Halstead Ace High, high pressure rise to 2.5bar when heating is on?

Please help, all rads bled okay, hot water demand fine.
But pressure on gauge shows a rise from 0.5 to 2.5bar when heating comes on, and when the themostat swiches off the heating, the pressure drops quickly to 1bar, and then back 0.5bar when cold, No water loss.
New R/V fitted because it was loosing pressure out of overflow originaly. Expansion vessel air checked 1bar okay no water noted when this valve depressed.
Note: I think if I set the manual water heating control setting to max and 25degrees on room thermostat. pressure would rise to 3bar and overflow. checks = Fan,Pump. okay