Unvented Cylinders & Megaflo Advice
Common issues, servicing and repairs for unvented hot water cylinders and Megaflo systems from Gas Safe Registered engineers.
Unvented hot water cylinders — including Megaflo systems — deliver mains-pressure hot water to your taps and showers without a cold water tank in the loft. They are efficient and powerful, but because they operate under pressure they require annual servicing by a qualified engineer and specific safety controls.
This page gives an overview of common issues. For a same-day repair or service, book a Gas Safe engineer.
Common unvented cylinder problems
- No hot water — often caused by a failed immersion heater, thermostat, or a tripped reset button.
- Water not hot enough — a faulty cylinder thermostat or scale build-up on the heating element.
- Discharge from the tundish — indicates a pressure or temperature relief valve issue that needs immediate attention.
- Lukewarm water — possible airlock, blending valve fault, or insufficient recovery time.
Why annual servicing matters
Unvented cylinders operate under mains pressure and have safety devices (temperature and pressure relief valves, expansion vessels) that must be checked annually. A missed service can lead to unsafe pressure build-up or inefficient heating. Servicing is a legal requirement under G3 of the Building Regulations and should only be carried out by a suitably qualified engineer.
When to call an engineer
Any work involving the cylinder's safety controls, immersion heaters, or expansion vessel must be carried out by a qualified engineer. If you see water discharging continuously from the tundish, turn off the power and water supply and book an engineer immediately.
FREQUENTLY ASKED QUESTIONS
How often should an unvented hot-water cylinder be serviced?
Unvented cylinders contain safety and pressure-control components — temperature and pressure relief valves, expansion vessels — that must be maintained in accordance with the manufacturer's instructions. Annual servicing is a legal requirement under G3 of the Building Regulations and should only be carried out by a suitably qualified engineer.
Why is water dripping through my tundish?
Water visible through the tundish indicates discharge from part of the unvented system's safety arrangement. Possible causes include expansion-control issues, pressure problems, or safety-valve discharge. Because the tundish forms part of a safety arrangement, persistent discharge should be investigated by a qualified engineer rather than ignored.
Why is my Megaflo leaking?
A Megaflo or other unvented cylinder may appear to leak for several reasons — discharge from a safety valve, expansion issues, leaking connections, or pressure-control faults. The source should be identified before any repair is attempted. Turn off the power and water supply if water is escaping and book a qualified engineer.
Still need help?
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