Expansion Vessel Pressure for Hot Water Cylinder: Setting Guide

The expansion vessel is the single most-failed component on a London unvented hot water cylinder, and the one most often misdiagnosed by general plumbers. A dripping tundish, a hammering hot tap, a T&P valve that discharges on every heating cycle — nine times out of ten the vessel pre-charge has dropped or the rubber diaphragm has perforated. Set it right and the cylinder runs silent for a decade. Get it wrong and the safety relief group works overtime to mask the fault, until eventually the diaphragm gives up and the whole assembly waterlogs.

This guide is the engineer's version of what most product manuals only summarise: the correct pre-charge pressure (3.5 bar on virtually every modern UK cylinder), how to read it properly with a calibrated low-pressure gauge, how to top up an in-line vessel, how to spot a perforated diaphragm in under sixty seconds, and when the job leaves DIY territory and becomes G3-only work under Approved Document G3. It applies to every common London brand — Megaflo, Telford Tempest, Joule Cyclone, Range Tribune HE, Gledhill StainlessLite, OSO Delta and the Mixergy X smart cylinder.

What the Expansion Vessel Actually Does

Water expands by roughly 4% by volume as it rises from a 10°C cold mains inlet to a 65°C cylinder setpoint. On a vented cylinder that expansion vents harmlessly back up the open vent pipe to the loft cistern. On a sealed unvented cylinder there is no open vent — the system is pressurised against the mains — and the expanded volume has to be absorbed somewhere or the pressure inside the cylinder climbs until the T&P valve dumps it to drain.

The expansion vessel is a steel pressure vessel with a flexible rubber diaphragm dividing it into two chambers. One chamber is pre-charged with dry nitrogen or air to a known pressure (3.5 bar is the modern UK default). The other chamber is open to the cylinder hot-water circuit. When the water heats and expands, it pushes against the diaphragm, compresses the gas chamber, and stores the expansion energy until the next hot-tap draw-off releases it. No drip. No hammer. No T&P discharge.

The component is one element of the safety relief group required by Approved Document G3 on every unvented cylinder over 15 litres. The other elements are the cold mains pressure-reducing valve (PRV, typically 3.0–3.5 bar), the temperature-and-pressure-relief valve (T&P, typically 7 bar / 90°C), the single-check valve and the line strainer. All five are mounted as a pre-assembled safety set on the cold inlet, and all five are G3 work to replace or rebalance.

The Correct Pre-charge Pressure

The rule is simple and the rule has one exception:

• Pre-charge must match the cold mains PRV setting within ±0.2 bar.
• On virtually every modern UK domestic unvented cylinder the PRV is set to 3.0–3.5 bar, and the vessel ships from the factory at 3.5 bar.
• The exception is older or low-rise installs where the PRV has been wound down to 2.0 or 2.5 bar to protect legacy distribution pipework — in which case the vessel pre-charge must be reduced to match.

Read the manufacturer plate on the vessel itself, not the cylinder data badge — they sometimes disagree. The plate states the factory pre-charge and the maximum working pressure (commonly 6 bar). Brand defaults:

• Heatrae Sadia Megaflo Eco — internal 18L vessel, factory pre-charge 3.5 bar, PRV setpoint 3.5 bar.
• Telford Tempest / Hurricane — internal 18L–24L vessel by capacity, factory pre-charge 3.5 bar, PRV 3.5 bar.
• Joule Cyclone — internal vessel sized to capacity, factory pre-charge 3.5 bar, PRV 3.0 bar (typical).
• Range Tribune HE — external in-line vessel on smaller models, internal on larger, 3.5 bar pre-charge.
• Gledhill StainlessLite Plus — external in-line vessel as standard, 3.5 bar pre-charge.
• Mixergy X — external vessel mounted alongside the smart cylinder, 3.5 bar pre-charge.

Sizing is the second half of the rule. BS 6700 and the Approved Document G3 sizing tables both put vessel volume at 10% of cylinder capacity for indirect systems and 15% for direct (immersion-only) systems, based on a 10–65°C temperature rise. For a typical 180L indirect that is an 18L vessel; for a 250L indirect, 24L; for a 150L direct, 18L. Brand-supplied replacements match these sizes — only deviate if the manufacturer's own sizing chart says otherwise.

How to Check the Pre-charge Pressure

This is the procedure every G3 engineer runs at the annual BS 7593 / Benchmark service. It takes about ten minutes on an external vessel and twenty on an internal one. The critical point is that the vessel must be drained on the water side before the gauge reads — a vessel under system pressure will read falsely high.

