Expansion Vessel Replacement London
Boiler pressure rising when the heating runs? Pressure relief valve dripping? We diagnose and replace expansion vessels across London — Gas Safe registered, same day, fixed price. External and internal boiler vessels.
✅ Normal boiler pressure — what to expect
Signs Your Expansion Vessel Has Failed
These six symptoms all point to the same root cause — a failed expansion vessel diaphragm. Identifying which applies to you helps us confirm the diagnosis faster.
Pressure Rises When Heating Runs
Cold pressure 1–1.5 bar. After 30 min of heating, pressure exceeds 2.5–3 bar. This is the primary sign of expansion vessel failure.
Pressure Relief Valve Dripping
Small pipe exiting through the wall drips or weeps water when heating is on. PRV is doing its job venting overpressure caused by failed vessel.
Needing to Top Up Pressure Often
If you're filling the system regularly (more than once a month), the PRV is venting water every cycle. Fix the vessel to stop the pressure cycle.
Boiler Cuts Out on Overpressure
Modern boilers have an overpressure safety cutout at 3+ bar. If the boiler locks out with a pressure-related error code after running, this is why.
PRV Pipe Hot to the Touch
The discharge pipe from the pressure relief valve should be cold. If it's warm or hot, the valve has been weeping hot water regularly — vessel failure confirmed.
Gurgling / Banging in Pipes When Heating Starts
Pressure surges from a failed vessel can cause water hammer in the pipework as the system heats up and pressure spikes.
Expansion Vessel Types
| Vessel type | Notes | Replacement cost | Expected lifespan |
|---|---|---|---|
| Internal expansion vessel (inside boiler) | Built into the boiler casing — standard on all modern combi boilers. Requires boiler disassembly to replace. Not accessible for re-gassing without specialist tools. | £220–£320 inc. vessel + labour | 8–12 years |
| External expansion vessel (on heating pipework) | Fitted externally on the flow or return pipework. Can be re-gassed with a standard Schrader valve. Easier to replace than internal vessels. | £180–£240 inc. vessel + labour | 10–15 years |
| Potable water expansion vessel (for unvented cylinders) | Used on unvented hot water cylinders to handle thermal expansion of domestic hot water. Different specification from heating vessels — must be potable-water-approved. | £180–£250 inc. vessel + labour | 8–12 years |
Expansion Vessel & Pressure Repair Prices — London 2026
| Job | Typical time | Price |
|---|---|---|
| External expansion vessel re-gas (nitrogen pre-charge) | 30–45 min | £85–£120 |
| External expansion vessel replacement | 45–90 min | £180–£240 inc. vessel |
| Internal expansion vessel replacement (inside boiler) | 60–120 min | £220–£320 inc. vessel |
| Potable water expansion vessel (unvented cylinder) | 45–90 min | £180–£250 inc. vessel |
| Pressure relief valve (PRV) replacement | 30–45 min | £95–£130 inc. valve |
| Pressure gauge replacement | 20–30 min | £75–£100 inc. gauge |
| Full boiler pressure system diagnosis | 30–45 min | £85 (deducted if repair proceeds) |
Gas Safe registered engineers. Fixed quote before starting. No call-out fee. 7 days a week.
Expansion Vessel Repairs Across London
Gas Safe engineers across all 33 London boroughs. Same-day attendance.
Expansion Vessel FAQs
What does an expansion vessel do?
It absorbs the pressure rise when water in a sealed central heating system expands as it heats. Water expands ~4% when heated. Without the vessel, every heat cycle spikes the pressure and triggers the PRV. The vessel contains a rubber diaphragm pre-charged with nitrogen — as pressure rises, the diaphragm compresses and absorbs it.
How do I know if my expansion vessel has failed?
Signs: boiler pressure rising significantly when heating runs (above 2.5–3 bar), pressure relief valve dripping when heating is on, needing to bleed pressure regularly, or boiler cutting out on overpressure. When the vessel diaphragm fails it can no longer absorb expanding water and pressure spikes on every heat cycle.
How much does expansion vessel replacement cost in London?
External vessel replacement costs £180–£240 including the vessel and labour. Internal vessel replacement (inside the boiler) is £220–£320 depending on the boiler model. Re-gassing a vessel with a failed pre-charge (if diaphragm is intact) is £85–£120. Fixed quote before we start.
Why does my boiler pressure keep rising?
Pressure that rises when the heating runs (from 1–1.5 bar cold to above 3 bar when warm) is the classic sign of expansion vessel failure. The vessel can no longer absorb thermal expansion, so all that pressure goes straight to the system. The PRV then vents water to bring it back down — a cycle that continues until the vessel is fixed.
Can I re-gas an expansion vessel instead of replacing it?
If the rubber diaphragm is intact, re-gassing restores function for £85–£120. But if the diaphragm has failed, gas just escapes into the water side and re-gassing won't help. We test the vessel on arrival to determine which applies. If the vessel is over 10 years old, replacement is almost always more cost-effective.
Is it dangerous if my pressure relief valve keeps weeping?
It's a symptom, not a cause — the PRV is doing its job. But it shouldn't be ignored. The underlying cause (failed expansion vessel) needs fixing. If the PRV keeps weeping, scale builds up on the valve seat and eventually prevents it closing fully — you then need both vessel and PRV replaced.
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Boiler Pressure Rising? We'll Fix It Today.
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