Why Does My Boiler Keep Losing Pressure? 3 Easy Fixes

Last updated on December 26th, 2024

A boiler losing pressure regularly means you have a problem with the boiler or the central heating system. The most common reason is a leak, which could be anywhere on the system.

I have been a full-time heating engineer for over a decade, and a boiler losing pressure is normally easy to fix.

Why Does My Boiler Pressure Keep Dropping?

If you have recently bled the radiators this will cause your boiler pressure to drop, but if it keeps dropping, there is something else causing it.

Your boiler losing pressure regularly is most likely for one of these reasons:

• A Leak on the central heating system
• The expansion vessel has lost its air
• Pressure relief valve passing water
• The filling loop is slightly open

A Leak on the Heating System

The most common problem I get called out to when a boiler is losing pressure is a leak somewhere on the central heating system.

This could be anywhere on the full system: a radiator leak, a radiator pipe under the floor, the boiler, literally anywhere on the heating system.

How to find a leak in a heating system

To find a leak in the central heating system, the first thing to do is check all the radiator valves. The most common place for a heating system leak is on a radiator valve. Check them all for any signs of water, or old water damage like rust or stains.

Radiator leak fix

Sometimes you can fix a leaking radiator valve by tightening one or some of the nuts on the valve. There are at least three places a radiator valve can leak from that can be fixed by tightening the nut with an adjustable spanner.

There are two main nuts, one on the pipe at the bottom, and one on the tail coming out of the radiator. Then there is the gland nut right on top where you turn the valve to open and close it.

You might need to replace the radiator valve if the leak won’t stop after tightening the leaking nut. After fixing the leak, you should bleed the radiators and repressurise the boiler to 1 bar on the pressure gauge

If the leak is not on a radiator, you should check under the boiler and any other heating pipes on show around the house. If you can’t find the leak anywhere on your heating system, it could be under the floor.

This means the floor will have to come up, or you can try adding some leak sealer to the system. Leak sealer will only work if your boiler is losing pressure slowly as it will only fix small leaks.

No Air in Expansion Vessel

Your pressurised central heating system needs an expansion vessel to take the expansion of the water when the heating is turned on. If your system or combi boiler pressure is dropping with no leaks, then a faulty expansion vessel is the most likely cause.

Expansion vessel location

We can find these either inside the boiler or somewhere on your heating pipes, depending on what kind of system you have. The expansion vessel is full of air, so when the heating is on it takes some of the pressure increase away from your radiators and pipes.

If it loses air (quite common), then your boiler pressure will rise too high when you turn the heating on, causing it to reach over 3 bar of pressure. This will cause the pressure relief valve (safety device), to release all the water (pressure) outside and take your pressure to 0 bar on the pressure gauge. This is why your boiler is losing pressure.

How to fix the expansion vessel problem

To fix this problem, you need to either recharge your expansion vessel by pumping it up with a pump, or if that doesn’t work, and the diaphragm inside the vessel is split, you will have to replace the expansion vessel.

Pressure Relief Valve Passing

All combi boilers and pressurised heating systems must have a safety device called a pressure relief valve (PRV).

A PRV protects your central heating system from building up too much pressure and causing a radiator or boiler to blow up. PRVs can leak because they get little bits of debris or dirt stuck in them when they open to release the high pressure.

Slow Pressure Drop? If your boiler pressure keeps dropping slowly and it’s not rising to 3 bar first, you should check your copper blow-off (overflow) pipe outside for dripping water. If it’s dripping, you will need to replace the pressure relief valve.

You need to make sure the pressure is not rising before replacing the pressure relief valve because the new PRV will leak if something is causing the pressure to rise.

Filling Loop Letting By

Another reason for losing pressure could be your filling loop being open slightly or broken. This will cause the pressure to rise to 3 bar and the PRV to blow the system water outside.

Sometimes the valves on the filling loop are not closed properly after topping the pressure up, causing the system to slowly fill with too much water and pressure.

Or, maybe the valve just doesn’t close fully anymore because of dirt or damage and the filling loop valve needs to be replaced.

I have been called out to fix a boiler where the filling loop was left fully open and the customer thought it was closed. The water was constantly blowing off outside through the PRV pipe like a tap, but the customer hadn’t noticed as the heating was still working. There was still enough pressure because the mains water pressure constantly flowed through the PRV.

Before replacing the PRV, always make sure you have fixed any other problems first, as you might be wasting your time. If your boiler is losing pressure because of the expansion vessel or filling loop, then you might have to replace the PRV as well. Once the PRV has blown water out, sometimes they never quite close properly again.

Pressure Rising First

If your boiler pressure is rising to 3 bar, then having a leak is not causing the boiler to constantly lose pressure. This is almost always caused by your expansion vessel not doing its job.

The pressure rising to 3 bar could also be caused by the filling loop constantly letting water into the heating system, but this is less common. Make sure your filling loop valves for repressurising the boiler are fully closed (but they might need replacing).

When your pressure rises to 3 bar it will cause your boiler to keep losing pressure. The pressure relief valve is set to let the water out of your central heating system when it reaches 3 bar.

This is a safety device which means the pressure relief valve is doing its job and telling you there is a problem elsewhere, causing the boiler pressure to keep rising before it drops.

Conclusion

If your boiler loses pressure constantly then finding out the cause is the first step to fixing it. You might need to call a heating engineer if you need to replace any of the parts mentioned but here are 3 potential easy fixes without having to replace parts:

1. Close the filling loop fully
2. Recharge the expansion vessel
3. Add a bottle of leak sealer to the system to fix a small leak

Feel free to ask any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.

FAQs

Can a boiler lose pressure without a leak?

Yes! A boiler can lose pressure without a leak. A leak is the most common cause of a boiler losing pressure, but it could also be caused by an expansion vessel problem, a pressure relief valve problem, a filling loop problem, or by bleeding the radiators.

Can a dripping tap cause boiler pressure to drop?

No, a dripping tap can not cause boiler pressure to drop. A tap is not connected to your central heating system, so a kitchen or bathroom tap dripping cannot affect your boiler pressure.

How often should a boiler lose pressure?

A boiler should never lose pressure unless there is a problem. If your central heating is losing pressure, then you might have a leak on the heating system.

Is low boiler pressure dangerous?

No, low boiler pressure is not dangerous. If your boiler pressure is too low then the boiler will not work, but it will not make it dangerous. You should repressurise the boiler if you have low pressure to stop it from breaking down.