1. Isolate the cold mains feed to the cylinder at the inlet stopcock or the isolation valve on the safety set.
2. Open the lowest hot tap in the property (usually the kitchen sink or downstairs bathroom basin) and leave it running until water stops flowing — that drains the cylinder side of the vessel diaphragm.
3. Remove the Schrader-valve cap on top of the vessel.
4. Fit a calibrated low-pressure gauge (0–10 bar range, 0.1 bar resolution).
5. Read. The needle should sit within 0.2 bar of the manufacturer plate value — typically 3.3–3.7 bar.
6. If water emerges from the Schrader pin as you depress it, the diaphragm has failed — go to the replacement section.
7. If the reading is correct, remove the gauge, replace the cap, close the lowest tap, restore the cold supply slowly, and bleed any trapped air through a hot tap until flow is steady.

The full annual service includes a T&P valve manual lift-test, a PRV pressure check, a discharge-pipe flow test through the tundish and an immersion-thermostat verification. Our unvented cylinder service in London covers all five points on every visit, with the Benchmark logbook updated and a fresh G3 service record left with the property file.

How to Top Up a Low Vessel

If the pre-charge has fallen but the diaphragm is intact (no water at the Schrader), top-up is straightforward on an external in-line vessel:

1. Repeat the isolation, drain and gauge-fit steps from the previous section.
2. Connect a foot pump or compressor to the Schrader valve via a tyre-style adapter.
3. Pump slowly. Read the gauge every few strokes — do not exceed the manufacturer plate value.
4. Stop at the plate value (typically 3.5 bar). Disconnect, recap.
5. Restore the cold supply and bleed through a hot tap.
6. Check the cylinder pressure gauge over the next full heating cycle. If it climbs above 5 bar or the tundish drips, the diaphragm is failing on a slow leak — book a replacement.

On an internal vessel — fitted inside a pre-plumbed Megaflo, Tempest or Cyclone cylinder case — top-up requires removing the cylinder cover to access the Schrader. Under G3 that cover-removal is engineer-only work because the safety set is exposed once it comes off; an owner-occupier doing it themselves voids the cylinder warranty and gives the insurer grounds to refuse any subsequent water-damage claim. For internal-vessel cylinders, call a G3 unvented cylinder engineer.

Diaphragm Failure — The Signs

The rubber diaphragm separating the gas and water chambers is the wear part. Over 7–12 years of expansion-and-contraction cycles in London water, it perforates. Once it has, the vessel is waterlogged — the water side fills completely, there is nowhere for the heating expansion to go, and the safety relief group has to dump the expansion to drain on every heat cycle.

The five symptoms, in rough order of how often we see them on first attendance:

• Tundish drip on every heating cycle. A clear plastic tundish on the discharge pipe runs with a steady drip or trickle whenever the cylinder reheats. This is the most common presenting symptom and the one homeowners notice first because the drip leaves a stain on the floor.
• T&P valve discharge. The temperature-and-pressure relief valve discharges audibly into the tundish each time the cylinder reaches setpoint. Often mistaken for a faulty T&P; in fact the T&P is doing its job because the vessel is not doing its job.
• Hammer or pressure surge on hot taps. Open a hot tap and the first half-second of flow comes through hard, with audible knock in the pipework, before pressure normalises.
• Water at the Schrader valve. Depress the pin to check pre-charge and water emerges instead of air. Diagnostic on its own — vessel is finished.
• Cylinder gauge climbing. The pressure gauge on the safety set climbs above 5 bar on heating, sometimes hitting the 6 bar maximum before the T&P discharges.

None of these can be cured by re-pressurising. A perforated diaphragm cannot hold gas charge — pump it up to 3.5 bar and within hours the next heat cycle drives water through the perforation again. The only fix is replacement.

Adjacent failure modes worth ruling out before you condemn the vessel: a stuck-open T&P valve discharging continuously regardless of vessel state (replace the T&P — see our T&P valve replacement service); a failed immersion thermostat boiling the cylinder past 90°C and tripping the T&P thermal element (our cylinder thermostat replacement service covers both wet-side and electric); and a corroded immersion heater leaking through the boss seal (handled under our immersion heater replacement).

Replacing the Expansion Vessel

Replacement is a G3-engineer job under Approved Document G3 on any unvented cylinder over 15 litres. Typical London London-area job time is 90 minutes for an external vessel, 2.5–3 hours for an internal one. The fitted price from Emergency Repairs London starts at £265 for an external swap and £345 for an internal one, both including:

• New WRAS-approved expansion vessel sized to BS 6700 (12L, 18L or 24L typical).
• Pre-charge set on-site to match the PRV reading within 0.2 bar.
• T&P valve manual lift-test and PRV pressure verification — the rest of the safety set checked while access is open.
• Tundish and discharge pipe inspected for blockage and correct fall.
• Benchmark logbook updated and a fresh G3 service record left for the property file.
• 12-month parts and labour warranty on the new vessel and the workmanship.

