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G3 qualified engineer replacing an expansion vessel on an unvented hot water cylinder in London
G3 Qualified — Same-Day Vessel Swaps

Expansion Vessel Replacement London

From £265 fitted, same-day across London. Tundish dripping, T&P discharging, mains cold pressure dropping when the cylinder heats? That is a waterlogged expansion vessel — we carry 12L, 18L and 24L WRAS-approved potable vessels on the van. Building Regulations G3 notification included on every swap.

Diagnosis, drain-down, vessel replacement, pre-charge to manufacturer spec, recommissioning and benchmark logbook update — 75–90 minutes on a standard cylinder. Megaflo bubble-top resets also covered.

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£265 From, fitted
G3 Qualified engineers
90min Typical on-site time
WRAS Approved vessels
2yr Parts warranty
Quick answer

A waterlogged expansion vessel on an unvented hot water cylinder is the single most common cylinder fault in London — it shows up as a dripping tundish, hissing T&P valve or pressure spike during heat-up. Like-for-like replacement is £265 fitted on the same visit, takes 75–90 minutes, and includes Building Regulations Part G3 notification.

What is an expansion vessel and why does it fail?

An expansion vessel is a sealed pressure chamber split internally by a flexible EPDM rubber diaphragm. One side carries a pre-charged volume of air (typically 3.0 bar), the other connects to the cold inlet of the unvented hot water cylinder. As stored water heats from 10°C to 60°C it expands by roughly 4% — on a 210L cylinder that's around 8 litres of extra water that has to go somewhere. The vessel diaphragm absorbs the expansion by compressing the air pocket; when the water cools, the air pushes the water back out.

Two things go wrong over time. Either the air pre-charge slowly leaks out across the Schrader valve and seals — fixable by a clean drain-down and a recharge with a calibrated gauge. Or the diaphragm ruptures and water fills the air side, drowning the diaphragm — at that point the vessel is dead and needs replacing. The diagnostic is the same in both cases: depressurise the cylinder, press the Schrader valve. Air means top-up. Water means replace.

When the vessel can no longer absorb expansion, the only relief path left is the T&P safety valve and the dedicated expansion relief valve on the cold inlet group. Those valves discharge through the tundish into a copper pipe terminating outside. So the customer-visible symptom of a failed vessel is always the same — a dripping tundish or a hissing inlet valve, every time the cylinder heats. The underlying physics is non-negotiable. For full background see our unvented cylinders London guide.

Signs your expansion vessel has failed

The six warning signs below are the order in which a failing vessel typically presents. The earliest sign is mains pressure variation during heat-up — the latest is constant tundish discharge that puddles in the airing cupboard.

Tundish on an unvented hot water cylinder with discharge pipe in a London airing cupboard

  • Tundish dripping when hot water runs

    The single clearest sign. As the cylinder heats, expanding water has nowhere to go and the T&P or expansion relief valve discharges through the tundish to relieve pressure.

  • T&P or expansion valve hissing or weeping

    A continuous trickle from the safety valves on the cylinder body or inlet group, especially during boiler firing or immersion heat-up.

  • Pressure spikes above 6 bar on the inlet gauge

    If a gauge is fitted, working pressure should sit at 3.0–3.5 bar with momentary spikes under heat. Sustained readings above 6 bar mean the vessel cannot absorb expansion.

  • Banging or kettling noises from the cylinder

    Trapped pressure makes the cylinder shell flex audibly as it heats and cools — sometimes confused with a kettling boiler.

  • Mains-fed cold taps lose pressure on heat-up

    When the cylinder back-pressures into the cold inlet, the pressure-reducing valve closes and starves nearby cold outlets.

  • Schrader valve test fails

    Depressurise the cylinder, press the Schrader (car-tyre) valve on the vessel. Water spits out instead of air — diaphragm has ruptured, vessel is waterlogged, replacement is the only fix.

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Expansion vessel types — which one fits your cylinder?

Vessel sizing tracks cylinder volume. Most London 150–180L cylinders run a 12L external vessel. 210–250L cylinders step up to 18L. 300L+ and HMO setups take a 24L. Megaflo Eco GP cylinders use an internal bubble-top instead of an external vessel — different fix, similar cost.

