If your boiler keeps losing pressure, you are not alone.
This is a very common boiler repair job that I get called out to on a regular basis.
There are many possible reasons why this can happen – I’ll cover the most common here.
1. A Leak
The most common problem I get called out to when a boiler keeps losing pressure, is a leak somewhere on the central heating system.
This could be anywhere on the full system – a radiator, a radiator pipe under the floor, the boiler, literally anywhere on the heating system.
Check all the Radiator Valves!
The most common place a heating system leak happens is on a radiator valve.
Check them all for any signs of water, or old water damage like rust or stains.
Sometimes this can be fixed by tightening one or some of the nuts on the radiator valve.
There are at least three places a lockshield radiator valve can leak from that can be fixed by tightening the nut with an adjustable spanner.
There are the two main nuts on the pipe and tail coming out of the radiator.
Then there is the smaller nut right on top where you turn the valve to open and close it.
2. Expansion Vessel
Your pressurised central heating system needs an expansion vessel for when the heating is turned on.
These can be found either inside the boiler, or anywhere on your heating pipes, depending on what kind of system you have.
Gravity fed, non pressurised heating systems do not have expansion vessels.
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’s broken, then your pressure will rise very quickly when you turn the heating on, causing it reach over 3 bar of pressure.
This will cause the safety device, called a pressure relief valve, to release all the water (pressure) to outside and take your pressure to 0 bar.
This is why your boiler keeps losing pressure.
To fix this problem, you either need to recharge your expansion vessel by pumping it up with a pump, or if that doesn’t work, you will most likely have to replace it.
3. Leaking Pressure Relief Valve
All combi boilers and pressurised heating systems must have a safety device called a pressure relief valve (PRV).
These will protect your central heating system from building up with too much pressure and causing radiators and boilers to blow up.
PRVs can leak because they get little bits of grit or dirt stuck in them when they open to release the high pressure.
Slow Pressure Drop?
If your boiler keeps losing pressure slowly, and it’s not rising to 3 bar first, then you should check your copper blow off pipe outside for dripping water.
If it’s dripping, you will need to replace the PRV.
You need to make sure the pressure is not rising first before replacing it because the new PRV will leak also if something is causing the pressure to rise.
4. Filling Loop Open
Another way your boiler keeps losing pressure could be caused by your filling loop being open or broken.
This will cause the pressure to rise to 3 bar and the PRV to blow it off 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 pressure.
Or maybe it just doesn’t close fully any more because of dirt or damage inside and needs replaced.
I have been called out to boiler repair jobs where the filling loop was left fully open when the customer thought it was closed.
The water was constantly blowing off outside through the PRV like a tap, but the customer hadn’t noticed as the heating was still working.
There was still enough pressure because of the mains water pressure constantly flowing through.
Why Your Boiler Keeps Losing Pressure Summary
Before replacing the PRV, always make sure you have fixed any other problems first, as you might be wasting your time.
If your boiler keeps losing pressure because of the expansion vessel or filling loop, then you might have to replace the PRV as well.
Once it has blown the water out, sometimes they never quite close properly again.