The fitted cost compares against around £180–£220 for a non-G3 swap that voids the warranty and the buildings-insurance position — a saving that disappears the first time the insurer asks for a compliance certificate. The full expansion vessel replacement service for London covers all 32 boroughs with a 2-hour Central London SLA.

If the cylinder itself is also over 10 years old, has a slow weep at the boss seal, or is throwing repeat faults across several components, the economic case usually tips toward whole-cylinder replacement rather than vessel-only swap. Our hot water cylinder replacement service starts from £1,495 fitted for a 120L and £1,745 fitted for a 180L, including the new safety set, G3 commissioning and Building Control notification.

Internal vs External Vessels by Brand

Where the vessel sits matters because it changes who can service it and how. The trade-off is access vs aesthetics — internal vessels look neat behind a single white case, external vessels look like a tank on the wall but anyone with a gauge can service them.

• Internal (inside the cylinder case) — Heatrae Sadia Megaflo Eco, Telford Tempest / Hurricane, Joule Cyclone, Worcester Greenstore. Pros: single neat unit, no extra wall fittings, smaller airing-cupboard footprint. Cons: cover removal is G3-engineer work, internal vessels are harder to inspect, and replacement requires partial cylinder disassembly.
• External (in-line on the cold inlet) — Gledhill StainlessLite Plus (standard), Range Tribune HE (smaller models), Mixergy X, OSO Delta, retrofit installs on older cylinders that have been upgraded with a separate vessel. Pros: easy to inspect, top up and replace; pre-charge check takes ten minutes; replacement is a 90-minute job. Cons: extra fitting on the cold inlet, additional pipework joints, visible tank on the wall.

For larger HMO installs and the bedsit-style buildings we cover under our HMO hot water cylinder service, we now spec external vessels as standard because the annual licence inspection requires a documented pre-charge reading, and external vessels make that reading a five-minute check rather than an engineer-only twenty-minute job.

G3, WRAS and Benchmark Compliance

The compliance set on an expansion vessel job is fixed — none of these are optional:

• Approved Document G3 — Anyone working on the safety relief group of an unvented cylinder over 15 litres must hold a current G3 qualification (BPEC HWSS or City and Guilds 6189), renewed every 5 years. The vessel is part of the safety relief group. Non-G3 work is non-compliant under the Building Regulations.
• WRAS — The replacement vessel itself must be WRAS-approved under the Water Supply (Water Fittings) Regulations 1999. All major UK domestic vessel brands (Zilmet, Flamco, Reflex, Heatrae Sadia OEM) are WRAS-listed; check the label before fitting.
• BS 6700 / BS EN 806 — Vessel sizing follows the BS 6700 tables (since superseded by BS EN 806 + BS 8558 but the sizing arithmetic is identical): 10% of cylinder capacity for indirect, 15% for direct.
• Benchmark — Every unvented cylinder ships with a Benchmark logbook. Every service visit and every safety-set component replacement should be logged with date, engineer name, G3 ticket number, action taken and components fitted. A blank Benchmark book is a red flag at the next sale or remortgage.
• BS 7593:2019 — Treatment of water in central heating and indirect hot-water systems. The vessel service is one item on the annual BS 7593 check; the rest of the check covers system inhibitor dosing, pump operation and corrosion-product analysis.
• Competent Persons Scheme — We self-certify expansion vessel replacement work through our WaterSafe / BESCA scheme membership. The Building Regulations Compliance Certificate is posted to the address within 30 days at no extra charge.

A failed expansion vessel rarely fails alone in the long term. We almost always find that the T&P valve has been overworked by years of compensating for the dead vessel, and the immersion thermostat shows nuisance-tripping in the service log. Replacing the vessel without testing the rest of the safety set is a half-job; we replace the vessel, lift-test the T&P, verify the PRV and inspect the discharge pipe on every attendance.

FAQs

The FAQ schema at the foot of this page covers: the correct pre-charge pressure, how to spot a failed vessel, whether you can top up the pressure yourself, expected vessel lifespan in London water, sizing rules under BS 6700, and the G3 requirement for replacement.

For the wider picture on how the expansion vessel sits inside the unvented system — including the T&P valve, PRV and discharge route — read our briefings on unvented cylinders in London and the parent boiler repair service that handles the heat-source side of the system. If the cylinder is connected to a wider sealed system, our central heating service covers the heating-circuit expansion vessel that is a separate component from the hot-water one discussed here.

Save the number now — 0207 046 1363 or WhatsApp 07456 975436. The first sign of a vessel problem is a drip you can hear from across the room; the second sign is a phone call you should have made a week earlier.

John Alexander N. — Director, Emergency Repairs London