The radiators are hot, but the shower stays cold or lukewarm? On a combi boiler, heating and domestic hot water share the same burner but not the same parts. Here is where to look.
Checks you can do yourself
- The boiler mode: is it set to "heating + hot water" and not heating only?
- The hot water temperature set on the boiler: 50 to 55 °C is suitable in most cases.
- The pressure: between 1 and 1.5 bar when cold.
- All taps: if only one tap is cold, the problem is the mixer tap, not the boiler.
- The flow rate: open the tap fully. Too low a flow does not trigger the boiler.
The most common causes
A stuck diverter valve
This valve sends hot water either to the radiators or to the hot water heat exchanger. If it stays stuck on the heating side, you get hot radiators and cold water (and sometimes radiators heating up when you run a tap). It is the number one fault.
A scaled plate heat exchanger
Limescale, very present in the water in Brussels and much of Wallonia, gradually blocks the hot water heat exchanger. Symptoms: lukewarm water, fluctuating temperature, decreasing hot water flow. The exchanger must be descaled or replaced.
The flow sensor or the hot water sensor
The flow sensor tells the boiler that a tap is open; the temperature sensor measures the water temperature. If either fails, the boiler does not start in hot water mode or regulates badly.
On a boiler with a cylinder
If you have a hot water cylinder, check its set temperature. A faulty cylinder sensor or charging pump may also be to blame.
The opposite case: hot water but no heating
First look at the thermostat (batteries, set temperature, schedule), then the radiator valves. If everything is fine, the diverter valve or the pump may be stuck.
When to call a technician?
As soon as the simple checks give nothing. Replacing a diverter valve or descaling a heat exchanger requires draining the boiler and opening the hydraulic circuit. Our technicians carry out this repair in Brussels and Wallonia within 24 to 48 hours, on all brands.
Frequently asked questions
Why do I have heating but no hot water?
The most common cause is a diverter valve stuck on the heating side. A scaled plate heat exchanger or a faulty sensor are also possible.
Why does my hot water go lukewarm and then cold?
This is typical of a scaled hot water heat exchanger: limescale prevents heat transfer. It must be descaled or replaced.
What temperature should the boiler hot water be set to?
Between 50 and 55 °C on a combi boiler: comfortable, economical and it limits scale build-up.