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Why Is My Combi Boiler Losing Pressure
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Why Is My Combi Boiler Losing Pressure?

So its been a while since my last post, but I've had a lot of issues recently with customers complaining about their boiler losing pressure. 9 times out of 10 this issue is easily fixed.

The boiler could be losing pressure for a couple of reasons. The pressure gauge on a boiler is indicating the pressure of the central heating system, so this is going to be an issue with the central heating side of your system.

Most boilers now have safety devices inside which prevent the boiler from working because low pressure could cause damage to vital components inside.

A boiler losing pressure can be caused by a number of things.

You might have a leak on your central heating system if your combi boiler is losing pressure.

A leak on your central heating system can cause your combi boiler to lose pressure. Be on the look out for any obvious signs of leakage. This could be a stain on the ceiling, or a damp patch appearing on your floor.

Some leaks are so small that they are barely noticeable. Commonly when a radiator develops a pinhole from corrosion. The water can evaporate before it has time to settle into a wet patch.

A faulty expansion vessel

If you have a pressurised heating system then you will have an expansion vessel. The expansion vessel is there to accommodate any expansion created when water is heated up. Within the expansion vessel there is a balloon, when the water expands the balloon will absorb the increase in volume.

The expansion vessel balloon will eventually lose air through a process called osmosis and needs to be pressurised. This should be done as part of an annual service which will prevent this from happening. Its a relatively straight forward repair but I don't recommend this type of repair to be made by anyone other than a gas safe engineer. unfortunately these balloons don't last forever and eventually may split, in which case a new expansion vessel would be needed.

If there is no room for expansion because the balloon has lost its charge, the increase of the water volume within the boiler and central heating system will cause a safety valve (PRV) to disperse the extra water. This is to maintain a safe central heating pressure but can then another fault. Which leads me seamlessly on to the next cause.

Faulty relief valve (PRV)

If you have had a continuous flow of water running through your relief valve (PRV) then there's a good chance that some debris may have got lodged inside. This will result in the boiler losing pressure. They not the most expensive part within a boiler so no need to panic just yet. Some valves can be dismantled and cleaned.

I hope you have found this post useful, If you have please share it on Facebook and Twitter to help your friends who might need find this helpful.

Jamie Cureton
Jamie Cureton
Jamie is an extremely passionate heating engineer. He particularly enjoys the technology aspect of the industry especially how new heating controls work to help people save money and keep warm.
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