Vessel typeDetailsPrice
External 12L potable vesselThe default on most London 150–180L unvented cylinders. Wall-mounted next to the cylinder via a service tee. Pre-charge 3.0 bar. White potable-rated rubber EPDM diaphragm.£265 fitted
External 18L potable vesselStandard sizing for 210–250L cylinders and dual-bath properties. Same fit-up as 12L with a larger air chamber. Pre-charge 3.0–3.5 bar to manufacturer spec.£295 fitted
External 24L potable vesselLarger 300L+ cylinders, HMOs and homes with secondary circulation. Always wall-bracketed to take the weight when full.£345 fitted
Internal bubble-top (Megaflo Eco GP)Megaflo cylinders use a captive air gap inside the cylinder head instead of an external vessel. Recharged via a drain-down procedure — no replacement part needed.£165 service
Heatrae Sadia internal air-gapSome Heatrae cylinders use a similar captive-air design. Resettable by trained engineers, no external vessel maintenance required.£165 service
Combi boiler heating-side vesselDifferent fault profile — heating circuit vessel inside a combi or system boiler. Same diagnostic, different access. Quoted separately on boiler repair visits.From £185

Expansion vessel replacement cost London — 2026 prices

Fixed prices, given on site after a 10-minute diagnostic. No call-out fee on quoted work, parts on the van for all common London cylinder brands, full G3 notification included.

JobWhat's includedFitted price
External expansion vessel — like-for-like (12L)Diagnostic, isolation, drain-down, replacement 12L potable vessel, pre-charge to 3.0 bar, leak test, recommissioning, benchmark logbook update.£265
External expansion vessel — 18L (larger cylinders)All of the above with an 18L vessel sized for 210L+ cylinders. Includes new flexi tail and isolation valve.£295
External expansion vessel — 24L (300L+ / commercial)Wall-mounted 24L vessel, bracket, isolating servicing valve, copper tail repipe if required.£345
Internal bubble-top / air-gap recharge (Megaflo)Drain-down through tundish, manual reset of internal air gap on Megaflo Eco / GP cylinders, recommissioning. No new parts required.£165
Vessel + PRV combined serviceExpansion vessel replacement plus pressure reducing valve cartridge swap — common pairing on 8-10 year old cylinders.£395
Vessel + T&P valve combined serviceNew external vessel plus temperature & pressure relief valve replacement. Common after a sustained discharge event.£415
Vessel pre-charge top-up onlyVessel diagnosed as serviceable. Drain-down, repressurise to manufacturer spec, leak test, certificate update.£135
Out-of-hours emergency callout (eve/weekend)Same-day attendance outside standard hours, diagnostic, first hour labour. Parts and onward repair quoted on site.From £165

* All prices include VAT and Building Regulations G3 notification. Full price list on the pricing page.

How we replace an expansion vessel

Every vessel swap follows the same five-step sequence. The procedure protects the cylinder warranty, satisfies Building Regulations Part G3, and keeps the benchmark logbook intact for property sale and insurance purposes.

1

Diagnose the fault

10 min

Vessel pre-charge tested with cylinder depressurised. Schrader valve checked for water carryover. PRV outlet pressure measured to rule out a secondary fault before committing to a vessel swap.

2

Isolate and drain the cylinder

15 min

Cold mains isolated at the inlet group. Hot tap opened at the lowest outlet to break the vacuum. Drain hose run from the cylinder drain cock to the nearest gulley.

3

Remove the failed vessel

15 min

Vessel servicing valve closed. Old vessel unscrewed at the flexible tail. Bracket inspected for corrosion and re-fixed if required. Worn isolation valve replaced.

4

Fit and pre-charge the new vessel

20 min

New WRAS-approved potable vessel mounted, fresh PTFE-taped joint, new flexi-tail. Air pre-charge set to manufacturer spec (usually 3.0 bar) with a calibrated gauge before water is admitted.

5

Recommission and certify

20 min

Cylinder re-pressurised, hot taps run to vent, T&P witnessed-lifted clean, working pressure logged at 3.0–3.5 bar, Building Regulations G3 notification updated, benchmark logbook signed off and emailed within 24 hours.

