Saunier Duval Combi Boiler Pressure Rises?

Hi

Hopefully someone can answer this problem for me please?

I have a Saunier Duval Thema Condens F30e Boiler which recently had the heat exchanger replaced (intermittent hot water). However, since it was replaced the pressure keeps rising in the system. The water and CH work fine but within about two hours the pressure will rise from 1.5 bar to around 2.9. This cuts out the CH but the tap water will still heat on demand as required.

If the mains water is isolated the pressure in the system remains steady. When the isolator is opened the pressure slowly increases (as described above), whether taps are used or not. The pressure always increases i.e. it doesn't fluctuate up and down, just slowly increases.

The items changed so far:
2 x thermistor sensors and a thermostat overheat (trying to solve the original problem)
Heat Exchanger (was the original problem)
3 way valve
Main Pump

Can anyone sugggest what the problem may be please?

Thank you