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Combi Boiler Problems

Combi Boiler Problems

This page is about combi boiler problems, however we will start with a brief intro to combi boilers.

A combi boiler is one that provides heat and hot water on demand. There is usually no hot water cylinder as hot water is produced instantaneously. This modern space saving technology has it’s benefits and problems.

The main additional parts contained in a combi boiler compared to a regular boiler are as follows (please note there are now newer innovative combi boilers that may not contain some of these parts):


Divertor valve

Plate heat exchanger

Central heating pump

Integral automatic bypass

Expansion vessel

Pressure Guage / Sensors

Air pressure sensors.

The most common problems affecting combi boilers are as follows:

Boiler shows low water pressure

Usually a combi boiler will cut of below a certain pressure. This pressure can vary. A common pressure setting is 1.2 bar. Usually as long as the system stays between 1 and 2 whilst cold and running, you will have a perfectly functioning system. If you are having to continuously top up pressure, then you have a leak in the system. It would be wise to stop leaks as continuously filling the system can cause other problems.

Radiators working but no hot water(water running but cold):
This can be caused by a faulty divertor valve, faulty water flow sensor.
Radiators not working but hot water working:
If hot water is working, then essentially the core of the boiler is working. Reasons for radiators then not working could be divertor valve stuck / faulty, demand signal not reaching boiler, blocakages in the radiator flow and return pipework, although usually you will also see problems on the hot water side.
Condensate blocked / frozen:
In cold weather if the condensate freezes the boiler will stop working. Also similarly at any time if the condensate is blocked or waste pipework blocked then the boiler will also stop working. So make sure condensate trap is clear and pipework is also clear. Usually correctly installed pipework will prevent combi boiler problems associated with condensate pipework.
If your boiler sounds like a kettle, with sometimes slight banging or noises, you likely have sludge or limescale build up within the boiler. This can be flushed out using chemicals and machines. A flushing machine may be required.

Hot Water running cold hot cold hot:

This combi boiler problem is usually caused by limescale build up in the secondary plate heat exchanger. The way to solve this is to replace the plate heat exchanger or descale it if possible.

Disclaimer: Information on this website is provided for informational purpose therefore we cannot accept liability for subsequent problems. All repair tasks should be carried out by qualified trades people. We may also receive commission payments when you are taken to external websites and complete purchases from third party sellers, once you leave our site, we cannot be held liable for your purchases.