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Heat load – definition, influencing factors and calculation

Photo: © Irena Maslova /

A heating system functions optimally when it is correctly designed and sized. That way, it is pleasantly warm during the heating season and works energy efficiently at the same time.

If, for example, the system output is too low for the building and the conditions, the premises will be undersupplied at peak times. Always bear in mind that moisture damage and mould growth can occur at certain room and wall surface temperatures. Likewise, a heating system can also be oversized, causing the burner to cycle due to the mismatch between demand and supply. Frequent cycling increases wear on burner components.

To prevent this, it is important to determine the heat load of the building. There is a standardised calculation for this. We explain precisely what heat load is, how it differs from heat demand and what is included in a heat load calculation.

Heat load and its significance

The heat generator is selected according to the calculated heat load. Domestic hot water heating must also be taken into account. The heat load is specified in kW. The output must be high enough to compensate for heat losses, including through the building envelope, and to keep the building comfortably warm during the heating season. Accordingly, it is important that a predefined internal temperature be maintained. This requires a certain boiler output.

The standard heat load can be determined either for sizing the heat generator or room by room for the design of individual heating surfaces. In existing buildings, the boilers and heating surfaces are often too large. In addition, small changes to the building envelope can still have an effect on the heat load. Even replacing windows can have a significant impact. This makes it all the more important to have the heat load determined when you are considering modernising your heating system. The heat load must always be determined for a new build.

The heat load is not the heat demand

In addition to the heat load, you will often hear the term heat demand. Be careful not to lump these together as they are not the same. While the building heat load tells you how high the boiler output needs to be, the heat demand indicates the amount of energy that is required per square metre over the course of a year.

The heat demand or building energy demand is therefore a criterion for describing the energy quality of a building. It is calculated in kilowatt hours per year and compared to one square metre of net floor space. We cannot use the heat demand to determine boiler output.

Heat load calculation – procedure and influencing factors

When planning a heating system for a new build, it is essential to calculate the heat load. The calculation should also be performed when undertaking modernisation projects, otherwise the heating output of the system will be inconsistent with the current conditions of the building. Subsequent readjustment or balancing is almost impossible. If the system is over or undersized, beyond a certain point hydronic balancing cannot help either. This makes it all the more important to perform the calculation accurately. It is not a job for an amateur. When undertaking modernisation projects, contact a Viessmann trade partner who will take care of the calculation during the planning stage. In a new build project, the calculation is part of the tasks of the system design engineer. There are some online calculators that perform heat load calculations, but their results are merely approximate and give only an idea of the heat load in the preliminary planning phase.

Standardised heat load calculation procedure

The calculation of the heat load is based on the DIN standard EN 12831 'Heating systems in buildings – Method for calculation of the design heat load'. The heat load is made up of the following elements:

Transmission heat losses: losses through the components of the building envelope

Heat losses via ventilation: ventilation flow rate, leaks and hygiene-related minimum air change rate

Additional heat-up output: output of the heating system to provide heat for a short time for heating up after it has been shut down

In a calculation, the sum of the transmission and ventilation heat losses is added to the additional heat-up output.

Building-specific influencing factors

The following variables and values have a significant influence on the heat load of a heating system:

Required temperature: the DIN standard specifies standard temperatures for indoors and outdoors.

U-values: this value indicates how much heat escapes through one square metre of the building envelope when there is a temperature difference of one degree. This applies to all relevant surfaces: external walls, windows, exterior doors, roof surfaces, ceilings and floors of unheated rooms as well as interior doors of less heated rooms.

Air change: this value shows how often the room air volume is exchanged by natural or mechanical ventilation.

Storage mass: among other things, the building weight influences the reheating performance of a heating system after it has been shut down for an extended period.

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