Combi boiler - v low pressure when heating off

turtle99 · 25 September 2018 at 11:58AM

Hi

Our Worcester combi boiler (Greenstar 25i) is behaving a bit weirdly. Pressure is about 0.5 bar when the heating is off, and then rises rapidly when the heating is on, up to about 2.5 bar (possibly higher sometimes).

For the initial low pressure, we tried repressurising a few times up to 1.5 bar, but it soon returns to about 0.5 bar. No leaks are apparent.

What I don't understand is the mix of too low when off, then too high when on...

Thanks in advance for any advice as to what is going on , before we make the call to the engineer...

nofoollikeold · 25 September 2018 at 12:09PM

Most likely:
1. The internal expansion vessel has either lost its air charge or has failed.
2. Boiler fires, water heats and expands. Nowhere to go, so opens pressure relief valve and dumps excess. Water cools, pressure falls.
3. Call a heating engineer.

turtle99 · 25 September 2018 at 12:12PM

Thanks for the quick reply, that's extremely useful

Brummie85 · 25 September 2018 at 12:20PM

This sounds like a burst expansion vessel membrane or faulty pressure relief valve (PRV).

Depending on how confident you are, you could take the cover off and see if you can get access to the expansion vessel. There will be a test/inflate schrader point that if you push briefly, air should come out. If any water comes out as well, that's an indicator that the expansion vessel has failed. If no water comes out then it might be okay, but you would need to recharge it. The manual will contain the manufactures spec for this.

If the PRV has stuck open, that's a bit more difficult.

This guy has a good video on it all

https://www.youtube.com/watch?v=4meZgMAUecw