Variable boiler pressure

We have a baxi combi boiler - not sure of the precise model, anyway the pressure can range from anywhere between 1.5 - over 3.0 - BF has drained the system and it remained around 1.5 for a little while, haven't had the CH running for a few months but when I did test it the system started making a banging noise (pipes) and the pressure seemed to go up instead of down, can have washing machine and dishwasher on and it's fine other days it won't be - any ideas and suggestions would be much appreciated.

Sounds to me as if the expansion vessel has burst or needs re-pressurised.

The Expansion Vessel in the boiler takes up the extra pressure in the system created by the expansion of water as it heats.

The expansion vessel has a diaphragm which seperates compressed air on one side and the system water on the other. If the vessel has burst or if it needs a charge of air then there is no-where for the water to expand to and the pressure guage will rise. The indicator that this has happened is that the safety pipe outside the building will run.

This requires an expert to replace or recharge so if the BF isn't then get someone in who is.

One point to bear in mind is that the filling loop (the bit where you add pressure) may be leaking water into the system. shut off both valves and remove the pipe connecting them. If this cures the problem then you need a new filling loop kit, again get an expert if the BF is unsure.

Yes and actually when you take flexible connection out leave it out and only put it back when you need to top up, it belongs on top of the boiler. It has easy wing connections both sides because it is intended for replacement only when required and removal when not.