Pressure rises too much when the boiler heats up — how to check the expansion vessel

The characteristic symptom

The pressure is normal when the system is cold, but increases significantly as soon as it heats up.

What the expansion vessel does

It absorbs the increase in water volume as it heats up. Inside, it has a membrane and gas under pre-pressure.

What happens if pre-pressure is lost

There is not enough available volume for water expansion. The pressure rises sharply and the safety valve may open.

What happens if the membrane is torn

The vessel fills with water and effectively stops functioning correctly.

Practical indication

A large pressure difference between a cold and hot system is a strong indication that the vessel needs inspection, rather than just "a little air".

Safety

Pre-pressure checking is performed with the vessel isolated and the water side without pressure, in accordance with the manufacturer's procedure.