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Why does my boiler keep losing pressure?

Pressure loss is one of the most common faults with boilers and central heating systems, but it’s not always the boiler itself that needs repairing. 

It’s worth having a look at the most likely causes before you decide whether to call a plumber or a boiler engineer.

Knowing your pressure

Your boiler pressure should usually be at around 1–1.2 bar, with 1 bar being atmospheric pressure (the pressure of the air around you). The boiler and heating system need to have just enough pressure to operate, that is, to ensure the water is pumped around the system.

Also, if pressure falls below 1 bar, then the system would suck air into itself if a leak formed, and that would cause its own problems.

All boilers have some way of showing the pressure inside the system. In older models it will be a dial with a pointer, but more modern units have a digital display, although you sometimes have to find the pressure through a series of menus.

Most people don’t check the pressure regularly, and only realise there’s low pressure when the boiler trips out, but it’s always worth checking every now and again. If everything’s normal, you’ve probably got nothing to worry about.

Causes of boiler pressure loss

The most likely cause of pressure loss in the system is a leak. The central heating system is a closed loop, rather like a tyre, but it’s filled with water. If there’s a hole anywhere in the system, water will leak out and the pressure will drop.

The loop consists of the boiler and all its internal parts, pipework, joints, valves and radiators. Any of those can leak, but it’s the joints that are the weakest point. If there’s water coming from the system, nine times out of ten it will be from a joint that’s either burst or has not been tightened up enough.

The leak might turn out to be somewhere you can see it, such as around the boiler or next to a radiator. But that’s not necessarily the case – there’s plenty of pipework under the floorboards, and a leak could be anywhere. Look for damp patches on walls, floors and ceilings, or secondary signs such as mould or flaky paintwork. That could be your sign.

Another potential cause of pressure loss is a fault with the boiler, and in that case, the expansion vessel is the most likely culprit.

This is a sealed tank with a flexible diaphragm inside it, a bit like a syringe or plunger, and its job is to accommodate the difference in pressure between hot water (when your system is on) and cold (when it’s off). These do sometimes fail, and need replacing.

Other issues could crop up with the relief valve, pump or some other internal malfunction. Diagnosing this is not a job for an unqualified person, however.

What to do

If you do see evidence of a leak, it’s worth having a plumber or central heating specialist come and fix it for you. It probably won’t be a big job, and might just need something tightening up.

If you see no evidence of a leak, however, it’s time to call a boiler engineer to have a look at your system. They will be able to do some tests and get to the bottom of the problem pretty quickly. It’s possible you might need a new boiler, but it could well be a repair job. Either way, you’ll be guaranteed a healthy heating system – and no more automatic shutdowns.

Published: 9 September 2021

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