The heating does not get warm – what can you do?

© Unconventional /

As soon as it gets colder outside, we turn up the heating to keep cosy and warm inside. But what if you, as a system owner, discover: the heating does not get warm? Faults in the system and radiators that stay cold are most often noticed at the beginning of the heating season, when the heating system has been running in summer mode for a long time to save energy and heating costs.

We explain in which cases you should ask your heating contractor to carry out a repair. Also find out what you can do yourself to ensure that heating faults are remedied as quickly as possible or, ideally, avoided.

Identifying faults and instructing the heating engineer correctly

When should you ask your heating contractor to carry out troubleshooting? It is best to do this immediately after you have noticed the failure and/or determined the possible fault. In order to do this as effectively as possible, we have listed common faults below, for which you should notify the heating engineer. In individual cases, the causes may of course differ from the situations shown. Which troubleshooting measure is ultimately the right one depends on the technology used and the specific conditions on site. We recommend that you always seek the advice of a heating contractor in the first instance if your heating is not working properly.

If your heating is not getting warm, always check the heating system itself. A fault code appears on the display of your Viessmann heating system for certain faults. You will find this listed in a table in the service guide for your Viessmann heating system. So that the heating engineer can rectify the fault as quickly as possible, we recommend that you make a note of the displayed fault code. Simply inform the engineer of this when placing the repair order. This enables them to better prepare for the repair job.

Are the radiators not getting warm or is the entire heating system not working?

If you turn up the thermostats fully and your radiators do not get warm, there are two basic cases to distinguish:

  1. The heating system is running properly, but no hot water is getting into individual or several radiators in the heating circuit. This manifests itself in the radiators not getting warm or only getting warm at the top.
  2. If the heating does not work, there may be a fault with the boiler and/or the entire heating system.

Check the setting of your heating system:

  • Is it still set to summer mode?
  • Does the heating run exclusively on DHW heating?
  • Is the system time set correctly?
  • Has the system jumped back to the default setting after a power cut?

These settings can be read directly from the display of your Viessmann heating system. With most Viessmann systems, they can be changed via a mechanical controller or via the display's touchscreen. For older heating systems, it is highly recommended to call in an installer to avoid causing damage with the incorrect settings.

In this case, the heating engineer will probably have to free up a jammed thermostatic valve or replace the thermostat. Reason: the thermostat has been set to zero or the frost protection position has been unchanged for a long time in summer, so the valve has become jammed and no longer allows heating water to flow into the radiator.


The heating engineer has to bleed the radiator. Reason: there is air in the heating circuit. You may be able to bleed the radiator yourself. Our guide to bleeding the heating system explains how to do this and what equipment you need. However, a check is necessary afterwards and it is best to consult a contractor if you are in any doubt.

The heating engineer has to carry out hydronic balancing. Reason: the water in the heating circuit is not evenly distributed, so that the pressure ratios between individual radiators differ excessively. This can occur, for example, after replacing several radiators or renewing the boiler. The German Federal Office for Economic Affairs and Export Control (BAFA) subsidises hydronic balancing by 30  percent.

Reason: The heating circuit pump is not functioning properly. To check this, you can hold the tip of a screwdriver against the pump housing and listen for vibrations or flow noises at the handle of the screwdriver. If nothing can be heard, the heating circuit pump is not running. Notify your heating contractor of the problem; they will repair or replace the pump.

A fault in the boiler can have various causes. The heating system installer can determine the fault from the fault code that appears on the display of your Viessmann heating system. They then delete the code, make the necessary repairs and, in the best case, simply switch the boiler on again. It is best to have the code ready when you contact the contractor. Then they can be better prepared.

The reason for the failure is most likely a lack of fuel. Check the fuel supply to your heating system:

  • Is the gas tap open or closed?
  • What is the fill level of my oil or gas tank?
  • Is the feeder in a pellet or wood chip heating system jammed or blocked?

Have a contractor check your heating oil system, your LPG tank or gas connection –– depending on which heating system you have in operation. Oil or gas tanks must never run completely empty. If the level indicators fall below the necessary filling level –– usually 15 to 20  centimetres for oil –– the burner is already getting too little fuel or no fuel at all.

If the entire heating system can no longer be started, this may be due to the emergency switch being pressed by mistake, for pellet heating systems, oil heating systems or wood chip heating systems. A small light on the emergency switch shows whether the switch is in the "on" or "off" position. Press the switch again and restart the boiler. If in doubt, consult your heating contractor.

When the temperature drops outside and the heating is not working, this can quickly become an inconvenience. The worst case scenario is a threat of uncomfortable days and nights. But serious frost damage is also possible, for example due to burst water pipes. Photo: © Zoonar GmbH /

How to avoid costly repairs

When they notice that the heating is not getting warm, system owners should consult their heating contractor immediately. It is best to remember to check the heating in advance. Be sure to plan enough lead time, as contractors tend to be particularly busy at the beginning of the heating season.

We also recommend that you have the heating system serviced regularly. An annual service of your heating system identifies problems at an early stage and prevents them from becoming more serious. Costs for such a measure are money well spent, as a heating system that is serviced annually runs more economically. Our Guide to heating system maintenance: Benefits, intervals, costs, maintenance contract tells you everything you need to know about the Viessmann service range.

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