Replacement expansion vessel and Schrader valve on an unvented cylinder cold inlet group

G3 compliance and Benchmark commissioning

Replacing an expansion vessel on an unvented cylinder over 15 litres is a controlled service under Approved Document G3. The work is notifiable to building control and must be carried out by a G3 qualified engineer — BPEC, LCL Awards or City & Guilds 6035, renewed every five years. We self-certify via our competent person scheme and email the Building Regulations Compliance Certificate within 30 days.

The benchmark logbook is updated on every visit — pre-charge pressure recorded, working pressure measured, T&P witnessed lift logged, engineer signature and G3 ticket number. The logbook is the manufacturer's warranty record and the buyer's evidence of compliance on a future sale or remortgage. A vessel swap with no benchmark entry is not a compliant repair.

G3

Approved Document G3 — Hot water supply

Replacing the expansion device on an unvented hot water cylinder over 15 litres is a controlled service under Building Regulations Part G. The work must be carried out by a G3 qualified engineer and notified to building control via a competent person scheme.

BS EN 12897

Unvented hot water storage product standard

Defines the four layers of safety required on every UK unvented cylinder — working thermostat, energy cut-out, expansion device and T&P relief valve. The expansion device is one of the four and must be replaced like-for-like in size and pre-charge.

WRAS

Water Regulations Approval Scheme

Every replacement vessel must carry a WRAS approval for potable water — the diaphragm material must be food-grade EPDM. Heating-circuit vessels are not WRAS approved and must never be used on the hot water side.

Water Regs 1999

Water Supply (Water Fittings) Regulations 1999

Statutory backflow, fluid category and material rules administered by the local water undertaker. A vessel swap that changes the system arrangement is notifiable to the water company.

Real London vessel jobs — worked examples

Five anonymised expansion vessel jobs from the last twelve months. Diagnostic, work done, time on site, final figure.

Victorian conversion, Camden — Megaflo Eco 210L tundish dripping

Customer reported a constant drip from the tundish over a five-day period. Diagnosed waterlogged internal air gap (the Megaflo bubble-top had collapsed after eight years of service). Cylinder isolated, drained, bubble-top reset using the manufacturer procedure, T&P witnessed-tested, system recommissioned to 3.2 bar working pressure. No new parts. Time on site: 75 minutes. Final invoice: £165 inc. VAT.

1990s mid-terrace, Wandsworth — external 12L vessel waterlogged

Schrader valve test confirmed diaphragm rupture — water carried out of the air port instead of air. New 12L WRAS-approved potable vessel fitted, pre-charged to 3.0 bar, flexi-tail and servicing valve renewed at the same time. Tundish dry on full heat-up cycle. Time on site: 90 minutes. Final invoice: £265 inc. VAT.

Top-floor flat, Kensington — vessel plus PRV failure

Tundish discharge plus working pressure recorded at 6.8 bar — both the expansion vessel and the pressure reducing valve had failed. 18L potable vessel and a new PRV cartridge fitted together. Working pressure reset to 3.3 bar, inlet strainer cleaned. Time on site: 2 hours 15 minutes. Final invoice: £395 inc. VAT.

New-build 2-bed flat, Hackney — annual service caught early failure

Annual G3 service flagged vessel pre-charge dropped to 1.4 bar (target 3.0). Diaphragm still intact, no water carryover. Pre-charge topped back to 3.0 bar with a calibrated gauge. Vessel flagged for swap at next service if it drops again. Time on site: 70 minutes including full service. Final invoice: £135 inc. VAT for the top-up alone.

HMO licensed house, Tower Hamlets — repeat vessel failures

Third vessel failure in five years — root cause was an out-of-spec PRV letting mains pressure spike past 6 bar overnight when no draw-off occurred. Replaced vessel, replaced PRV, fitted a non-return check valve and an inline pressure gauge so the landlord can spot drift. Time on site: 3 hours. Final invoice: £465 inc. VAT.

Vessel failed? Tundish dripping?

