Why does my boiler keep losing pressure?

If your boiler keeps losing pressure, you might be able to fix the problem simply yourself, using this advice. (Picture: Getty)

We’re in the thick of winter at the moment, which means it’s the perfect time for your boiler to break down (not).

A common question connected with maintaining boilers is ‘Why does my boiler keep losing pressure?’ Once you know the ‘why’, learning the ‘how’ in order to re-pressurise your boiler and fix the problem is just as important.

But don’t worry, it’s not as difficult as it sounds. We asked the experts to explain exactly how it’s done so you can avoid a costly (and maybe even unnecessary) engineer call out.

Why does my boiler keep losing pressure?

The normal pressure for a boiler is between 1-1.5 bars. So if your boiler keeps losing pressure, it could be for one of these reasons, as explained by Lorraine Taylor, claims manager at boiler breakdown insurer Prominence Support:

‘Correct boiler pressure is vital for normal functioning. If the needle on the boiler’s pressure gauge drops overnight repeatedly, it is likely to be one of the following reasons:

• The pressure relief valve (PRV) is not working correctly.
• The auto air vent is leaking pressure.
• There are leaks in: the towel rails, radiators, radiator valves or the pipe themselves.
• The radiators are full of air.
• The expansion vessel is broken.
• Old joints have rusting and are leaking.
• The pressure gauge on the boiler isn’t working correctly.

‘If the boiler only loses pressure when the heating is on, this suggests there’s a small leak. The added pressure caused by increased heat places an extra burden on the leaking component. If the boiler appears to lose pressure even when the heating and boiler is off, this indicates a larger leak.

To find the pressure on your boiler, look for a gauge on the front of the appliance near the other controls. (Picture: Getty)

‘A boiler will switch off if the water pressure inside it becomes too low. This is a built-in safety feature of the appliance to prevent the water overheating and potential damage (and explosions).

‘When a heating system is originally installed, the engineer will have pressurised it correctly, but over time it can and will lose pressure. While a sudden loss of pressure can signify more serious problems like a leak in your central heating system, a slow depressurization over the years can simply be not bleeding your radiators.

‘To find the pressure on your boiler, look for a gauge on the front of the appliance near the other controls. This is usually a dial and may have green and red sections signifying safe and unsafe pressures. Make sure the needle on the dial is square in the middle of the green part of the gauge.

‘Don’t panic if it isn’t. Often low pressure is an issue that can be resolved easily by simply turning a tap/valve on the boiler. Check your boiler manual for details on how to re-pressurise the system.’

How to repressurise your boiler

Would you know what to do if your boiler kept losing pressure? Many people don’t, but luckily re-pressurising your boiler is fairly simple and it’s likely that you won’t need to spend the time (or money) calling out an engineer to do the job for you.

Hometree Founder & CEO Simon Phelan says: ‘To return the boiler’s pressure to the normal level of 1-1.5 bars when cold, you need to know how to re-pressurise a boiler.’

‘First, you need to know how to increase boiler pressure or how to top up the pressure on the boiler. Before you do this, it is important to check your boiler manufacturer’s manual, which may be in hard copy, or accessed online. Check the boiler is one that can be re-pressurised by the householder. The process for re-pressurising the boiler may be in the manual too, but the process below will work for most boilers.

• You need to find the filling loop, which is usually underneath the boiler. It is a flexible, metallic pipe with usually two tap-like valves.
• You need to find the pressure gauge on the boiler. It is important that you can see the pressure gauge when you are increasing or topping up the boiler pressure.
• The boiler and central heating system need to be cold.
• Open both valves on the filling loop. This allows mains cold water to enter the central heating system.
• Let the cold water flow into the filling loop and the boiler until the boiler pressure reaches 1.5 bars. On some boilers, the ideal pressure level will be indicated on the pressure gauge.
• Turn off the valves on the filling loop.
• Check the boiler pressure is steady at 1.5 bars.
• Turn on the boiler and monitor the pressure levels.
• If the drop in pressure was not due to an ongoing component fault, the pressure should remain at the normal level. If you find that repressurising your boiler isn’t a permanent fix, you will need to contact a gas safe engineer to help you fix the issue.

Just remember, if you’re unsure about any aspect of repairing your boiler or heating then seek out a qualified heating engineer to ensure work is safely carried out.

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