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G3 qualified plumber servicing an unvented hot water cylinder in a London property
G3 Qualified — Building Regs Part G3

Unvented Cylinders London — Install, Service & Repair

G3 qualified engineers across all 32 London boroughs. Megaflo, Gledhill, Heatrae Sadia, Kingspan and OSO specialists. Annual service from £140, full new cylinder install from £1,400. Building Regulations Part G3 notification included on every job.

Same-day callouts for leaks, no hot water, T&P discharge or expansion vessel failure. Fixed-price quotes given on site before any work begins.

0207 046 1363WhatsApp 07456 975436Book online
£5M Public liability
G3 Unvented qualified
24/7 Emergency cover
BS EN 12897 compliant
60min London response
Quick Answer

An unvented hot water cylinder stores mains-pressure hot water with no loft tank — strong flow to every outlet, less maintenance and freeze-proof. Installation and service in London is a controlled service under Approved Document G3 of the Building Regulations and must be carried out by a G3 qualified engineer. Annual servicing costs £140–£220 in London; a new install runs £1,400–£3,200 depending on size.

What is an unvented hot water cylinder?

An unvented cylinder is a sealed pressurised hot water store fed directly from the cold mains. There is no open vent and no loft tank — the system runs at mains pressure (typically 3.0–3.5 bar after the inlet pressure-reducing valve) and feeds every hot outlet at the same pressure as the cold. That is why unvented showers feel as strong as the cold tap, and why a properly sized cylinder can run two showers at once without dropping flow.

The trade-off is safety — a sealed cylinder being heated to 60°C is by definition a pressure vessel, and Approved Document G3 of the Building Regulations treats it as a controlled service. Every install must have four layered safety devices: a working thermostat, a non-self-resetting energy cut-out, an expansion device (vessel or bubble-top) and a temperature & pressure (T&P) relief valve discharging to a safe visible location. All four must be tested annually.

Across London the dominant brands are Heatrae Sadia Megaflo Eco, Gledhill StainlessLite Plus, Kingspan Albion Ultrasteel, OSO Super S and Range Tribune. All are duplex stainless steel with 25-year tank warranties when serviced annually. The vast majority of unvented work in London is either annual service (£140–£220), expansion vessel replacement (£180–£280) or full cylinder swap (£1,400–£3,200).

Unvented cylinder cost London — 2026 prices

Every quote is a fixed figure given on site after a 10-minute diagnostic. No call-out fee on quoted work, no parking add-ons, no out-of-hours premium on scheduled annual services.

JobWhat's includedTypical cost
Annual G3 service (single cylinder)Expansion vessel pre-charge check, T&P valve test, PRV test, anode inspection, thermostat verification, full commissioning certificate refresh.£140–£220
Expansion vessel replacementLike-for-like external expansion vessel, isolation, drain-down, recharge to manufacturer spec, leak test.£180–£280
Temperature & Pressure relief valve (T&P)Replacement T&P valve (typically 7 bar / 90°C), discharge line check, witnessed test, recommissioning.£150–£230
Pressure reducing valve / inlet control setReplace failed PRV or full inlet group (check valve, PRV, strainer), set system to 3.0–3.5 bar working pressure.£170–£260
Immersion heater element replacementDrain to immersion boss, replace element + thermostat, refill, vent, electrical safety check.£160–£240
New cylinder install — 150L direct (electric)Supply & fit direct unvented cylinder, all inlet group, discharge pipework, commissioning, Building Regs G3 notification.£1,400–£1,900
New cylinder install — 210L indirectSupply & fit indirect cylinder for heat-only or system boiler, full G3 kit, primary connections, discharge to outside, certificate.£1,800–£2,400
New cylinder install — 300L (large home / HMO)Supply & fit 300L stainless steel cylinder, full G3 group, secondary circulation if specified, commissioning, certificate.£2,400–£3,200

* Prices include VAT and Building Regulations G3 notification where applicable. Full price list on the pricing page.

Need a quote on a specific job?

Call 0207 046 1363

What we check on every visit

Every diagnostic and service runs the same 12-point checklist, in the same order. Nothing is judged by eye — everything is measured against the manufacturer's commissioning data and the BS EN 12897 / G3 minimums.

Megaflo unvented hot water cylinder with expansion vessel and T&P safety devices in a London airing cupboard

  • Expansion vessel pre-charge

    Tested with the system depressurised. Reset to manufacturer-stated air pressure (commonly 3.0 bar).

  • Temperature & Pressure relief valve

    Witnessed lift test of the 90°C / 7 bar safety device. Discharge line confirmed clear to outside.

  • Pressure reducing valve (PRV)

    Inlet pressure measured and set to balance the cold supply. Strainer inspected for debris.

