Why is my hot water not hot

No hot water in spite of the functioning heating at the gas boiler

Why does the gas boiler not produce hot water when the heating is working?

Gas boilers, which are designed both for heating the living space and for hot water production, are called combi gas boilers. With such devices it can always happen that either the heating fails and the hot water still works or vice versa. If your combi gas boiler is heating, i.e. the burner is OK, but the drawn water remains cold, this could be due to the following reasons:

  • Setting to heating mode only
  • Changeover valve defective
  • Defective heat exchanger
  • Defective sensor

Setting to heating mode only

Especially with sensitive devices such as a gas boiler that is under water system pressure and filled with highly explosive gas, you should first of all rule out very trivial causes. If possible, you should avoid unnecessary work with associated risks. So first see whether the combi-thermal bath is simply set to heating mode.

Changeover valve defective

In older gas boilers in particular, a frequent cause of a hot water failure is a defective or jammed switchover valve. This 3-way switch valve is necessary for combined gas boilers so that it is possible to switch back and forth between heating mode and DHW heating mode. If the valve is defective, it may get stuck in one position - i.e. in heating mode if the hot water preparation no longer functions. The switching valve can be exchanged. But it is better to leave the change to professionals.

Defective heat exchanger

The heat exchanger can also be the problem, especially when problems with the hot water preparation gradually creep in and there were times with only lukewarm water before a complete failure. Over time, it can become calcified and then no longer perform its service properly or not at all. It may be enough to descale and clean it. To remove the device, however, it must be depressurized. If you are unsure, it is better to let professionals do it.

Defective sensor

If the combi gas boiler is coupled to a hot water reservoir, the sensor that sends the gas boiler the message to reheat if the temperature is too low may also be defective. It is possible that it is no longer firmly connected to the cabling due to external mechanical influences and only needs to be straightened.

Caroline Strauss
Article image: Maridav / Shutterstock
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