Worcester Greenstar 12i/24i boiler. Loss of pressure. No obvious leaks under the radiators. Pressure holds up on hot water but if CH has been on pressure drops back to zero. Builds up again when switched on with heating so pulling water from roof tank??
There is some water (dripping) from pressure release valve when boiler goes off. not a massive amount
The dripping from the pressure relief valve will be the problem.
Are you able to see what pressure it will hold when you refill it?
Ok, the PRV seems to be doing its job, but there is a problem with the expansion vessel
Do you have a tyre pressure gauge?
Yes we do have a gauge
but how do we check the vessel with it please??
If you empty the system, then look at the top of the boiler behind the frame, you should see the pressure gauge and the shraeder valve to check it
Do you mean we need to empty system completely?