Same-day callouts across London for waterlogged expansion vessels, hissing T&P valves and pressure-driven cylinder faults. Fixed quote on site, vessel on the van, work completed the same visit on 95% of jobs.

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Areas covered across London

Same-day expansion vessel replacement across every London borough. Heaviest call volumes from Camden, Westminster, Islington, Wandsworth and Kensington & Chelsea.

CamdenWestminsterIslingtonHackneyTower HamletsSouthwarkLambethWandsworthHammersmith & FulhamKensington & ChelseaHaringeyGreenwich

Frequently asked questions

What is an expansion vessel on a hot water cylinder?
An expansion vessel is a sealed chamber containing a flexible diaphragm with a pre-charged volume of air on one side and a water connection on the other. As the unvented cylinder heats up, water expands by roughly 4% of stored volume between 10°C and 60°C. That expansion has to go somewhere — on a vented system it pushes back up into the loft tank, on an unvented system it presses into the vessel diaphragm, compressing the air pocket. Without a working vessel the only relief path is the T&P safety valve, which is why a failed vessel always shows up as a leaking tundish.
How do I know if my expansion vessel needs replacing?
The textbook symptom is a tundish dripping every time the cylinder heats. Other clear signs are the T&P relief valve hissing, mains-fed cold taps losing pressure during heat-up, banging noises from the cylinder body and visible weeping at the expansion relief valve. The definitive test is depressurising the cylinder and pressing the Schrader (car-tyre) valve on the vessel — if water comes out instead of air, the diaphragm has ruptured and the vessel is waterlogged. That vessel must be replaced, not topped up.
Can a waterlogged expansion vessel just be re-pressurised?
Only if the diaphragm is intact and the vessel has simply lost air over time — which can happen on a 5-7 year old vessel that has lived under good conditions. The fix is a drain-down, a top-up with a calibrated foot pump or compressor to the manufacturer pre-charge (commonly 3.0 bar), and a leak test. If the Schrader valve carries water across, the diaphragm has split and no amount of pumping will fix it — only a new vessel will. We always do the Schrader test before quoting either job.
What is the correct pre-charge pressure on an expansion vessel?
Always check the manufacturer label first. As a working rule, the vessel pre-charge should match the working cold-water pressure delivered by the pressure reducing valve — typically 3.0 bar on UK Megaflo, Gledhill, Heatrae Sadia and Kingspan installs, sometimes 3.5 bar on OSO Super S and high-flow setups. Pre-charge is set with the system depressurised — you cannot measure a true pre-charge with water under pressure on the wet side of the diaphragm, the gauge will simply read system pressure.
How much does an expansion vessel replacement cost in London?
A like-for-like external 12L vessel replacement is £265 inc. VAT, fitted on the same visit, with a fresh G3 commissioning entry in the benchmark logbook. An 18L vessel is £295, a 24L is £345. A pure pre-charge top-up where the diaphragm is intact is £135. Megaflo internal bubble-top resets are £165 — no part is fitted but the procedure takes an hour of skilled work and includes a benchmark sign-off. Out-of-hours emergency callouts start at £165 for the first hour.
How long does it take to replace an expansion vessel?
A like-for-like external vessel swap in a London airing cupboard takes 75–90 minutes from arrival to certificate. That covers the diagnostic, isolation, drain-down, vessel removal, new vessel installation, pre-charge with a calibrated gauge, refill, vent, heat-up cycle, T&P witnessed test and benchmark update. Tight access — under a stair, in a bulkhead, behind a kitchen unit — can add 30 minutes. Bubble-top resets on Megaflo cylinders take a similar 75 minutes because the drain-down is the slow part.
Why does my expansion vessel keep failing?
If a vessel fails inside two years there is almost always a secondary fault. The usual culprits are a sticking pressure reducing valve letting mains pressure spike overnight, a closed isolation valve between cylinder and vessel that blocks the diaphragm working, or a vessel sized too small for the cylinder. We always check working pressure, isolation valves and vessel sizing before fitting a replacement — fitting a third vessel on the same setup without finding the root cause is the kind of work that costs landlords money and doesn't fix anything.
Can I replace the expansion vessel myself?