  • Tundish gap & discharge pipework

    Visible 300 mm air gap confirmed, discharge pipe falling continuously to a safe visible termination.

  • Immersion heater & thermostat

    Resistance check on each element, thermostat cut-out tested at 60°C / 65°C set point.

  • Anode protection (where fitted)

    Sacrificial magnesium or impressed-current titanium anode condition recorded for corrosion warranty.

  • Cylinder body & insulation

    External inspection for staining, distortion, condensation, jacket integrity, pipe lagging.

  • Primary coil (indirect cylinders)

    Flow / return temperatures logged against boiler set point. Indirect coil thermal performance benchmarked.

  • Cold mains pressure & flow

    Static pressure (bar) and dynamic flow rate (L/min) recorded — both must meet manufacturer minimums.

  • Thermostatic mixing valves

    TMV2 / TMV3 outlet temperatures verified, especially on HMO and care setups (≤43°C bath outlets).

  • Legionella control

    Stored water held above 60°C, distribution above 50°C within one minute, in line with HSG274 / L8.

  • Electrical safety of immersion circuits

    RCD protection confirmed, immersion isolator switch fused to manufacturer rating, bonding to BS 7671 18th Edition.

How it works — booking to certificate

1

Call or message

5 min

Call 0207 046 1363 or WhatsApp 07456 975436. Describe the cylinder (brand, size, age if known) and the fault. Photo of the data plate speeds up parts sourcing.

2

Same-day diagnosis

Same day / next day

G3 engineer attends, isolates the cylinder, runs the full diagnostic — pressure readings, vessel pre-charge, T&P lift, discharge gap, electrical safety on the immersion.

3

Fixed-price quote

On-site

Repair or replacement priced in writing on the spot. No hidden parts mark-up. Around 60% of common faults (expansion vessel, T&P, immersion) are fixed on first visit from van stock.

4

Works carried out

1–6 hours

Annual service averages 60–90 minutes. Component swaps 1–3 hours. Full cylinder swap is a half-day to full day depending on access and pipework re-route.

5

Commissioning & certificate

Same visit

System pressurised, balanced, run-up to 60°C, safety devices witnessed-tested, Building Regulations G3 notification filed, benchmark logbook completed and emailed within 24 hours.

Standards we work to

Unvented hot water in the UK sits on a small stack of statutory rules and British / European product standards. Every job is documented against them in the benchmark logbook that ships with each cylinder.

G3

Approved Document G3 — Hot water supply and systems

Every unvented hot water storage system over 15 litres in England and Wales is a controlled service under Building Regulations Part G. Installation, commissioning and modification must be carried out by a person competent in unvented systems (the G3 ticket). The work must be notified to building control either directly or via a competent person scheme.

BS EN 12897

Water supply — Specification for indirectly heated unvented closed storage water heaters

The European product standard that governs cylinder construction, the layered safety devices required (PRV, expansion device, non-self-resetting energy cut-out, T&P relief valve) and the labelling on every UK unvented cylinder.

BS 6700 / BS EN 806

Design, installation and maintenance of services supplying water

Sets the rules for cold-water inlet groups, isolation, backflow protection, secondary circulation and acceptable materials for hot and cold water distribution.

Water Regulations 1999

Water Supply (Water Fittings) Regulations 1999

Statutory backflow and fluid-category rules administered by the local water undertaker. Every cylinder install is notifiable to the water company when it changes the system arrangement.

HSG274 / L8

Legionnaires' disease — control of legionella in water systems

HSE guidance covering stored water temperature, distribution temperature and dead-leg management. Stored water must be held at or above 60°C and reach outlets at 50°C within one minute, 55°C in healthcare.

BS 7671

18th Edition Wiring Regulations (2018+A2:2022)

Governs the electrical work on every immersion heater, PCB-driven cylinder and unvented stat circuit — RCD protection, isolator rating, cable size, earthing and bonding.

Temperature and pressure relief valve and tundish on an unvented hot water cylinder

Real London jobs — worked examples

Every London property is different. Here are six anonymised jobs from the last twelve months, with the diagnostic, the work done and the final figure.

Victorian terrace, Islington — Megaflo dripping from tundish

210L Megaflo Eco SystemFit installed 2018. Constant drip from tundish, expansion relief blowing under heat-up. Diagnosed expansion vessel waterlogged (pre-charge collapsed). External 18L vessel fitted, recharged to 3.0 bar. Total time on site: 90 minutes. Final invoice: £210 inc. VAT, parts and labour.

1970s mid-terrace, Hackney — old Gledhill Stainless Lite, no hot water

Twin immersion element failure — bottom element open-circuit, top element earth-leaking and tripping RCD. Both elements and thermostats replaced, anode inspected (50% spent — replaced as goodwill upsell). Recommissioned, certificate updated. Cost: £290 inc. VAT.

