

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Edited by eCMS on 12.11.2026 05:05

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