Plumbing Questions? Ask a Plumber for Answers ASAP
Hello and welcome to JustAnswer! This is James, I'm a Plumbing Technician with over 10 years of experience, and I'll be assisting you today
Sorry to hear your appliance is having issues, please stand by while I find the answer for you, this is not an immediate service however I will try and respond as soon as possible.I assume this is a red expansion vessel for your central heating which is showing 3 bar, and after the boiler being switched off for half hour or so has the pressure started to reduce again or remian high?
In reality the boiler will have a high pressure cut out if it exceed pressure beyond tolerance, and there is also a pressure release valve which will open to vent water. The cause is most likely the expansion vessel needs re-pressurising (if working fine previously) or undersized for the property (and need replacing if you a new to the property).
Its nothing which should give you concerns to use the boiler tonight but would get a heating engineer to come out and service the boiler (if not done recently) or at least test the expansion vessel.
It was a pleasure to assist you today. I hope the above answers the question. If you have any follow up questions then please let me know.
Correct the unit will release the pressure if it does exceed 4, but it is something which needs looked into.