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.
Help I have had expansion tank and pressure valve changed. The boiler is 5 years old. Keeps losing pressure, the guys that looked at it say everything is fine. Obviously NOT as it loses pressure daily
I am a lady on my own am at my wits end.
Any idea
Good Morning ,
We have been having issues with our ideal combi 30 for a while now as it keeps dropping pressure we have had two ideal plumbers look at it and another independent plumber they all stated there is a leak we rented a leak detector which did not show any concerns in this area , we finally managed to get the plumber that fitted our system and he stated that if we had a leak there would be evidence of damp etc in our bungalow all my husband is doing is topping up the boiler to just above 2 bar and today let the boiler go cold and closed flow and return valve unplugged boiler and it has dropped just over 1 bar in 25 min. When heating is kept on nothing drops please can you give us some advise .
Many thanks
Christine
Hi Christine,
Check the copper blow off pipe outside for water coming out, it sounds like it might be the pressure relief valve causing your boiler pressure to keep dropping.
Morning I hope you can help. I’m a maintenance engineer working on a campsite. We have one troublesome boiler in a lodge that keep’s loose pressure every it about a day and a half. I’ve been under the lodge and checked all the speed fit connections and pipes and no sign of a leak. All the radiators inside seem fine the inside of the boiler shows no signs of a leak. I’ve changed the evaporator. Then I turned the boiler off for the night and this morning it’s still lost pressure I’m at a loss of things to check now any advice would be appreciated thanks Stan
Hi my boiler keeps dropping after the heating has finished being on. I cant see any leaks but I replenish daily for the pressure any suggestions thanks Kev
Hello ,I have a vaillant boiler and it has been running for 10 years without a single problem,albeit serviced every year.In the last year or so , it started losing pressure.The very first time I found it out was after the boiler stopped working after making unusual noise. Realising the pressure was nearly zero , I topped it up to 1.5 bar and the Boiler was back to normal. However , I kept repeating this every 4/5 weeks.But recently , the 4/5 weeks pressure topping up frequency has become 1 week.I called my servicing engineer and after a thorough check he topped up the expansion tank and replaced the RV as it was leaking according to him.All seemed fine but the next day the pressure was down to zero. I rang him and he advised me to to top up again , which I did , the problem remains in that the pressure drops from 1.5 bar to 0.5 bar in few days i.e worse than before (before any change was made). There is no obvious tell tale on any of the ceiling for any obvious leak and no leak on any of the radiator valves.
The only thing I noticed when changing the carpet on the top floor was a patchy discolouration of the chip board but nothing on the ceiling below. This is just for info.
Could you please suggest or advise on what could be the reasons for the loss of pressure.
A prompt response will be appreciated.
Many thanks.
Hi Pitouz,
It sounds like it could be an expansion vessel problem causing the pressure to drop if the PRV was leaking. It might need to be replaced if it’s lost its air again. Or sometimes just the Schrader valve on the expansion vessel can be replaced if that’s letting the air out.
It could also be a leak on the radiator pipes under the floor downstairs.
You should check for the boiler pressure rising too high when you turn the heating on, if it does then you’ll know that it’s the expansion vessel causing the pressure to keep dropping.
Hi there, asking for a friend as he is really stressed out. He had his whole house refurbished with brand new boiler etc just 2 months ago. Few days ago, he noticed a whistling noise from one of his radiators and so he bled it to release the trapped air! Next day he noticed that the pressure on his boiler has dropped. His wife increased the pressure but by mistake, she overdid it to around 3.0. Since then the boiler is not retaining the pressure and losing it continuously. He spoke to the installer and he came around and checked everything but could not find any fault with the boiler or any visible leak. He applied the leak sealer through the system and that seemed to have made no difference as the boiler dropped the pressure again the very next day. Its a brand new boiler! He is very concerned and doesn’t know what to do. He is concerned that if the leak is the under the floor, so does that mean everything will have to be stripped? What would be your best expert advise? how should he take this forward to resolve this? I mean he has just finished the complete refurbishment of the whole house so you can imagine. Is there any other convenient way around this??
Hi Suhel,
Yes, you’ll probably have to pull the floor up downstairs to find and fix the leak if it’s definitely not a boiler problem. Leak sealer won’t do anything unless it’s a really slow leak taking weeks or months for the boiler to keep losing pressure, not overnight.
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?