Heating curve

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The blue heating curves have slopes in the range from 0.5 to 3. Like the middle red heating curve, their base point is at the intersection of the x and y axes at 20 ° C outside temperature and 20 ° C flow temperature. In a windproof and insulated building, it is usually not necessary for the heating to switch on as soon as the outside temperature falls below 20 ° C (since people and other heat sources also contribute to the heating). Therefore, the heating curve would be shifted downwards in parallel: With the lower red heating curve, the heating would only switch on at an outside temperature of 17 ° C.
Due to the large heated area, underfloor and wall heating react much more strongly to an increase in the flow temperature. The heating curve should therefore be less steep with surface heating. In some cases a slope of 0.1 is sufficient. A steepness above 1.5 is usually only required for historic buildings that are not windproof. Because of the chimney effect , the heated room air is drawn out to the outside through the chimneys and other openings in the building envelope when the outside temperature is low.

A heating curve ( also known as a heating curve ) describes the relationship between an outside temperature and the flow temperature associated with a heating circuit . In order to heat the rooms of a building to a continuous temperature level with different outside temperatures, the heating surfaces through which the heating water flows must each be supplied with a certain flow temperature. Since the heating curve depends on various factors, it differs from case to case. In some buildings there are several different heating curves (e.g. floor and radiator heating circuit).

The heating curve is set on a controller . This changes the level of the flow temperature with the help of an outside temperature sensor and the corresponding settings; it also takes the wind speed into account if an appropriate measuring device (pinwheel) is available. Very large, recently built systems with their own building management systems often take on other influencing variables such as the energy that may be stored in the building envelope and solar radiation; in some cases this is even done via a weather forecast.

The course of the heating curve is slightly curved, since the heat output of the heating surfaces is not linear at different temperatures.

A correctly set heating curve ensures reduced heat losses, improved room temperature control and thus saves energy.

Parameters of the heating curve

The heating curve can be influenced with various parameters. The individual parameters are difficult to determine by calculation; this is expediently done by trying them out while the system is in operation. There are also controllers that support the system operator with automatic adjustment.


The slope determines how much a change in the outside temperature causes a change in the flow temperature. Typical values ​​for conventional heating are 1.4 ... 1.6. A value of 1.5 means that a 1 K change in outside temperature results in a 1.5 K change in the flow temperature. The slope depends on the heating system used and the heating requirements of the rooms. A flat heating curve with values ​​of 0.5 is typical, for example, for underfloor or wall heating with medium thermal insulation .

Parallel shift

With the parallel shift, the level of the flow temperature can be influenced over the course of the heating curve.

Night reduction

The night setback causes a parallel shift downwards. There are no typical values ​​because every building is insulated differently and there are locally different differences between day and night temperatures (large in the mountains, small in cities).

Heating limit

If the outside temperature exceeds the heating limit , the controller switches off the heating system.

Setting the parameters

The steepness of the heating curve should be set at outside temperatures below 0 ° C, the parallel offset setting at temperatures above 5 ° C. To do this, all radiator valves are set to the desired room temperature. During a period of 1 to 2 days, a check is carried out to determine whether the desired internal temperature is just reached; if necessary, the parameters are changed. The slope and parallel shift are reduced or increased accordingly if the internal temperature is too low. It should be noted that if the parallel offset changes, the flow temperature at the end of the heating curve also changes, so the slope should be reduced if it is increased.

The flatter the curve and therefore the lower the flow temperature, the lower the system losses and thus also the energy consumption.

Since the heating curve is often only set to the standard values ​​intended for safety when the system is created, the system operator should absolutely check the heating curve and adjust it subsequently.

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