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:
- Close the filling loop fully
- Recharge the expansion vessel
- 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.
Hi there, I bled all my radiators this week for the first time in a VERY long time. There was a LOT of air in the system. I repressurised the boiler throughout (often had to do this after each radiator) and afterwards. But today, the pressure keeps dropping on the boiler and I have to keep topping it up regularly. Is this normal the following day after bleeding the radiators?
Hi Lucy,
You could potentially have air still in the system causing problems so keep bleeding the air out and repressurising the boiler for now.
It sounds like you might have a leak which is causing the pressure to keep dropping, or no air in the expansion vessel which is causing the pressure to rise too high when the heating is on (3 bar+) then the pressure (water) will be released out of the copper pipe outside.
Hi Steven
I have an oil fired combi boiler with underfloor heating.
I had problems last year with high pressure in my boiler. I have had the expansion vessel and filling loop replaced as well as the air vent valves.
Now I have a problem with low pressure. I would really value your thoughts as I am losing patience with my heating engineers. The main features are:
1. it loses almost all of the pressure within 2 to 3 weeks of re-setting it.
2. it is not very evident that there is a pressure issue when the boiler is on as the pressure reads anywhere between 1.0 and 1.5 and it never goes above this.
3. I have looked very carefully several times and there are no signs of any leaks anywhere.
4. when I switch the boiler off, the pressure guage goes down to almost zero after the boiler has fully cooled down (this takes about 15 hours).
5. I notice that if:
a. the boiler is switched on
b. the switch in the boiler is set to ‘water off’.
c. I run a hot water tap
the pressure guage in the boiler drops alarmingly and would go to zero if I run the hot water for a minute or so.
I know this is not a normal practice but I thought it might help with the diagnostics.
6. There does not seem to be an issue with the pressure relief valve. It does sometimes feel a bit damp (condensation maybe?) but there is no sign of dripping and it is usually dry even when the boiler is running.
Thank you
Rob
Hi Steve
I have a Combi Boiler over 10 years old and I am experiencing frequent pressure loss without any identifiable leaks on Radiators. My engineer put in an external expansion vessel which seemed to help for a short while but pressure loss has returned again.
The engineer advised isolating the boiler which I did and the pressure did drop but I realised that the the system was very hot and the cooling may have dropped the pressure. So I topped up again and this time no pressure was lost. I thought the problem must in the system but when I opened up the isolation valves the pressure still didn’t drop! My engineer says I should replace the boiler as he believes the problem is there but I’m wondering if he should be trying to replace some parts first.
Many thanks
Hi, I’ve had my boiler checked yearly. This last time they said all working great. A few days later another engineer rang to say he will be with me shortly. I ask why he said to give me a quote on a new boiler. I never asked for one. Then a few days later the pressure went down. I’ve had 3 engineers who’ve checked everything and all is working great. But every couple of days I have to put the pressure up again. Why did all the engineers say all is fine but it keeps going. I’m a widow and a pensioner so can’t afford a new one. My boiler was fitted in 2014 so it shouldn’t need replacing. I’m at my wits end and fed up keep putting the pressure up. There’s no leaks in the boiler or radiators. All was checked thoroughly. I do hope you can help me?
Hi Steve
No pressure drops when system is on water only
No pressure drops when the heating is on, as soon as we turn the heating off at night the pressure drop straight away and there is a noise from the boiler as if it is draining
Hi Steven,
My boiler is continually losing pressure over approx 5 to 7 hours. My glowworm combi boiler was new a year ago and I’ve had glowworm engineer out to check and he changed the PRV (water and staining found in this) and also changed tge AAV. He said the boiler was now working properly but it didn’t mean there wasn’t a leak somewhere. The pressure started falling again to 0 within hours of him completing the work and I was constantly doing that for 2 weeks prior to a plumber coming out on the 6th Jan. They couldn’t find a leak anywhere and resorted to using the self sealing liquid in the pipes. Since Friday the pressure has again consistently dropped to 0. Could it be something else wrong with the boiler or does it mean there’s a definite leak. I’m so fed up with it and a lady living on my own with a boiler in the loft so it means going up into the loft at least once a day. Please can you help?
Thank you
Jo
Hi Jo,
Yeah sounds like a leak, they can be very frustrating. It’s probably under the floor downstairs as you would most likely see it if it was somewhere else. You can isolate the boiler from the system to confirm it’s not on the boiler.
Thanks for your response Steven. My plumber ended up using a leak sealant which after reading articles I hope it won’t damage anything? This was 4 days ago now and my heating stayed on all day yesterday but it didn’t fire up this morning and pressure was down to zero. The only floor they haven’t pulled up downstairs is the hall as it’s wood floor but I guess that needs to be investigated. What would you advise? Thanks very much,
Jo
Sounds like the floor is going to have to come up if you’re sure it’s on the heating system. You can sometimes spot leaks from above the floor with a thermal imaging camera but not many people have them and it might not find it.
Hi Steven,
Your post and comments are awesome. Thank you for advising. My boiler pressure drops slowly over a few weeks. It is not the boiler but the heating system. I have a manifold system connected to pipes for the lower floor. One of the joints on the manifold has significant limescale and green stuff and looks like it has been leaking but is not wet at the moment. Is this likely to be a cause of pressure loss as a pin prick leak perhaps. I have had all valves replaced, all pipes around rads inspected but no engineer has even considered or looked at the manifold. Thanks for the advice. Naomi
Hi Naomi,
Thanks! If the manifold isn’t wet after repressurising the boiler and having the heating on then it’s probably not that causing the pressure to drop. When it’s green like that it’s usually a sign of not cleaning the flux off after soldering the pipe connections but you’d know straight away if there was a pinhole leak.
Dec 9th – Boiler no pressure. Dec 10th heating engineer re-pressurised system, checked the boiler. Within 2.5 hours boiler pressure zero. Dec 13th Engineer returned found no issue with boiler – suspected leak somewhere – left filling loop slightly open to maintain pressure. Dec 16th following call to insurers company engineer visit to perform trace and access process. Initially performed moisture check around all radiators and skirting board – nothing detected. Then drained system and applied gas injection to detect leaks – no leaks detected. Refilled system, bled some radiators but boiler error code says air-lock. Engineer left. Dec 17th Air lock gone I refired the boiler. The boiler pressure goes from 2.1 (my setting) to 0.3 over a period of 8.5 hours. I am now repressurising daily – every 8 hours. Over the first 2.5 hours pressure drops from 2.1 to 1.1 – over next 5.5 hours drops from 1.1 to 0.4. Any advice on what action to take next most appreciated. Thanks in advance for your time. Best regards Mike.
Hi Mike,
The first thing I would do is verify if it’s a boiler or system problem. I’d do that by topping the pressure up to 2 bar when the system is cold, then isolating the flow and return valves on the boiler. Leave it overnight and check if the pressure has dropped