Expansion vessel replacement in Macclesfield
In short
An expansion vessel absorbs the expansion of water as it heats. When its air charge is lost, pressure spikes on every heat-up and escapes through a relief valve — which looks exactly like a leak and is not one. Testing and recharging takes minutes; replacement is a short job.
Also called: expansion vessel recharge · pressure vessel failure · water discharging outside · boiler pressure spiking
Signs you need this
- Boiler pressure spiking when hot, low when cold
- Water discharging outside or at the tundish when the cylinder heats
- Needing to top the system up repeatedly with no visible leak
- A pressure relief valve that has started to weep
- Hammering or banging as the system heats
There are usually two of them in a modern house and they do the same job in different places. One inside the boiler, serving the sealed heating circuit. One next to an unvented cylinder, serving the stored hot water. Both are steel vessels divided by a rubber diaphragm, with pressurised air on one side and system water on the other.
The failure is gradual and invisible. Air migrates slowly through the diaphragm, so the charge falls over years until there is no cushion left. Once that happens, water heating up has nowhere to expand into, pressure rises sharply, and the relief valve does its job by dumping water. On a heating system you see the gauge spike hot and read low cold; on an unvented cylinder you see water at the tundish or dripping outside every time it reheats.
It is routinely misdiagnosed as a leak, and people spend money lifting floors looking for water that was never escaping into the building. Testing the charge with a tyre gauge on the schrader valve, with the system depressurised, takes five minutes and should be near the front of any diagnosis of pressure loss.
Recharging works when the diaphragm is intact. Where the diaphragm has perforated — the tell-tale is water coming out of the schrader valve when the charge is checked — the vessel is finished and needs replacing. On a boiler, that sometimes means fitting an external vessel alongside rather than dismantling the appliance, which is a perfectly good solution and often cheaper.
Vessels are consumables with a service life of several years, and checking the charge is part of any proper boiler or cylinder service. A system that has needed topping up 'for years' very often has a vessel that failed early on and was never checked.
What the job covers
- Charge testing on heating and hot water vessels
- Recharging to the correct pressure for the system
- Vessel replacement, including external vessels where internal ones cannot be renewed
- Pressure relief valve testing and replacement where it has been passing
- Refilling, venting and rechecking
- Root cause explanation so it is not misdiagnosed again
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your plumber has to honour it. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
How do I know if the vessel has failed rather than something leaking?
The pattern gives it away: pressure high when hot, low when cold, with no water anywhere in the house. Depressurise the system and test the schrader valve — if water comes out instead of air, the diaphragm has gone.
Can a vessel be recharged rather than replaced?
Yes, provided the diaphragm is intact. It is a quick job and is part of a proper service. Only a perforated diaphragm requires a new vessel.
Need this doing in Macclesfield?
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