Last updated on February 18th, 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 then 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 central 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 central heating 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 me any questions in the comment section below and I’ll try my best to help.
Please share this post if you find it helpful.
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.
What should boiler pressure be?
Your boiler pressure should be around 1 to 1.5 bar when the heating is off and the system is cold. If it drops below 1 bar, you should repressurise your boiler.
When the heating is on, it’s normal for the pressure gauge to be around 2 bar.
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 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
Over this weekend of very cold weather, my combi boiler has lost pressure 4 times, it is located in the loft.
I can not find any sign of a leak on any pipe or radiator. No signs of any leaks on exposed pipes in the loft.
I have used the filling loop to refill the boiler to 1 bar each time..there is a few drops of water from the filling loop levers when opened but stop when the filling loop is closed.
To check how much condensate was being produced, I diverted the condensate pipe into a plastic bucket, and not as much as 2 litres an hour, but a fair amount was produced.
Any ideas what I should do while I wait to get a gas engineer out
You can only keep repressurising the boiler until the problem is found and fixed.
The filling loop leaking won’t be causing the pressure to drop but a leak could be anywhere on the pipes under the floor.
You should check if the pressure is rising to 3 bar before dropping
Thanks for the quick response, will keep filling as suggested until I can get a gas engineer. I just rechecked the filling loop. It’s wet on the cold mains side, not on the heating side. So I’m happy that the filling loop is not the problem.
It could still be a filling loop problem if it’s leaking at all. I thought you meant it’s only leaking when you top the pressure up. If it’s leaking all the time then it’s probably a faulty filling loop valve problem.
When the heating comes on the pressure guage goes to 1.5bar
When the heating goes off pressure guage goes back down to 0
The engineer from BG came and pumped water out of the expansion tank and said it should now be ok
The problem after a couple of days has reverted to the original situation as above .. BG can’t come back until the first week in January .
Is there anything I should do? Should I add water to the system?
Sounds like the pressure relief valve is letting the water outside if the pressure is not rising to 3 bar first. This is normal after having expansion vessel problems, sometimes the pressure relief valve doesn’t close properly after being used and it will need to be replaced.
You’ll just have to keep topping the pressure up until it gets fixed.
Thanks for such a speedy reply. I’ll mention it to the bg engineer. The next date they have given me is 21/12. In the meantime can I just keep topping up? I’m s vulnerable adult and need to have my heat on?
You’re welcome. Yes, you can keep topping the pressure up.
Good luck!
As a follow up I had the bg engineer come and fix a leak in the downstairs radiator. Another engineer came to finish the job as the nut was still leaking. There has been no further leaks but no one topped the water and a substantial amount of water came out of the radiator. But in the last 3 weeks the pressure issue has been a bigger problem
Hi Zuher,
Sounds like an expansion vessel or pressure relief valve problem causing the pressure to keep dropping, possibly both.
You should top the pressure up and turn the heating on at the maximum temperature setting on the boiler, then wait to see how much the pressure rises.
If it rises to 3 bar then you will need to recharge the expansion vessel, or possibly replace it.
If the pressure does not rise to 3 bar and starts leaking outside, then you will need to replace the pressure relief valve.
Hi
My boiler pressure keeps dropping. Yesterday I topped the water and after the first heat cycle and the second the pressure was down to 0. I have also noticed water dripping outside. Any suggestions?
Hi Steven, I changed heat plate but after few hours again pressure dropping, any idea what else I can do?
Hi Lukasz,
Are you sure you isolated the flow and return valves on the boiler properly?
If the leak is on the boiler then it will either be coming out of the blow-off pipe outside or the plate heat exchanger.
Yes, Steven, I close First one from the left and the last one, also I keep checking all the time blow off pipe and is nothing there, is this pipe? https://forum.buildhub.org.uk/uploads/monthly_2019_06/20190622_232022.jpg.d1d777b238b60f3a088305a775628fbb.jpg Heat exchanger plate is dry as well, do I have to top up expanssion vessel after I changed heat plate?
Yes, that’s the blow-off pipe. Did you top the boiler pressure up to 2 bar when everything was cold then isolate it overnight?