Well, on my own combi-fed central heating system, I have to keep checking the pressure occasionally by looking at a dial on the front of the combi. Every time the pressure drops below the recommended minimum, you have to manually top up the pressure using a removeable link to the mains water supply. What I'm thinking of is a device that will keep doing that for you whenever the pressure drops (say due to small leakage at pipe joins or whatever). My own system often needs its pressure topping up manually.
Frank