Combi Boiler Pressure Rising

[cwcw]

I have an oldish combi boiler, non-condensing, Alpha CB 24x. Recently the pressure has been rising and the PRV leaking outside. Even with heating not in use it still rises to the max. Sometimes it drops back to midway and sometimes it remains at max pressure, without intervention either way.

The first time I noticed the problem I bled a radiator, disconnected the filling loop (left it disconnected) and the pressure was down to 1, but within a day it was back up again. Heating and hot water both otherwise working fine. Any ideas before we get the plumber round? Thanks.

 

[45yearsagasman]

Abnormal pressure readings on combis are usually down to a lack of expansion vessel charge or blocked communication pipe, or possibly a plate heat exchanger passing internally. Easy to diagnose. Read FAQ's on Exp vessels.

 

[nortonj]

I would be firstly checking the expansion vessel, if it rises when heating on. If not as the filling loop is disconnected i would look at the plate heat exchanger passing.

 

[nortonj]

^^^ beat me to it.

 

[cwcw]

Just to add that, bizarrely, after the heating has been on the last hour and now cooled down again, pressure has dropped to 1.5. It will now rise to 3 slowly even though heating not in use. Can't understand why the pressure falls after heating on, although the PRV gushes like a tap!

 

[nortonj]

Turn the cold water main off if the pressure stops rising then you know its the plate passing.