Annual G3 service · Warranty preserved · £150–£220 fixed
Unvented Cylinder Service London
Annual G3 service for unvented hot water cylinders — 10-item safety checklist, expansion vessel pre-charge, thermostat verification, service report and G3 certificate for your manufacturer warranty file. Megaflo, Telford, Gledhill, Vaillant, Mixergy all serviced.
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Why the annual G3 cylinder service matters — three converging requirements
The annual G3 service on an unvented hot water cylinder is one of the highest-return maintenance items on any London home. £150–£220 spent once a year on the service protects a £2,000–£3,500 replacement cost by keeping the 25-year manufacturer warranty in force, discharges the Building Regulations G3 duty of care that sits with the property owner, and provides the HSE ACOP L8 Legionella-controls documentation that HMO and rental landlord licences require. Three independent regulatory and commercial requirements all converge on the same answer — annual service, documented, kept on file.
What makes the service worth its price is not the paperwork — it is what a competent G3 engineer catches during the visit. The three components that fail predictably on cylinders 8–15 years old — expansion vessel bladder collapse, T&P relief valve seizure, immersion element burn-out — all show warning signs to the engineer months before the outright failure. The vessel pre-charge reading is below spec; the T&P valve stiffens on the manual test; the immersion resistance drifts out of tolerance. Fixing these on service is a scheduled £75–£195 job. Fixing them on emergency dispatch after a flood or a scald is a £400–£600 job plus the property incident cleanup.
Every service we run follows the same disciplined 10-item checklist. T&P relief valve manual test, expansion relief valve test, expansion vessel pre-charge check against the manufacturer setpoint, pressure-reducing valve verification, tundish inspection, immersion element continuity test, cylinder thermostat setpoint verification, sacrificial anode inspection (where fitted), cold-feed isolation valve check, and a written service report with photographs and readings. The service report is what preserves the manufacturer warranty and satisfies the landlord licensing team.
Every service is delivered by a G3 competent engineer (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) with Water Regulations 1999 competency (WaterSafe registration, certificate 136356 same period). Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027. The G3 certificate on the service report is the document manufacturers require to keep the 25-year warranty valid.
The 10-item annual G3 service checklist
Every service we run covers every item. No shortcuts, no skipped tests. The service report shows the reading for each item.
Temperature and pressure (T&P) relief valve test
Manual lift-and-reset test on the T&P relief valve — the primary safety device on the cylinder. If it fails to open at the manual test or fails to reseat properly, replaced same visit. A stuck T&P valve is a G3 category-1 safety failure and the cylinder cannot be left in service.
Expansion relief valve test
Secondary safety valve, tested by manual actuation. Verifies the valve opens under overpressure and reseats when the pressure returns to normal. Replaced if the test fails.
Expansion vessel pre-charge check
The expansion vessel absorbs the volume increase as stored water heats from cold to 60°C. Correct pre-charge is 3.0–3.5 bar on standard installs. Reading below 2.5 bar means the vessel bladder has failed — replaced same visit. Pre-charge check requires isolating the mains, draining the vessel connection point, gauging with a calibrated gauge.
Pressure-reducing valve (PRV) verification
The PRV on the cold mains feed sets the operating pressure of the unvented system (typically 3.5 bar). Verified by pressure-gauge reading downstream of the PRV, and by manual adjustment if the reading drifts. A failed PRV allowing mains pressure spikes above the cylinder rating is a G3 safety failure.
Tundish inspection and clear
The tundish is the visible air-break in the discharge pipework from the T&P and expansion relief valves. Inspected for water residue (indicates a leaking safety valve upstream), scale build-up, and physical damage. Cleared and physically tested by feeding water through from above.
Immersion element check and continuity test
Every immersion element (primary and backup) inspected for scale build-up and tested with a multimeter for resistance across the element. Out-of-range readings indicate a failing element — customer advised for replacement on the same or a follow-up visit.
Cylinder thermostat setpoint verification
Cylinder thermostat setpoint checked against a calibrated thermometer reading at the cylinder top. Any drift beyond ±5°C is flagged — the thermostat is typically replaced on the same visit if it is the cause.
Sacrificial anode inspection (where fitted)
Copper and some stainless cylinders include a sacrificial magnesium or aluminium anode designed to corrode preferentially and protect the cylinder body. Inspected and photographed. Replaced when consumption exceeds 60% — extends cylinder life significantly on hard-water London installs.
Cold-feed pipework and isolation valve check
Every isolation valve on the cold feed, hot outlet, and immersion circuit tested for correct opening and closing. Any seized valve is replaced or scheduled for replacement — a working isolation valve is what allows the next emergency to be handled without draining half the property.
Service report and G3 certificate
Full service report with photographs of every test, every reading, and any items flagged for follow-up. Retained by the customer for the manufacturer warranty file and by AK for the property service history. This is the document that preserves the 25-year cylinder warranty.
Why service annually — the four reasons
Preserves the 25-year cylinder warranty
Every major cylinder manufacturer — Heatrae Sadia (Megaflo), Telford, Gledhill, Joule — makes the 25-year warranty conditional on documented annual G3 service. Without service records, warranty claims for cylinder failure are rejected. A £2,500 replacement cost avoided by keeping the annual service in place is exceptional value for money.
