Last updated on January 4th, 2025
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.
Causes for pressure dropping
Bleeding radiators will cause your boiler pressure to drop, but if it keeps dropping, there is something else causing it.
Most likely causes:
- A Leak
- Expansion vessel has lost its air
- Pressure relief valve passing water
- Filling loop letting by
A leak
The most common problem I get called out to on a boiler that keeps losing pressure is a leak. This could be anywhere; a radiator, a radiator pipe under the floor, the boiler, literally anywhere on the heating system.
The most common place for a heating system leak is on a radiator valve. Check them all for any signs of water. You can often fix a leaking radiator valve by tightening a nut on the valve. There are usually three nuts on a radiator valve that can leak and be fixed by tightening the nut.
If the leak is not on a radiator, check under the boiler and any other heating pipes on show around the house. If you can’t find the leak, 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 if it’s a small leak.
Expansion vessel
A pressurised central heating system needs an expansion vessel to take the expansion of the hot water when the heating is turned on. If your pressure is dropping with no leaks, a faulty expansion vessel is the most likely cause. These are either inside the boiler or could be in the hot water cylinder cupboard if you have one.
The expansion vessel should be full of air, so when the heating is on it takes some of the pressure increase away from the radiators and pipes. If it loses air (quite common), 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 gauge to 0.
To fix this problem, you need to recharge your expansion vessel by pumping it up with a pump. 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 pressurised heating systems must have a safety device called a pressure relief valve (PRV). A PRV protects your 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. Make sure the pressure is not rising before replacing it 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 slightly open 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, and sometimes the valve just doesn’t close fully anymore because of dirt or damage and the filling loop valve needs to be replaced.
Pressure rising first
If your boiler pressure is rising to 3 bar this is most often caused by your expansion vessel not doing its job. It could also be caused by the filling loop constantly letting water into the heating system, but this is less common. When your pressure rises to 3 bar the PRV will let the water and pressure out of your heating system. The PRV sometimes will not close fully after being used, so after fixing the pressure rising problem, you might have to replace the PRV also.
Conclusion
If your boiler loses pressure constantly, finding out the cause is the first step to fixing it. If there is no sign of water coming out of the copper pipe outside, it is most likely a leak. If the pressure is rising to 3 bar, it’s most likely the expansion vessel.
Have any questions? Drop them in the comments below, I’ll do my best to help! If you found this post helpful, don’t forget to share it with others who might benefit.
FAQs
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, you might have a leak on the heating system. If it loses pressure once or twice a year, I wouldn’t worry about it.
Is low boiler pressure dangerous?
No, low boiler pressure is not dangerous. If your boiler pressure is too low 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