No. Replacement of any safety device on an unvented hot water cylinder over 15 litres is a controlled service under Approved Document G3 of the Building Regulations. The work must be carried out by a G3 qualified engineer (BPEC, LCL Awards or City & Guilds 6035) and notified to building control. DIY work voids the manufacturer warranty on the cylinder, voids the home insurance, and exposes the owner to enforcement under the Building Act 1984. Topping up the air pre-charge with the system fully depressurised is borderline — but if you have to ask, call a G3 engineer.
Do I need a Megaflo vessel for a Megaflo cylinder?
Not necessarily. Most Megaflo Eco GP cylinders use an internal captive air gap (bubble-top) and have no external vessel at all — those are reset, not replaced. Older Megaflo CL cylinders that do use external vessels can be paired with any WRAS-approved potable expansion vessel of the correct size — 12L for 150–180L cylinders, 18L for 210–250L, 24L for 300L. We carry Aquasystem and Reflex vessels on the van; both are direct-fit equivalents and both preserve the cylinder warranty.
Will replacing the vessel restore my hot water pressure?
Sometimes. A waterlogged vessel can cause the inlet pressure reducing valve to close and starve the cold side, which feels like a hot water pressure drop. Replacing the vessel restores the PRV to normal operation. But if the PRV itself has failed — cartridge worn, set point dropping below 2 bar — the vessel swap alone will not fix pressure. We always measure static and dynamic pressure before and after the vessel swap and quote a combined PRV+vessel job (£395) if both need attention.
Is an external vessel better than an internal bubble-top?
Each has trade-offs. External vessels are universal, easy to swap, and a standard maintenance item with a clear replacement path. Internal bubble-tops are tidier, faster on commissioning and have no external parts to leak — but they require a full drain-down to reset and that takes time. Reliability is comparable when both are serviced annually. The choice is usually dictated by the cylinder brand, not by preference — Megaflo Eco GP uses internal, most other UK brands use external.
Do I need to notify Building Control when replacing the vessel?
Yes. Replacing a safety device on an unvented hot water cylinder is a notifiable Building Regulations event. Our engineers self-certify through our competent person scheme — the notification is filed with the local authority within five working days of the job and a Building Regulations Compliance Certificate is emailed and posted to you within 30 days. That certificate is the legal record of the work for property sale, mortgage and insurance purposes.
What warranty do you offer on a new expansion vessel?
Two years on parts (Aquasystem and Reflex potable vessels), twelve months on labour for the replacement and recommissioning. If the vessel fails inside two years from a manufacturing defect we replace it free. If it fails from an unfixed system fault — failed PRV, oversize/undersize selection — we identify the root cause and quote the full repair before fitting anything new. We do not refit on a known faulty system.
Will an expansion vessel replacement need a system flush?
No — the vessel sits on the cold water side of the cylinder and is only exposed to mains potable water. There is no glycol, no inhibitor and no heating system contamination involved. The drain-down volume is typically 30–80 litres from the cylinder body, which goes to drain and is replaced with fresh mains water on refill. The heating-side circuit is untouched. A vessel swap is one of the cleanest cylinder repairs available.
Can you replace the vessel on the same visit as a service?
Yes, and we usually do. An annual G3 service catches around 30% of vessels at the pre-charge stage before they fully fail — we top-up where the diaphragm is intact and replace where it has split. Pricing combines: a full annual service at £155 plus a vessel replacement at £265 becomes a single fixed bundle of £375 (saves £45 against booking separately). Benchmark logbook updated for both events on the same visit.

Question not answered? Call 0207 046 1363 and speak to a G3 engineer directly — no call centre, no triage.

Service area

Service Area — All London Boroughs

G3 qualified vessel engineers with same-day cover across all 32 London boroughs and the City. Average response time inside Zone 2 is 60 minutes for tundish-discharge emergencies.

Tundish dripping? Vessel waterlogged? Call now.

G3 qualified engineers across London. From £265 fitted, vessel on the van, certificate emailed within 24 hours. £5M public liability, 2-year parts warranty.

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London 247 Home Services Ltd · Company No. 17120057 · £5M public liability · G3 qualified · 24/7 emergency line

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