Hot water not hot enough: 3 possible causes & how they can be fixed

Hot water not hot enough? If you’ve got a combi boiler, you may be all too familiar with this problem. It’s even more frustrating when the water is lukewarm when the tap is open fully, and is only hot enough when it’s half shut. Tepid water is hardly ideal for doing the washing up. Plus, the frustration of trying to run a decent bath speaks for itself.

Hot water not hot enough explained

Firstly, if you have a combi boiler, this fault is likely to become evident very quickly. This is because combi boilers heat water from the mains on demand. There is no cylinder of hot water, heated in advance.

Secondly, the size or power of the boiler shouldn’t make any difference. A 24 kW combi boiler for a flat or small house should still be able to provide 55-60 degree hot water with the tap open at full bore, no different to a 42 kW boiler in a large house with 4 bedrooms. Even with the central heating on, the size of the house and amount of radiators do not matter. That’s because combi boilers prioritise the domestic hot water over the heating. Open a hot tap, and a sensor in the boiler will tell it to heat water for the taps instead of the radiators. Once all hot outlets are off, the boiler will carry on providing heat for the radiators.

So what causes it?

Faulty diverter valve

A faulty diverter valve can easily cause this problem. The diverter valve is a component inside the boiler which determines whether hot water should go through the radiators or through the plate heat exchanger. Hot water inside the plate heat exchanger transfers its heat to fresh mains water. This then goes off to the hot taps. The two bodies of water – the water for the radiators and the domestic hot water – never mix.

Turn your central heating off, and open the hot tap. If your radiators start to get hot even though the heating is off, a faulty diverter valve is quite likely. This may also be the case if your find yourself having to turn the heating on in order to have hot water. Faulty diverter valves must only be replaced by qualified heating engineers.

Faulty thermistor

The thermistor is a sensor fitted inside your boiler. It determines how much heat is needed in order to bring the water up to the desired temperature. A faulty thermistor can easily affect the temperature of the water.

If your radiators don’t heat up when the hot tap is open, this would suggest that the diverter valve is OK but there may be a problem with the thermistor. Once again, this may only be replaced by a qualified heating engineer


If you live in a hard water area or your combi boiler is quite old, limescale buildup on the plate heat exchanger may prevent the heat being transferred to the water effectively. Taps which will only deliver water that’s hot enough when partially closed are a likely indication of this problem.

Other clues of limescale problems may include gushing or boiling noises from the boiler. These sounds are known as “kettling”. A particularly strong indicator of limescale buildup being a problem is if the boiler cuts out while the hot tap is open – this is a safety feature to protect the boiler from being damaged by the excess heat which cannot be transferred to the water.

Unfortunately, while all of these problems should only be fixed by a qualified engineer, having an idea of what the problem might be may help them diagnose the issue faster and save you money.

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