BIASI 24s Combi Pressure variation

[TREVC2005]

When the boiler is cold the pressure shows as zero but when heating is turned on the pressure will rise (without any filling) to around normal operating pressure (although there is some pressure loss over time) as the temperature rises. It's also quite noisy when first turned on from cold. I'm fairly sure it's trapped air but have bled the radiators on a regular basis and find not great air escape.
Is this air trapped in the boiler somewhere and if so how do I get rid of it?

 

[plumbnuts]

The pressure in the sysytem should be around 1 bar when the system is cold. You will need to top up the pressure (See FAQ). The pressure will rise as the heating system warms up but you shouldn't expect it to rise to more than 2.5 bars.

 

[TREVC2005]

Thanks for that.
The bit I can't get away from is the pressure drops right down to zero when cold but will return to around the 1.5 - 2.0 mark when hot.
So is this some sort of functional fault, there's supposed to be some sort of auto air-vent which was replaced recently so is that not working?

 

[Agile]

The exppansion vessel is failed or needs recharging.

See the FAQ on this site for instructions to check.

Tony