How often should an expansion vessel be checked?

Your manufacturer's instructions and your risk assessment set the interval. Many sites have them checked every year alongside other engineer tasks.

Can I recharge an expansion vessel myself?

Not on an unvented hot water system. That work must be done by a qualified person.

Is water in the tundish dangerous?

It is a sign that something needs attention. The safety valves are releasing pressure as designed. Get an engineer to find the cause.

Expansion Vessel Problems: The Warning Signs

By the Germ3 Water team. Updated 20 October 2026.

What an expansion vessel does

Water expands when it is heated. On an unvented hot water system that extra water has to go somewhere. The expansion vessel gives it space. Inside is a rubber membrane with air on one side and water on the other.

Sealed heating systems have their own vessel too. This guide is mainly about vessels on the water you drink and wash in.

Why it is a legionella risk

Many vessels only have one connection. Water goes in and out of the same pipe and the water inside barely moves. It sits warm and still which is what legionella likes.

HSE guidance in HSG274 Part 2 recommends vessels designed so water flows through them. Where that is not possible the vessel should be flushed as part of your scheme.

The warning signs

Water dripping or running into the tundish below the cylinder.
Hot water pressure that rises and falls when taps are used.
Banging or knocking in the pipework.
Rust or corrosion on the vessel or its connections.
A vessel with no way of draining or flushing it.
No record of the vessel ever being checked.

Water in the tundish is the most common sign. It usually means the vessel has lost its air charge or the membrane has failed. The safety valves are doing their job by releasing the pressure.

What not to do

Never cap or block a tundish or discharge pipe. Never adjust or isolate the safety valves. They stop the cylinder from building up dangerous pressure.

Why this is engineer work

Checking a vessel properly means isolating it and draining the water side. The engineer then reads and resets the air charge. On an unvented hot water system the law requires that work to be done by a qualified person.

“If the task needs the system drained or a chemical or a tool other than a thermometer, it is engineer work.”
Compliance Buyer, Do I need a legionella risk assessment?

What an engineer will do

Isolate and drain the vessel safely.
Check and reset the air charge to the right pressure.
Replace the vessel or membrane if it has failed.
Fit a flow through vessel or a flushing valve set where needed.
Flush the vessel and record the work in your logbook.

What you can do

Look at the tundish when you do your monthly checks. Note any water in it. Make sure the vessel is on your schematic and your risk assessment. Then book a qualified engineer if you see any of the warning signs.

Expansion vessel problems: the warning signs