Firebird Oil boiler, pressure always increases?

Hello Firebird,

We have a Firebird Combi 90 and a few weeks ago we had thermostats put on our radiators and the water pressure in the system was refilled to the correct level of 1-1.5 bar when cold. For a few days everything was fine and then the pressure in the system increased in to the red, to about 3 bar. The safety valve outside is functioning fine and is releasing water, but pressure is staying in the red still.

The radiators have been bled a few times with no or little effect and the pressure keeps rising to the same level.

The plumber that fitted the thermostats has been back a number of times, he drained the system and adjusted the the pressure in the expansion chamber to make sure it was correct (I think thats what he did) and then put the water back in to the correct pressure, but then again the next day the pressure increased to 3 bar.

Yesterday he came out and took two of the radiator thermostats off (that he originally put on, one upstairs and one downstairs), he believes that would fix it, but hasn't.

Does anyone have any ideas on what the problem might be?

Many thanks

JamesJ