Baxi boiler pressure drops after heating

[Martin Pearce]

Hi, we have a back combi heater. Whenever the heating comes on it seems to lose pressure within a few hours and I have to refill each morning. The boiler seems to function well, and the pressure only drops with the heating and not the water being on. Ie it started last winter and was then fine all summer when only needed water.

There are no leaks and the radiators are all bled and function well. We have a water based underfloor heating system and I isolated this and the boiler pressure was still falling after heating.

I spoke to an engineer on a call and he suggested an expansion vessel could be fitted as he serviced the boiler in late summer and he said it was like new.

Any suggestions on tests/ checks I could perform to test.

Thanks

 

[oz-plumber]

Isolate and drain boiler.
Pump up expansion tank to required pressure with boiler drain open.
Probably change PRV at same time