Top 4 Reasons Your Boiler Keeps Losing Pressure


If your boiler keeps losing pressure, you are not alone.

This is a very common boiler repair job I get called out to on a regular basis.

There is a few different possible reasons why this can happen, I’ll cover the most common here.

1. A Leak

The most common problem when I get called out to a repair job when their 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.

boiler keeps losing pressure
Radiator valve – The three nuts you can tighten

The most common place a leak happens is on a radiator valve.

Sometimes this can be fixed by tightening one or some of the nuts on the radiator valve.

There is at least three places the radiator valve can leak from that can be fixed by tightening the nut with an adjustable spanner.

There is the two main nuts on the pipe and tail coming out of the radiator, and there is the smaller nut right on top where you turn the valve to open and close it.

After fixing the leak, you should bleed the radiators and top the pressure up to 1.5 bar.

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 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 have to replace it.

3. Leaking Pressure Relief Valve

All pressurised heating systems must have a safety device called a pressure relief valve (PRV).

These will protect your system from building up too much pressure and causing radiators and boilers to blow up.

If your boiler keeps losing pressure slowly, and it’s not rising to 3 bar first, then you should check your blow off pipe outside for dripping water.

If it’s dripping, you will need to replace it.

PRVs can leak because they get little bits of grit or dirt stuck in them when they open to release the high pressure.

If your PRV is leaking, 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 and there was still enough pressure because of the mains water pressure.

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 as once it has blown the water out, sometimes they never quite close properly again.


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