Fluctuating Pressure on Boiler

[stevepurley]

I have a potterton combi 100.

Had problems with heating and have bled the rads to get air out but now when the heating comes on the pressure goes up to 3.5 bar but when I turn the heating off the pressure falls back down to almost zero.

Hot water works fine.

Any suggestions please would be welcome.

Thanks

Steve

 

[ChrisR]

DO a search on pressure vessel - yours needs recharging, and you'll need a new p relief valve as yours has been letting the water out