Expansion Vessel Failure
Pressure that swings dramatically between cold and hot, often with a relief valve dripping outside..
Pressure that swings dramatically between cold and hot, often with a relief valve dripping outside.
What is actually going on
Pressure that swings dramatically between cold and hot, often with a relief valve dripping outside.
The realistic causes, in order
A perished rubber diaphragm allowing water into the air side of the vessel Air charge lost slowly through the Schrader valve over years of service A vessel sized for the original system that is now too small after radiators were added
What we test on site
Pre-charge measured with a gauge at the Schrader valve with zero system pressure Whether water emerges from the Schrader valve, which proves the diaphragm has failed Total system water volume against vessel capacity to confirm the sizing is right
Safe checks to make with a failed expansion vessel
- Record the pressure cold and again at full temperature — a swing over about 1 bar is the tell
- Note whether the outside discharge pipe has been dripping
- An external vessel is often the practical fix when the internal one is buried in a tight boiler case; either way it must be sized to the system volume, not guessed.
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Get a failed expansion vessel diagnosed properly
Recharge if the diaphragm is sound, replace if not. Either way the relief valve should be checked at the same visit.
Questions we get asked
What causes a failed expansion vessel on a gas boiler?
A perished rubber diaphragm allowing water into the air side of the vessel
What normally puts a failed expansion vessel right?
Recharge if the diaphragm is sound, replace if not. Either way the relief valve should be checked at the same visit.
Is it safe to keep using the system with a failed expansion vessel?
An external vessel is often the practical fix when the internal one is buried in a tight boiler case; either way it must be sized to the system volume, not guessed.