plumbing
Unvented Hot Water Cylinders in London: Repair, Service and Replacement
What Is an Unvented Hot Water Cylinder?
An unvented (pressurised) hot water cylinder stores hot water at mains pressure — unlike older vented systems that relied on a cold water storage tank in the loft. The result is powerful, consistent hot water pressure throughout the property without a header tank. Megaflo, Gledhill and Albion are common brands in London.
Why Unvented Cylinders Require Annual Servicing
Unvented cylinders store water under mains pressure at up to 65°C. If safety devices fail, they can become extremely dangerous. Building Regulations Part G requires that unvented cylinders are:
- Installed only by G3 qualified engineers
- Serviced annually by a G3 qualified engineer
- Fitted with specific safety devices (temperature and pressure relief valve, expansion vessel, pressure reducing valve)
Common Unvented Cylinder Faults in London
- T&P (temperature and pressure) relief valve discharging — safety device operating, indicates overpressure or overtemperature
- Expansion vessel failure — causes T&P valve to operate repeatedly
- Immersion heater element failure — no hot water from the cylinder
- Thermostat failure — water too hot or too cold
- Legionella thermal pasteurisation not occurring — thermostat preventing 60°C cycle
Frequently asked questions
Who can service an unvented hot water cylinder in London?
Only engineers holding a G3 (Unvented Hot Water Systems) qualification can legally install, commission or service unvented cylinders. This is in addition to Gas Safe registration for gas-heated cylinders. Working on an unvented cylinder without G3 qualification is illegal and invalidates insurance.
Why is my unvented cylinder dripping water outside?
A dripping tundish (the small funnel-shaped device with a pipe leading outside) indicates the temperature and pressure relief valve is operating. This is a safety device, not a fault in itself — but it means the cylinder is regularly exceeding safe pressure or temperature. Common causes: failed expansion vessel, thermostat set too high, or a failing T&P valve that needs replacement. Call a G3 engineer.
How long does an unvented cylinder last in London?
A well-maintained unvented cylinder in London typically lasts 15–25 years. Annual servicing, timely expansion vessel replacement (typically every 7–10 years), and attention to water quality significantly extend lifespan. London's hard water can accelerate limescale build-up on immersion elements, reducing efficiency.