System pressure range?Conflicting advice!

Get the engineer back if it's still under warranty, the expansion vessel diaphragm inside the vessel if faulty or split causing the boiler to over pressurise, the loss of water is being discharged outside the property from the pressure relief valve, ( which you might not notice) that's why you have a loss of pressure.....he'll need to replace the expansion vessel.....hope this helps

still under warranty, keep calling them back to fix it, 1.5 rising to 2.5 is ok,water expands as it is heated,the amount of expansion varies system to system depending on volume of water in system,faulty expansion vessel can lead to loss of pressure.

Expansion vessals usually come in 8 or 10 litre expansion capacity, so ok if yo have 7 large double panel type rads then to total volume of water in system is more than the 8 litre capacity usually fitted in a 24kw boiler , so keep pressure closer to 1 bar when setting (also heating to be cold) this will help expansion volume as less water in system , but I have a feeling you have a small leak as if pressure drops 0.5 bar weekly , if expansion vessal incorrectly charged or faulty the pressure gauge will rise rapidly when heating turned on , thus usually rising above the 3bar predetermined setting of the PRV and pressure loss is sudden and will be visible by observing blow off pipe external to boiler , if pipework not visible due to solid floors etc , try a good leak sealer as a starting point in your investigations to cure problem.

There are circumstances where the pressure can stray over the 2bar mark, if your boiler is on the ground floor and your property is three or more floors for instance. I would advise running the heating at highest setting on the boiler until the maximum temperature has been reached (this should be displayed on the boiler front) and then checking the copper discharge pipe at the rear of the boiler on the outside wall for any drips or evidence of wetness in the open end of the pipe. Generally any noise within the boiler in this case would be more likely to be air from the recent drain down and refilling rather than a expansion vessel issue, these are generally silent problems until the pressure reaches 3bar and the excessive pressure is vented through the aforementioned copper pipe, at which point the noise would only be a short burst rather than a constant sound while running.