Fluctuating central heating pressure on Combi Boiler

[MarkJamesH]

Hi
For the last few months we have had a problem with our boiler,
The central heating pressure gauge when set to 1bar can lose pressure to approx .8 but then go back up to one. Or other times continue to drop. I have checked everywhere for a sign of a leak but have been able to find nothing yet. We left the boiler off for a week and no pressure was lost. I can only presume the pressure fluctuation are caused by the air temp/weather. And the loss of pressure is a leak when its hot and expands or some valve not working correctly. I will end up having to call an expert to sort it but would prefer to have some idea of whats causing it.
Thanks

Mark

 

[Burnerman]

You need to test the air pressure inside the expansion vessel......with the system at zero pressure indicated on the gauge, check the air pressure inside the EV and set it to 10 psi as a rough figure. Use an ordinary car tyre pump for this.
If water escapes from the tyre type valve on the EV then the diaphragm within is split.
Pressurise the boiler back up to 1 bar when this is done.
John

 

[MarkJamesH]

Hi
Thanks for your quick reply.
EV - Escape Valve? Will it have its own pressure gauge on it? I haven't even take the outer housing of the boiler off I presume i will need to, to do this? I have all the docs/boiler diagrams from when it was installed.

Mark

 

[kevplumb]

expansion vessel View media item 24769
usually at the back between boiler and wall

 

[bengasman]

0.2 variation between hot and cold is perfectly ok.

 

[mysteryman]

The water in the system expands when it heats up, and the pressure rises.

This is perfectly normal, as long as there is no steady loss of pressure. Always check it with the system cold.