Boiler pressure rising to 4 when heating is on, PRV not kicking in, plumber thinks the pressure gauge is faulty, thoughts?

donnieberry · 20 Oct 2025

I have a boiler that is 12 years old apparently, a worcester boiler.

When the heating is off, it goes down to around 1 pressure, when the heating goes on, it rises to 4.

The plumber checked the expansion vessel and said it's fine as it sounds tinny which means it isn’t full of water, the PRV isn't releasing water when it hits 4 pressure, so he said the pressure gauge must be faulty.

What does everybody think?

I am a new home owner and I am absolutely terrified I need to replace the boiler. Thoughts?

terryplumb · 20 Oct 2025

donnieberry said:
The plumber checked the expansion vessel and said it's fine as it sounds tinny which means it isn’t full of water

Further investigation required to determine if the EV is actually functioning as it should.
What tests were carried out on the PRV ,if any ?

donnieberry · 20 Oct 2025

Basically, the plumber dropped the pressure to 0, refilled it, turned the heating on and watched it rise back up to 4.

I was told to go stand outside and keep an eye on the copper pipe that the water should discharge from, nothing happened the entire time except for 1 time, the pipe shook for maybe a second or two, thats it.

Apart from that, I am not sure what else he done to check it.

terryplumb · 20 Oct 2025

Did the plumber at any time remove the front cover from the boiler ?

donnieberry · 20 Oct 2025

I don't think so! As hes not a gas safe engineer, there was only so much he could do.

terryplumb · 20 Oct 2025

Get a proper GSR engineer to investigate , the plumber is out of his depth.

donnieberry · 20 Oct 2025

I've got one coming tomorrow, I'm just terrified I need to fork out money for a new boiler as I seriously cant afford that right now

terryplumb · 20 Oct 2025

Very unlikely that a replacement boiler is needed , replacement PRV is very likely ,the EV may just need recharging or at worst replacing.

donnieberry · 20 Oct 2025

Perfect thank you! I've actually just noticed the boiler has dropped from 1 pressure down to 0.8, its very up and down, no clue whats going on!

Also, about the EV, what would explain the PRV not working right? even when it was at 4 pressure, no water was coming out

Mister Banks · 20 Oct 2025

It’s likely to be either the gauge, the PRV or maybe both that is faulty but a competent gas safe engineer will easily fix it.

terryplumb · 20 Oct 2025

Does the boiler actually work for central heating and domestic hot water,or is it going to lockout ?

donnieberry · 20 Oct 2025

Mister Banks said:
It’s likely to be either the gauge, the PRV or maybe both that is faulty but a competent gas safe engineer will easily fix it.

Thank you!

To be honest, the only reason I noticed this is because I bled a radiator the other day and had to raise the pressure, so I was checking rhe boiler constantly, thats when it was at 4, so it could have been like this for a long time.

About the gauge, is it a somewhat common thing? the gauge failing?

Harry Bloomfield · 20 Oct 2025

donnieberry said:
About the gauge, is it a somewhat common thing? the gauge failing?

It's a simple job, to temporarily attach a gauge to the pipework and check it, against yours. If your is faulty a permanent one could be easily added. Even if the gauge is wrong, it does seem to be indicating a rather wide swing in pressure, which, if correct - suggests the EV is not doing its job.

donnieberry · 20 Oct 2025

terryplumb said:
Does the boiler actually work for central heating and domestic hot water,or is it going to lockout ?

works completely fine!

Leftum · 20 Oct 2025

Just fit a filling loop with a pressure gauge on- if that is possible, that way you can rule out the boilers own gauge.

Tesla Remote Filling Loop With Gauge

Use this remote filling loop to fill and pressurise central heating systems or heat interface units. The integrated gauge allows you to measure the pressure in the system, which is particularly useful when it's installed away from the boiler or cylinder.This Part L-compliant filling loop...

www.toolstation.com

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Boiler pressure rising to 4 when heating is on, PRV not kicking in, plumber thinks the pressure gauge is faulty, thoughts?

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