What is wrong with the pressure?

Hello

I have a Vokera Linea 24, for the past few months the pressure has been going up to nearly 4 each day.

Have to bleed the radiators morning and night to place back to 1.5.

I turn the water off through the day so the pressure stays the same till I get in.

We have had a part replaced which only cost £20 but that hasn't done anything.

Any help or advice would be much appreciated.

Thanks

Sam