How to Service an Unvented Cylinder — UK G3 Guide
Unvented cylinders need annual G3 servicing to maintain warranty and safety. Here is what a qualified engineer checks — and what happens if you skip it.
Step-by-step method
1. Test the expansion vessel
Isolate cold, open hot tap to release pressure. Check air charge on vessel schrader valve — should match manufacturer spec (typically 3.5 bar). Re-pressurise with a bike pump if low.
2. Check the pressure reducing valve
Working pressure should be 3.0–3.5 bar to cylinder inlet. Higher stresses the vessel; lower gives weak flow.
3. Test PRV and T&P
Manually lift both — they should discharge water for 2–3 seconds then reseal. If they weep afterwards, replace.
4. Check tundish and discharge
Should be dry unless a valve is discharging. Any regular drip = fault upstream.
5. Anode inspection
Some cylinders have a sacrificial anode — inspect every 3 years. Replace when 50% consumed.
Frequently asked questions
How much does an unvented cylinder service cost?+
£95–£140 for a G3 engineer. Often bundled with boiler service for £180 combined.
What happens if I don't service?+
Expansion vessel loses charge, water dumps through T&P (£50/month wasted water), warranty voided, and worst-case cylinder rupture.