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Adjusting the heating curve correctly

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The room temperature can be changed not only by means of the thermostatic radiator valve, but also with the help of the heating control unit. However, only heating system owners have this option. Those who live in rented accommodation still have to set the required temperature via thermostats. Adjusting the heating curve not only ensures that the required temperature is achieved; it can also increase thermal comfort and reduce heating costs.

The function of a heating curve

The heating characteristic curve, as the heating curve is called in technical jargon, is the graphic representation of a mathematical formula. It describes the relationship between the outside temperature and the flow temperature. This means that if the outside temperature changes, this has a direct effect on the flow temperature. The latter is the temperature of the heating water that flows from the heat generator to the radiator or to an area heating system. The flow temperature has a significant influence on the efficiency of a heating system and is therefore a key factor when adjusting a heating curve.


It should not be set too high or too low. When set too high, the flow temperature will increase heat losses and if set too low it prevents the house from getting sufficiently warm. To find the optimum flow temperature, it's worth trying out different settings. It is advisable to create a log to note down the settings, flow temperatures and outside temperatures. You should also consult a contractor to help you find the right values.

Heating curve adjustment is only carried out on heating systems with weather-compensated controls. With these systems, the outside temperature serves as the control variable to whose changes the flow temperature responds. To put it simply: the lower the outside temperature, the higher the flow temperature. These heating systems have one or more outside temperature sensors to record the outside temperature.

In addition to the outside temperature, the set temperature or set room temperature must also be taken into account when adjusting the heating curve. This is the specified required temperature for the rooms. If the temperature drops below this set value, warm heating water should flow to the radiators if the heating curve has been optimally adjusted.

Slope and level as key variables

To adjust the heating curve, system owners have two key variables at their disposal: the slope and the level of the heating curve. The heat generator uses the slope to determine how sharply it needs to change the flow temperature when the outside temperature changes. The slope can be very steep or very flat. This depends, among other things, on the required room temperature and the energy status of the building. If the house being heated loses little heat thanks to good insulation, a flat curve is sufficient. However, if there are substantial heat losses through the building envelope, the slope must be set steeper to prevent loss of comfort.

While the slope influences the behaviour of the heating curve, the level allows the flow temperature to be corrected evenly upwards or downwards. Specifically, this means that if the rooms are cooler than you want them to be, the level of the heating curve must be raised. If, on the other hand, it is too warm, lowering the level would be the correct thing to do.

How to adjust the heating curve correctly – video tutorial

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If you have a heat generator with weather-compensated controls from Viessmann, you can adjust the heating curve, and thereby influence the heating characteristics of the boiler, in just a few steps. The way you do this is virtually identical for all heat generators and is described in the respective operating instructions.   The tutorial is based on the Vitodens 300-W wall mounted gas condensing boiler, number 5817263, year of manufacture: 01/05/2018.

How to adjust the heating curve correctly

  1. Tap on "Menu" or the menu symbol
  2. Then click on "Heating"
  3. Now select the required "Heating circuit"
  4. Tap on "Heating curve"

Use the +/- button to change the slope and level of the heating curve. The factory setting is 1.4 for the slope and 0 for the level. You can accept the change with the arrow symbol.

The graph clearly shows the change in the heating curve as soon as you alter the value for the slope or level. The assigned set flow temperatures for the heating circuit (shown on the vertical axis) are shown as a function of different outside temperatures (shown on the horizontal axis).

Which value you set for the slope and level depends on many factors; therefore it's not possible to give a general answer to this. In addition to a building's thermal insulation, the type of radiators installed also plays a role.

Nevertheless, the following standard values for the slope may be used as a guide:

  • 0.3 to 0.5 in a well insulated house with underfloor heating
  • 1.0 to 1.2 for a well insulated house with radiators
  • 1.4 to 1.6 for an older, detached building with radiators

The following adjustments can be made for the heating curve level in an existing system:

  • If the room temperature is generally too low: increase the level
  • If the room temperature is too low, especially on cold days: increase the slope
  • If the room temperature is too low during spring/autumn, but sufficient on cold days: increase the level and lower the slope
  • If the room temperature is too high during spring/autumn, but sufficient on cold days: lower the level and increase the slope

By optimising the heating curve, system owners can ensure that their heat generator only produces as much heat as is needed to reach the required room temperature. In addition to the increase in comfort (no more being too cold or too hot) that this brings, there are above all financial advantages to be had. With less energy in the form of fuel being needed, heating costs are reduced in the long term. Anyone heating with fossil fuels such as gas or oil will also reduce the load on the environment.

To achieve the best result and avoid possible mistakes, it's advisable to consult a heating contractor when adjusting the heating curve for the first time. With our local partner search, you can find a suitable heating contractor near you in just a few steps. Alternatively, you can ask the Viessmann Community.

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