Hello, this sound like expansion and low on water pressure, assuming you have a 2 story house you should have a static pressure of around 1 bar, running pressure of around 1.2 bar and antifreeze mode of between 1 bar and 1.2 bar, the vessel should balance and absorb expansion.

To check this you will need to switch of the system and safely electrically isolate, isolate the expansion vessel and drain it, then using a pump with a gauge pump this vessel through the valve usually at the top to 1.5 bar, place this back into position and refill the system to 1 bar. then fire the unit back up.

If the vessel will not hold its pressure its likely that the bladder is broken and this will need replacing. there are some rules in the UK:

2. Servicing Requirements for Expansion Vessels

A. Regular Maintenance Checks

• Annual servicing is recommended, often carried out alongside a boiler service.

• A service should include:

  • Checking pre-charge pressure of the expansion vessel (usually between 0.75 - 1.5 bar).

  • Re-pressurizing if necessary using a foot pump or nitrogen (as per manufacturer’s instructions).

  • Inspecting for leaks, corrosion, or diaphragm failure.

B. Signs of a Failing Expansion Vessel

• Frequent pressure drops in the heating system.

• Water leaking from the pressure relief valve (PRV).

• Boiler pressure gauge rising too high when heating is on.

C. Replacement and Repair

• If an expansion vessel fails (e.g., diaphragm rupture), it must be replaced.

• Some vessels have external Schrader valves for easy re-pressurization, while others may need complete replacement.

3. Who Can Service an Expansion Vessel?

• For sealed heating systems, a qualified heating engineer or plumber can service the vessel.

• For unvented hot water cylinders, only a G3-qualified engineer can work on the expansion vessel.

4. Compliance & Best Practices

• Always follow the manufacturer's servicing guidelines.

• Ensure servicing records are kept, especially for landlords (required under the Landlord and Tenant Act 1985).

• Commercial systems may have additional requirements under The Pressure Systems Safety Regulations (PSSR) 2000.

Expansion is important for the system to run correctly so it may be worth getting a qualified engineer if you are not sure. 

@ashp-bobba thanks ! Ok so it's likely the expansion vessel. It's been replaced twice already. Last time about 2yrs ago. Will get in touch with a local heating engineer (again).

@alsocold123456 I would have thought it would last a bit longer than that. Its worth checking out, it may have just lost some air pressure, this is something your service engineer would have checked on the last service.