How to replace a pressure relief valve

How to Replace a Pressure Relief Valve on a Combi Boiler

Last updated on June 8th, 2024

A pressure relief valve (PRV) is a safety valve on a central heating system to prevent the pressure from getting too high and becoming dangerous.

When the boiler pressure is too high and reaches 3 bar, the PRV will release the water from the system and discharge it outside.

I have been repairing boilers full-time for over a decade and replacing a PRV is common.

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.

boiler pressure relief valve
PRV on a Baxi boiler

If it has a lever-type handle, you must pull the handle away from the valve and hold it open.

Pressure relief valve on a Vaillant boiler
PRV on a Vaillant boiler

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 need to be replaced.

I often open a PRV like this when I need to drop a boiler’s pressure for 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 Pressure Relief Valve

When you find water dripping from your pressure relief valve, it is a sure sign that it needs replacing. This will cause your boiler to keep losing pressure until it is replaced.

Boiler Leaking Water From Overflow Pipe
PRV blow-off pipe

When a PRV needs replacing, it’s usually a sign of another problem with the boiler as this often 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 replaced).

Recharging expansion vessel on combi boiler
Recharging expansion vessel

Water dripping from a pressure relief valve means it 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 inside. This stops the valve from closing fully and causes it to drip constantly.

How to Replace a PRV on a Baxi Combi

  1. Drop the pressure on the boiler by opening the pressure relief valve (you might need to drain the system)
  2. Use an adjustable spanner to undo the nut on the bottom of the PRV that connects the copper pipe
  3. Use a 2.5mm hex (Allen) key to loosen the grub screw on the bottom of the PRV
  4. Remove the pressure relief valve
  5. Fit the new PRV and tighten up the grub screw with the hex key
  6. 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)
  7. Repressurise the boiler and bleed all the radiators.
Removing a PRV nut with a spanner
Removing a PRV nut with a spanner

Removing a PRV grub screw with a hex key
Removing a PRV grub screw with a hex key

prv removed
PRV removed


Replacing a pressure relief valve on a Baxi, Potterton and Main (all the same) combi is easy compared to some boilers as the PRV is very accessible. Although other boilers can be very similar, some are a nightmare to access.

You must find out if the pressure is rising to 3 bar before it's dropping. If it is then you need to fix this problem first before replacing the PRV.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

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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 copper blow-off pipe from outside back to the boiler and see where it goes into the boiler.

It 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 valve should only need to be replaced when it is dripping. This usually means it has been opened and is not closing properly.

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 it’s leaking outside then this will cause the boiler pressure to keep dropping, when the pressure drops too much, the boiler won’t work.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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9 thoughts on “How to Replace a Pressure Relief Valve on a Combi Boiler”

  1. Rosemary Jeboo

    hi steven
    I have to look after the house myself – pensioner. I have a plan with Baxi to service my boiler and change parts when necessary. yesterday 30/10/2023 the boiler was leaking he will come back with a new expansion vessel and will fix the leaking pipe.
    when we went outside to check the condensate pipe he told me that the PRV (the safety piece of copper pipe was not there just a hole. he says that they are not allowed to fit this. I am worried as he says hot water comes out from there. please advise me on how i can get someone to put a new one? is it a difficult job. is it possible to attach from inside the boiler to the outside? is it a big job? i will appreciate your advice.

    1. Steven Reid

      Yeah you can fit a new piece of copper pipe from inside the house to outside, it’s an easy job most of the time.

  2. Adrian Milner

    Hi recently had some external gas main repairs outside the house. When the engineer came to turn everything back on, he checked the boiler and reminded that the pressure be dropped slightly. I drained some water off from the kitchen radiator and it reduced the boiler pressure. I bleed the radiator and the bathroom radiator as this is the highest in the house. Since this the boiler is not holding pressure and not kicking in. I top the system up and it kicks in. Next time around I have to do this again. Never had any issues before. Any advice would be gratefully appreciated. Thank you.

    1. Steven Reid

      Hi Adrian, sounds like a leak. Could be anywhere. Try checking the copper blow off pipe outside for dripping to see if it’s the pressure relief valve.

  3. Scott Kent

    Hi Steve,
    We have a Baxi 80e combi, recently had to change the fan and a new PCB, pressure will not hold on the boiler, pipe leading outside is constantly dripping and there is a small drip somewhere on the boiler? Could this be a PRV, Heater exchange or both?

    Many thanks.

    1. Steven Reid

      Hi Scott,
      Definitely a PRV problem if it’s dripping out of the copper pipe outside. Check if the pressure is rising to 3 bar when the heating is on as you might have an expansion vessel problem also. The drip in the boiler will probably not be from the PRV though, could be from anywhere.

      1. Thank you for your response Steve.
        Pressurised bolier to 3bar, turned bolier off and shut all valves, pressure still drops. Can hear slight trickle of water, copper pipe outside still leaking. Fired bolier back up pressure not going up and a slight whistling noise. Probably thinking its definitely PRV has gone?

        1. Steven Reid

          Yes, definitely pressure relief valve if it doesn’t stop dripping outside. You might have another leak inside the boiler that’s causing the pressure to keep dropping also but the PRV has definitely gone based on what you’ve told me.

          1. Thank you for your quick response and much appreciated help. New PRV it is and go from there ‍♂️

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