Grant Combi boiler, rising pressure and overflow

[Ballysillancfc]

This has probably been covered a million times on this forum but if someone could give me a bit of advice on this one I'd be very grateful.

I have a Grant Combi 90. On the pressure guage there is a wee red arrow pointing to 2 Bar. The pressure falls way down every so often and needs to be topped up from the water supply pipe every month or so using a manual valve.

This has been ok up to now, however a problem has now occurred. The black pressure arrow now rises above the red one when the boiler is turned on and water starts to discharge from the external overflow pipe.

I suspect this is a common problem but can someone tell me what it is likely to be and as that age old question goes...is it likely to be a big job to fix!

 

[cjb1971]

Check the pressure in the expansion vessel,i use a car foot pump,check MI but its usually about half a bar-if water comes out of the valve then it needs replacing-the prv will also most likely need replacing aswell.

 

[sambotc]

Also check the filling loop for let by. If you can disconnect it for a night and see if the pressure still rise's or you see any water coming out when the valve should be closed.

Sam

 

[gas4you]

Does this happen when it's on for h/w or heating or both?

 

[Ballysillancfc]

Does this happen when it's on for h/w or heating or both?

It's ok for h/w, the problem just occurs when the heating is on - does this narrow it down?