Discuss Unvented water heater in the UK Plumbers Forums area at Plumbers Forums

Hi, I have an unvented water heater which for some reason every year causes me issues.

At the end of 2023 I noticed that the pressure valve gauge was going up when the water tank was switched on. My local plumber came and re-pressured the expansion vessel as it was flat, afterwards we tested it and even with the heater switched on the pressure stayed at 3 bar, after about 20 minutes it and gone up to 3.5 bar and didn’t drop until I ran a tap, I called the plumber about this and they said it was normal.

This evening I noticed the pressure gauge was going up even though the heater hadn’t been switched on and hadn’t been for about 12 hours.

I’ve turned the water off and used the PRV to release the water, only a little came out.

I’m guessing that there is an issue with the expansion vessel and I need a new one? But just thought I’d ask for some advice as the heater hadn’t been switched on for hours.

The PRV’s and the intake valve on the gauge have all been replaced within the past three years so I’m guessing it’s not those.

Any advice welcome.
Without being there to test the cylinder components, I would suggest replacing the combination valve (which must be done by a qualified plumber).

Inside the valve is a pressure regulator that is set to 3 bar. If that is starting to fail the pressure in your cylinder will go to whatever your mains pressure is.

Hope this helps.
It seems like your diagnosis could be correct...

If the pressure in your unvented water heater keeps rising despite recent service on the expansion vessel, and considering the PRV (Pressure Relief Valve) and other components have been replaced recently, the expansion vessel might indeed be failing to absorb the pressure increases as intended. It's also possible there's an issue with the pressure reducing valve or a check valve causing backflow, contributing to pressure buildup.

Before replacing the expansion vessel, it might be worth checking these components and ensuring there's no thermal expansion from another source affecting the system. Consulting with your plumber to thoroughly assess these elements could help pinpoint the exact issue and avoid unnecessary replacements.

Good luck!
However it could also be a number of other things.
Needs someone on site to check it over, and they should be G3 registered in case it ends up being to do with the cylinder etc - which is not necessarily the case by the way!
Hi all, thanks for the replies. Just waiting for my plumber to come back with an appointment date.

I’m pretty sure it’s the expansion vessel as last night when the water was switched off at the mains the pressure still went up.

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