Last updated on September 23rd, 2023
A pressure relief valve (known as a PRV to plumbers) is a safety valve found on boilers and central heating systems to prevent the pressure from building up too high and becoming dangerous.
Pressure relief valves on combi boilers are set at 3 bar. This means when the boiler’s pressure is too high and reaches 3 bar, the PRV will release the water from the heating system and discharge it outside.
I have been repairing and replacing boilers for over a decade and replacing pressure relief valves is very common.
Here I’ll show you how to replace a pressure relief valve on a combi boiler based on my on-the-job experiences over the years.
How to Open Pressure Relief Valve
To open a pressure relief valve you must turn the head (usually red) anti-clockwise. It will automatically close again if you keep turning it so you will need to hold it open until you are finished.
If it has a lever-type handle, then you must pull the handle away from the valve and hold it open.
The problem with opening a pressure relief valve is that you risk it leaking when you repressurise the boiler because sometimes they don’t close properly and the PRV needs to be replaced.
I often open a pressure relief valve like this when I need to drain a boiler’s pressure when recharging expansion vessels and other boiler repair jobs, but I always keep spare PRVs on the van so I can replace them if I need to.
Water Dripping From Boiler Pressure Relief Valve
When you find water dripping from your boilers pressure relief valve this is a sure sign that it needs to be replaced.
This will cause your boiler to keep lose pressure until it is replaced.
When a PRV needs replacing, it’s usually a sign of another problem with the boiler as this means something is causing the boiler pressure to rise too high.
This problem is usually down to the expansion vessel needing to be recharged or sometimes replaced.
Water dripping from a pressure relief valve means the valve has done its job and released the water from the heating system.
However, sometimes (especially on old boilers) when the PRV releases the water, it gets little bits of grit or debris stuck in which stops the valve from closing fully and causes it to drip constantly.
How to Replace Pressure Relief Valve on Baxi Combi Boiler
- Drop the pressure on the boiler by opening the pressure relief valve (you might need to drain the water out of the system system)
- Use an adjustable spanner to undo the nut on the bottom of the pressure relief valve that connects the copper pipe
- Use a 2.5mm hex (Allen) key to loosen the grub screw on the bottom of the PRV
- Remove the pressure relief valve
- Fit the new pressure relief valve and tighten up the grub screw with the hex key
- Refit the nut and tighten it with an adjustable spanner. Make sure it has a half-inch fibre washer inside (you might need a new washer)
- Repressurise the boiler and bleed all the radiators.
Replacing a pressure relief valve on a Baxi, Potterton and Main combi boilers are one of the easiest boilers to replace them on as the PRV is very accessible.
Although other boilers can be very similar, some are a nightmare to access.
If your boiler pressure keeps dropping then you should check the pressure relief valve pipe outside to check for dripping water, if it’s dripping then you will definitely need to replace the pressure relief valve.
When replacing a pressure relief valve on a Baxi boiler it goes without saying you should know how to repressurise a Baxi boiler.
You must find out if your boiler pressure is rising to 3 bar first before releasing the pressure. If it is then you should fix this problem first before replacing the pressure relief valve.
Feel free to leave feedback or ask any questions in the comment section below and I’ll try my best to help.
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Replace Pressure Relief Valve FAQs
Where is the pressure relief valve on a combi boiler?
The pressure relief valve location on a combi boiler varies depending on the boiler. You will need to follow the pressure relief valve copper pipe back to the boiler and check where it goes into the boiler.
A pressure relief valve on a combi boiler is usually red and you can typically see them straight away when the casing is removed from the boiler.
How often should a pressure relief valve be replaced?
A pressure relief should only need to be replaced when it is dripping. This usually means it has been opened and is not closing fully.
How do you know if your pressure relief valve is faulty?
You will know your pressure relief valve is faulty when it is dripping out of the copper blow-off pipe outside, or possibly into a tundish.
If the pressure relief valve is leaking outside then this will cause the boiler pressure to keep dropping, when the boiler pressure drops too much then the boiler won’t work.
This is the first way you’ll know that your pressure relief valve is faulty.