Boilers & Heating
Unvented Cylinder G3 Inspection & Certificate
TL;DR
An unvented cylinder is a pressure vessel and needs a G3 annual safety service by a G3-registered engineer to maintain its warranty and — more importantly — its safety. A proper G3 service is £95–£140 including expansion vessel recharge, T&P valve test, PRV pressure check, discharge pipe inspection, and a Building Regs G3 certificate. Skipping it invalidates the 25-year cylinder warranty and, in rare cases, causes explosive failures — genuinely.
Unvented cylinders sit under mains pressure at 60°C. When the safety kit fails they can — and historically have — over-pressurise catastrophically, blowing the top off the cylinder like a rocket. The G3 safety devices (temperature and pressure relief valves, expansion vessel, tundish) are there specifically to prevent that, and they need annual servicing to stay reliable.
This page covers what a proper G3 service checks, what a valid certificate must contain, and why 'my mate the plumber did it' doesn't cut it for warranty or insurance.
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Why does an unvented cylinder need annual G3 servicing?
Three critical safety devices lose function over time. The expansion vessel loses its air charge — the vessel that absorbs pressure spikes as water heats and expands. The T&P (temperature and pressure) relief valve, which is the last-resort safety cut-out at 90°C or 7 bar, can seize or scale up. The PRV (pressure-reducing valve) on the cold inlet can drift out of setpoint. Any one of these failing silently is what leads to the very rare catastrophic failures the tabloids like to write about.
- Expansion vessel loses charge (silent) — every 12 months
- T&P valve can seize if never operated — every 12 months
- PRV setpoint drifts — every 12 months
- Tundish can be obstructed by scale or debris
- Discharge pipe termination can be blocked by insulation or a nest
- Immersion anode depletes (protects the tank interior)
What we do on a G3 service visit
Full pressure check on the mains supply, isolate the cylinder cold supply, drain enough to release residual pressure, recharge the expansion vessel to the correct pre-charge (usually 3.0 bar for a domestic unit), test the T&P valve by manually operating it and confirming it re-seats cleanly, check the PRV setpoint with a gauge and adjust if drift, inspect and clear the tundish, check the discharge pipe termination outside, test the cylinder thermostats and cut-outs, and inspect the immersion element(s). Everything is documented on the certificate.
What a valid G3 certificate must contain
Property address, engineer's name and G3 competent-person scheme registration number, cylinder make/model/capacity/serial number, date of service, next-due date, results of each check (expansion pre-charge before/after, T&P operating, PRV pressure, tundish condition, discharge integrity, thermostat setpoint), and any defects with the action taken. This is what the cylinder manufacturer (Megaflo, OSO, Telford, Gledhill) needs to see if you ever make a warranty claim.
Why 'my mate the plumber' isn't good enough
Fitting or servicing an unvented cylinder is a G3 notifiable job under the Building Regulations. Any plumber not on a G3 competent-person scheme is working illegally, cannot notify Building Control, and their certificate is not accepted by the manufacturer, your home insurer, or (for landlords) HMO licensing. G3 registration is a specific qualification — being 'Gas Safe' does not include it.
Typical prices
Real-world ranges from recent Northwest jobs. Your final price is confirmed in writing before any work starts.
| Item | Typical price | Notes |
|---|---|---|
| G3 annual service — standard unvented | 95 – 140 | Includes vessel recharge, T&P test, PRV check, certificate |
| G3 service — twin-coil solar cylinder | 115 – 160 | Additional solar circuit checks |
| G3 service + immersion element replacement | 195 – 285 | If element found to be scaled or failed |
| Expansion vessel replacement (if failed) | 125 – 195 | Vessel + labour |
| T&P valve replacement | 85 – 145 | Replaced if it fails to re-seat |
| PRV replacement | 95 – 155 | If pressure drift beyond tolerance |
| Building Control notification | 0 | Via competent-person scheme |
Which brand should you fit?
All G3 services carried out by engineers registered with a G3 competent-person scheme (BPEC / APHC / Blue Flame). Compatible with Megaflo Eco Plus, OSO Super S, Telford Tempest, Gledhill Stainless Lite Plus, and Heatrae Sadia. Certificates issued through Certi or Gas Tag. We install and service Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann and Alpha — none of them pay us commission and we don't push a "boiler of the month".
G3 service red flags
- What's your G3 competent-person scheme registration number?
- Are you actually recharging the expansion vessel, or just glancing at it?
- Are you physically operating the T&P valve, or ticking the box?
- Is the certificate a proper G3 Building Regs certificate, not just a plumber's invoice?
- Where does the T&P discharge terminate outside, and is it clear?
- Am I being registered with the manufacturer to maintain warranty?
- How long is the service (should be 45–60 minutes, not 10)?
Our promise on this job
Flat £50 diagnostic — deducted in full from any work booked. Fair, transparent quote before we start. If we can't fix the fault we diagnosed, we refund the diagnostic. All gas work is signed off by a Gas Safe registered engineer.
Talk to KJ about unvented cylinder g3 inspection & certificate
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