Expansion vessel recharging: a routine job often mistaken for a big one

What matters most with expansion vessel recharging

• Vessels are recharged with a system fully drained down, not under pressure
• The correct charge is usually printed on the vessel or the boiler manual
• A diaphragm that has actually split cannot be recharged and needs replacing
• Symptoms include pressure spiking noticeably when the boiler fires
• Internal vessels on combis are recharged the same way as external ones

What is actually going on with expansion vessel recharging

A vessel with the correct air charge keeps system pressure steady as the boiler heats and cools. Once the charge drops, the same job of absorbing expansion falls entirely on water compressibility, which barely exists, so pressure spikes when hot and drops when cold. Recharging means draining the system down, checking the Schrader valve with a tyre-style gauge, and pumping air back to the manufacturer's specified figure, usually matched to system height.

Where we deal with this

• Nailsea (BS48) · 14 mi
• Melksham (SN12) · 12 mi
• Oldfield Park (BA2) · 8 mi

Questions we get asked

How often does a vessel need recharging?

There is no fixed interval, but it is worth checking at every annual service since the air charge fades gradually and invisibly.

How do I know if the diaphragm has split rather than just lost air?

If pressure keeps climbing and water appears at the vessel's air valve, the diaphragm has failed and the vessel needs replacing, not just recharging.