New-build 2-bed flat, Wandsworth — annual G3 service

Heatrae Sadia Megaflo CL 170L on a 2021 install. Annual landlord-required service. Pre-charge dropped to 1.8 bar (target 3.0), reset. T&P lifted clean. PRV outlet pressure 3.4 bar (within range). Cost: £165 inc. VAT, certificate emailed same day.

Listed townhouse, Westminster — full cylinder upgrade

Replaced a 1990s vented cylinder + loft tank with a 300L OSO Super S indirect for a 4-bath property after a power-shower upgrade. Full G3 group, new discharge through external wall to drain, primary repipe in copper, building control notified, two-day install. Cost: £2,950 inc. VAT.

HMO licensed property, Tower Hamlets — TMV failure causing scald risk

Reported by tenant after a 65°C bath outlet reading at change-of-tenant inspection. TMV2 cartridge seized open. Two TMV2 cartridges replaced, all bath/basin outlets re-tested to 43°C, written confirmation for HMO licence file. Cost: £240 inc. VAT.

Kensington maisonette — discharge pipe non-compliant

Insurance survey flagged the T&P discharge pipe running uphill into a void with no visible termination. Re-routed in 22 mm copper from tundish, continuous fall, terminating to outside above the gully with a 300 mm air gap. G3 paperwork updated. Cost: £380 inc. VAT.

Need a G3 engineer this week?

Same-day callouts for leaks, no hot water, blowing T&P, or scalding water. Annual services booked within 3–5 working days. Building Regulations G3 notification included on every install.

