Vaillant eco tech plus pressure rising

[Timlouth]

Hi,
I have a Vaillant eco tech plus combi boiler.
It works okay for hot water but I am having problems with heating.
When the heating is on the system pressure starts to rise slowly. I have it set midway in the grey zone on the gauge but with the heating on it slowly creeps up through the red zone and just keeps rising. I bleed the excess pressure off through one of the radiators. It seems to have happened quite quickly so I don't think both valves in the filling loop are passing.
Is there any testing I can do to work out what the problem is.
The diverter valve was leaking and changed around a year ago.

 

[terryplumb]

If the pressure only increases when central heating is on ,stays constant when boiler is not in use ,then the expansion vessel would be the first suspect . Any evidence of water coming from the prv pipework outside ,or have you been keeping a close eye on your pressure and releasing it via radiator bleed before it gets to around 3 bar ? ( upper end of red section)

 

[Timlouth]

Thank you Terry. Only turned the heating system on last night for a test run so only noticed it last night. I have left it off until I get home from work then I will try it again to seen if there are any other clues. Surely a pressure vessel defect would cause a pressure drop?

 

[terryplumb]

What pressure vessel ? If you mean expansion vessel failure ,that results in your system pressure gauge showing increasing pressure. As your central heating water expands when heated, thus increases pressure in system ,the ev accomodates this expansion ,or doesnt if its not charged ( with air pressure ) properly or is defective.

 

[Timlouth]

Thanks Terry. Does the ev have a Schraeder valve so I can check the air pressure?

 

[terryplumb]

Yes ,bit like a car tyre.

 

[Timlouth]

Thank you. I will check it this evening. Do you know what the pressure is supposed to be?

 

[terryplumb]

Around 1 bar ,heating off and cold .

 

[Timlouth]

Thank you Terry, I will check it tonight.