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.
My heating system only stays good for a couple of days max then drops down to 0 , where should I start first??
Check if the pressure rises to 3 bar when the heating is on, and check the copper blow off pipe outside for any signs of water coming out.
Hi Stephen
My combi boiler is losing all pressure over about 30 minutes. I cannot find any leaks anywhere including under the floor. It takes about 2 minutes to top up the system again suggesting that a lot of water is going somewhere. Still I cannot find leaks. It is not going out of the “overflow” pipe that goes outside. That is dry. I have put some sealing fluid in the system via the ladder radiator in the bathroom and still the problem persists. What can I do please.
John
Hi John,
I would recommend repressurising the boiler and then isolating it from the system by closing the flow and return valves under the boiler. If the pressure keeps dropping then you’ll know it’s a boiler problem (possibly plate heat exchanger passing), and if it doesn’t drop then you know it’s a leak on the system.
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.