Expansion vessel testing and replacement
The expansion vessel absorbs the volume change as system water heats. When its diaphragm fails or its air charge leaks away, pressure spikes on every heating cycle and drops back as the system cools — the classic symptom that sends people topping up needlessly.
Testing before replacing
A vessel is tested cold with the system depressurised, checking the Schrader charge against the required figure. Water emerging from the valve confirms a ruptured diaphragm. Where the charge is simply low, recharging restores it — no part needed. It's a five-minute test that gets skipped surprisingly often.
What replacement involves
- System drain-down to the vessel, or isolation where valves allow
- Correct vessel sizing for system volume, not a like-for-like guess
- External vessel fitted where the internal one can't be accessed
- Recharge, refill, re-inhibit and full pressure test before handover
Frequently asked
- How long do vessels last?
- Commonly eight to fifteen years. Systems with frequent pressure cycling wear them out faster.
- Is this covered under boiler warranty?
- On a boiler within its manufacturer guarantee and serviced annually, usually yes — we'll tell you if a warranty claim is the right route.
- Do unvented cylinders have one too?
- Yes, and it needs annual checking as part of the cylinder service, alongside the expansion relief valve and tundish.
Test first, then decide
We'll always test and recharge before proposing a replacement. If the diaphragm has genuinely gone, you'll get the reading that proves it.
Related solutions
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New Worcester boiler with a 10 year guarantee*
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