Main 30 HE losing pressure after central heating heating been on

[AndyC666]

Good Afternoon folks. Thanks in advance for any advice.

I have a main 30 HE Combi. It loses pressure down to zero after the central heating has been on. If I start the day on 1 bar, it holds fine. Run the hot water/have a shower etc pressure goes up, then returns to 1 bar when turned off and holds.

If I put the central heating on the pressure goes up to just over 2 bar. Will stay for about 30 minutes then start going down. When heating finishes cycle the pressure keeps going down to zero. Fill it up via the loop to 1 bar when cold and it holds perfectly.

Can't find any leaks. Was wondering if anyone had any ideas.

 

[ShaunCorbs]

Have a look outside there should be a copper pipe that’s roughly exiting near the boiler rubber band with a plastic bag over the end

does it fill up with water ?