Call 0207 046 1363WhatsApp 07456 975436Book online

Frequently asked questions

Why is my unvented cylinder leaking from the T&P valve?
A T&P (temperature and pressure relief) valve discharging is almost always a symptom — not a fault in the valve itself. The most common cause in London cylinders is a waterlogged expansion vessel: when its air pre-charge collapses, expanding hot water has nowhere to go and the T&P lifts as the designed safety route. Less common causes are a stuck PRV letting mains pressure spike inlet pressure past 6 bar, a failed thermostat allowing the stored water past 90°C, or scale on the T&P seat. Diagnosis takes 15 minutes — vessel pre-charge first, then inlet pressure, then thermostat function.
Do I need a G3 ticket to work on an unvented cylinder?
Yes. Approved Document G3 of the Building Regulations makes unvented hot water installation, commissioning and modification a controlled service. The engineer must hold a current G3 (BPEC, LCL Awards or City & Guilds 6035) Unvented Hot Water Systems certificate and the work must be notified to building control directly or through a competent person scheme. Anyone working on the cylinder without a G3 ticket invalidates the manufacturer warranty, the home insurance and the building control trail — and exposes the owner to enforcement under the Building Act 1984.
How often does an unvented cylinder need servicing?
Annually. Every major UK manufacturer (Megaflo, Gledhill, Heatrae Sadia, Kingspan Albion, OSO, Range) specifies an annual service as a condition of the cylinder warranty — typically a 25-year tank warranty plus a 2-year parts warranty. The service must include expansion vessel pre-charge, T&P witnessed lift, PRV outlet pressure, thermostat cut-out test, anode inspection where fitted, and a refreshed commissioning entry in the benchmark logbook. Skipping a year voids warranty and is a red-flag finding on insurance and EICR-style inspections.
My hot water pressure is too low — is the cylinder the problem?
Low hot water pressure on an unvented system usually points to four places: a partially-closed inlet isolation valve, a clogged PRV strainer, a failing PRV (set point dropping below 2 bar), or a furred-up expansion relief valve restricting flow. A 30-second diagnostic measures static pressure at the cold inlet and dynamic flow at the nearest hot outlet. London hard-water cylinders over five years old commonly need a PRV cartridge swap (£170–£260) to restore performance — the cylinder itself is rarely the bottleneck.
Hot water is scalding hot or boiling — what's wrong?
Stored water above 65°C suggests the thermostat has failed in a closed position. Unvented cylinders are fitted with a non-self-resetting energy cut-out as a second layer of protection — it trips at 85–90°C and stops the heat source. If the cut-out has tripped, the cause must be found and corrected before reset (that is the G3 rule). On indirect cylinders the cause is usually a stuck motorised valve or failing cylinder stat. On electric direct cylinders it's almost always a failed thermostat on the immersion element.
How big is the discharge pipe and where must it go?
The discharge pipework from the tundish must be at least one pipe size larger than the inlet to the T&P valve (typically 22 mm from a 15 mm valve), with a continuous fall, no more than three bends, and a maximum equivalent length per the manufacturer's table. It must terminate to a visible, safe location — usually a gully or external wall — at low level. An internal discharge into a hopper, soil pipe or void is a Building Regulations breach and is the single most common fail we find on inherited installs.
Can an unvented cylinder explode?
It is extraordinarily rare and only happens when all four layers of protection are removed or have failed: the working thermostat, the high-limit cut-out, the expansion device and the T&P relief valve. Properly serviced cylinders cannot fail catastrophically because the layered safety design is exactly what Approved Document G3 enforces. The 2016 Cheshire incident widely reported on consumer media involved an installation with the T&P valve plugged off — every protection layer had been deliberately defeated.
Megaflo, Gledhill or Heatrae Sadia — which is best for a London property?
All three are duplex stainless steel cylinders with broadly equivalent warranties (25 years tank, 2 years parts). Megaflo Eco (Heatrae Sadia) has the strongest UK service network and the easiest parts supply in London — vessels, PRVs and elements are stocked on most plumbers' vans. Gledhill StainlessLite Plus has a slightly better recovery time. OSO Super S is the premium choice for high-flow multi-bathroom homes. For most London 2–4 bed properties, a 170L–210L Megaflo Eco or Heatrae Sadia Megaflo CL is the right call.
How long does a new cylinder install take?
A like-for-like swap in a flat with accessible pipework is a 4–6 hour single-day job. A first-time unvented install replacing an old vented system with a loft tank is a 1–2 day job because all the supply, discharge and electrical work is new. The big variables in London are access (rooftop cylinder cupboards in mansion blocks), discharge route to outside (often the hardest part in a mid-floor flat), and isolating the mains in shared buildings.
What size cylinder do I need?
Rule of thumb in London: 35–45 litres of storage per person, with an extra 50 litres for properties with a bath used daily. A 1–2 bed flat with a shower is fine on 150–170L. A 3-bed family home with one bath is 210L. A 4–5 bed home or any property with two or more bathrooms wants 250–300L. For HMOs and homes with secondary circulation, sizing is calculated against simultaneous-use loadings using the manufacturer flow curves, not a rule of thumb.
Does an unvented cylinder need an annual gas safety check?
The cylinder itself does not — it has no gas connection. But the boiler feeding an indirect cylinder does need a CP12 / Gas Safety Certificate every 12 months in any rented property under the Gas Safety (Installation and Use) Regulations 1998. Most landlords ask us to book the CP12 and the cylinder G3 service together to keep the compliance calendar tidy. Combined visits are quoted at a discount on our PPM contracts.
Will a new cylinder save energy?
If it's replacing a 15-year-old cylinder, yes — modern factory-foamed cylinders have around half the standing heat loss of pre-2010 units. Replacing a vented system also removes the lukewarm losses from the loft pipework and cold-water tank. On a 4-person London home a like-for-like upgrade typically saves 200–350 kWh per year on hot water, with a payback of 7–10 years on the cylinder cost. The bigger gain is comfort and flow, not energy.
Can I have an unvented cylinder in a top-floor flat?
Yes — they are common in London top-floor flats because they avoid loft tanks, header tank noise and freeze risk. The two checks are mains inlet pressure (must be at least 1.5 bar dynamic at the inlet group) and a viable discharge route to outside. In some mansion blocks the discharge route is the hardest engineering question — the cylinder may need to live closer to an external wall than the previous airing cupboard, or the discharge piped in a fire-rated metallic duct.
What do you do during an annual service?
Each visit follows a 12-point checklist: cold inlet pressure recorded, PRV outlet pressure recorded, inlet strainer cleaned, expansion vessel pre-charge tested and recharged, T&P witnessed-lifted, energy cut-out function checked, thermostat set-point verified, immersion element resistance measured (if direct or backup electric), anode inspected, discharge pipe checked for free fall and clear termination, all isolation valves operated, benchmark logbook updated and emailed. Average time on site is 60–90 minutes.
What if my cylinder is older than 25 years?
Most duplex stainless cylinders run 20–30 years before the tank itself fails. Cylinders made between 1990 and 2005 (Gledhill EnviroFoam, early Megaflo CL Indirect, early Range Tribune) are at the end of their economic life — by the time the expansion vessel, PRV and T&P have all been replaced two or three times, the smarter spend is a new cylinder with a fresh 25-year tank warranty. We'll always tell you straight whether to repair or replace, with the cost of both routes on the same quote.

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

Service area

Service Area — All London Boroughs

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

Completed G3 unvented cylinder commissioning certificate

Unvented cylinder fault or annual service due?

G3 qualified engineers across London. Fixed-price quote on site, work carried out the same visit where possible, Building Regulations G3 notification filed. £5M public liability, fully insured.

0207 046 1363WhatsApp 07456 975436Book online

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