Meets Building Regulations G3 duty of care
G3 does not expire at install — the ongoing responsibility for cylinder safety sits with the property owner or landlord. Annual G3 service documents the discharge of that duty of care. In an insurance claim following a cylinder-related incident, the service record is the single strongest evidence of reasonable maintenance.
HSE ACOP L8 Legionella compliance for landlords
HMOs and rental properties above 3 tenants require documented Legionella risk assessment under HSE ACOP L8. Annual cylinder service including thermostat setpoint verification (≥60°C) is a core component of that documented control. Included in every landlord property service report.
Catches failing components before failure
The three components that fail predictably on cylinders 8–15 years old — expansion vessel bladder, T&P relief valve, immersion element — all give warning signs in service that are visible to a G3 engineer months before the outright failure. Fixing on service is a scheduled £150 job. Fixing on emergency dispatch after a flood or scald is a £500+ job plus incident cleanup.
Cost — unvented cylinder service
| Scope | Price (inc. VAT) | Includes |
|---|---|---|
| Standard unvented cylinder annual service | £150–£195 | Full 10-item service checklist, safety valve tests, expansion vessel pre-charge, immersion continuity, thermostat verification, tundish clear, service report + G3 certificate for warranty file |
| Twin coil cylinder service (both coils tested) | £175–£220 | Both coil performance tests, twin thermostat setpoint verification, primary strainer clean on each coil, glycol antifreeze check on solar loops |
| Mixergy or smart cylinder service | £175–£225 | Standard G3 items plus controller firmware check, sensor calibration verification, app connectivity test, tariff schedule verification |
| HMO landlord service package (annual, per cylinder) | £135–£175 | Volume discount for portfolios above 5 cylinders. Consolidated service report for landlord licence file. L8 Legionella controls documented. |
| Emergency service call (thermostat, immersion, T&P valve) | £195–£320 | Same-day dispatch, diagnosis, component replacement, cylinder retest, service report |
| Sacrificial anode replacement (where fitted) | +£65–£95 | Magnesium or aluminium anode replacement, cylinder body inspection for corrosion, refill and test |
Real London cylinder services
Five recent services. What the service caught, what it cost, what it saved.
Wandsworth 4-bed home — 12-year-old Megaflo service catches failing vessel
Annual service on a 2013 Megaflo Eco 210L. Expansion vessel pre-charge measured 1.8 bar — well below the 3.0 bar factory setting, indicating bladder failure imminent. Vessel replaced same visit before failure occurred. Cost £185 service + £145 vessel replacement (total £330). Avoided £600+ emergency dispatch on a Sunday if the vessel had ruptured and dumped mains pressure into the T&P discharge.
Ealing HMO 8-bed — annual portfolio service for landlord licence
Landlord portfolio contract for annual G3 service across five HMOs (12 cylinders total). Standardised checklist, consolidated report per property, single invoice for accounting. L8 Legionella controls documented per cylinder. Property licence renewal approved with the service records as evidence. Volume-priced £135 per cylinder inc. VAT.
Islington flat — Telford Tempest service catches sacrificial anode consumed
Annual service on a 2015 Telford Tempest 180L. Sacrificial anode inspected — 75% consumed, cylinder body clean of corrosion behind it. Anode replaced same visit at £75 add-on. Extended expected cylinder life by 6–8 years. Total £220 including standard service.
Chiswick townhouse — first-year service on new Mixergy install
First-year service on a 2025-installed Mixergy 250L Pro. Controller firmware updated to latest version, sensor calibration verified against calibrated thermometer, Octopus Agile scheduling reviewed and optimised. Small drift on the bottom-of-cylinder sensor identified and re-zeroed. Total £215 including full G3 items plus Mixergy-specific checks.
Notting Hill — 20-year-old Ariston cylinder condemned during service
Annual service on a 2005 Ariston 210L. Expansion vessel completely failed (0 bar pre-charge, ruptured bladder). T&P relief valve seized (would not open on manual test). Cylinder body showed external corrosion staining at the immersion boss. Cylinder condemned on G3 category-1 safety grounds — cannot continue in service. Customer briefed with a written condemnation report and cylinder replacement quote. Cylinder replaced within 5 working days. Total £195 service + £2,450 replacement Megaflo Eco Indirect.
Annual cylinder service across every London borough
Frequently asked questions
How often does an unvented cylinder need to be serviced?
What happens if I skip the annual service?
What certification do you hold for the service?
How long does the annual service take?
Do you service smart cylinders (Mixergy, Sunamp, tepeo)?
What if a component fails during the service — do you fix it same visit?
Do you carry public liability insurance?
Can you service cylinders that were not originally installed by AK?
Do I get a certificate I can show to the manufacturer for warranty?
Do you offer service contracts for landlord portfolios?
Related services
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Full cylinder replacement when service reveals a failed unit.
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Same-day thermostat swap when the service identifies failure.
HMO hot water cylinder
HMO-specific service including L8 Legionella controls.
G3 unvented cylinder engineer
The G3 certification behind every service call.
Book your annual cylinder service
£150–£220 fixed price. Service report and G3 certificate for your warranty file.
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