Last updated on December 31st, 2024
A pressure relief valve (PRV) is a safety device on a pressurised heating system that prevents the pressure from getting too high and becoming dangerous. When the pressure reaches 3 bar, the PRV releases the water from the system and discharges it outside.
I have been repairing boilers full-time for over a decade and replacing a PRV is a common fix.
Water dripping from the PRV pipe
When you find water dripping from your pressure relief valve pipe outside, it is a sure sign that the PRV needs replacing. This will cause your boiler to keep losing pressure until it is replaced. This is also usually a sign of another problem that is causing the pressure to rise too high and then drop too low. This problem is often because the expansion vessel needs to be recharged (or replaced).
Water dripping from a PRV pipe means the PRV has done its job and released the water from the heating system when it rose too high. 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
The photos here are of replacing a PRV on a Baxi boiler but the process is the same for all boilers. The valve might be slightly different and it might be in a different location.
1. Close the flow and return valves on the boiler. They don’t always hold and sometimes you can’t turn them, so you might need to drain the system and leave them open.
2. Drop the pressure on the boiler by opening the pressure relief valve. Keep it held open until the water stops coming out of the copper blow-off pipe outside.
3. Use an adjustable spanner to undo the nut on the bottom of the PRV that connects the copper pipe. Turn it anti-clockwise.
4. Use a 2.5mm hex (Allen) key to loosen the grub screw on the bottom of the PRV. Some are held in with a clip that must be removed.
5. Remove the pressure relief valve by pulling it out.
6. Fit the new PRV and tighten up the grub screw. Or fit the clip if you have one.
7. Refit the pipe and nut and tighten it with an adjustable spanner. Make sure it has a half-inch fibre washer inside (you might need a new washer)
8. Open the flow and return valves if you closed them.
9. Repressurise the boiler.
Summary
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 any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.
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 copper blow-off pipe from outside back to the boiler and see where it goes into the boiler. It’s 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 this will cause the boiler pressure to keep dropping, when the pressure drops too much, the boiler won’t work.
i have a baxi duotec 33 the pressure goes above 3bar bur PRV does’nt appear to operate
sounds like you need to replace it. Pressure shouldn’t be going to 3 bar though so you should fix that problem first.
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.
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.
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.
Adrian
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.
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.
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.
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?
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.
Thank you for your quick response and much appreciated help. New PRV it is and go from there ♂️
Hi Steven . We have an oil boiler with an expansion vessel with PRV separate from boiler .
Been losing pressure over the weekend . The expansion vessel was only replaced a month ago . The PRV was not just refitted . Do you think this is the problem ? We live in bungalow and have access to underneath and definitely no leaks from pipe joints or fittings . Kindest Regards
yeah it’s common for the pressure relief valve to need replaced after having expansion vessel problems.