Megaflo and similar unvented hot water cylinders are excellent systems when properly maintained — they deliver mains-pressure hot water to every tap simultaneously, with no need for a loft tank. But when something goes wrong, they can be confusing, alarming, and occasionally dangerous if the problem isn't dealt with by a G3-qualified engineer.
As a G3-registered engineer servicing unvented cylinders across Orpington, Bromley, Sidcup, Sevenoaks and the wider Kent area, I see the same handful of issues come up again and again. Here are the seven most common — what they mean, what causes them, and what should be done.
Important: Under Building Regulations Part G, unvented hot water systems must only be worked on by a G3-qualified engineer. DIY repairs are illegal and dangerous — these systems hold pressurised hot water at temperatures that can cause serious injury.
1. Water Dripping from the Outside Discharge Pipe
What you see: A copper pipe coming out of an external wall, periodically dripping or running with hot water.
What it means: Either your temperature & pressure relief valve (TPRV) or your expansion relief valve has activated. Both are safety devices designed to release water if pressure or temperature exceeds safe limits.
The likely cause: Most often it's a failed expansion vessel. The vessel inside or alongside your cylinder loses its air charge over time, leaving nowhere for the heated water to expand into. Pressure rises, and the relief valve dumps water externally to keep things safe.
The fix: A G3 engineer will isolate the system, drain the expansion vessel, recharge it to the correct pre-charge pressure, and verify the relief valves are still functioning correctly. If the vessel itself has failed, it needs replacing.
2. No Hot Water at All
What you see: Cold water from hot taps, despite the boiler or heat pump appearing to be running.
What it means: Either the cylinder is being heated but heat isn't transferring, or it isn't being heated at all.
The likely causes:
- Failed immersion heater element (if used as backup or primary)
- Thermostat failure on the cylinder
- Programmer or timer not calling for hot water
- Two-port motorised valve stuck closed
- For ASHP-fed systems: heat pump fault preventing the system from heating the cylinder
The fix: Diagnosis starts with confirming the heat source is calling for hot water, then working backwards through the chain. This is genuinely engineering work — guessing wastes money on parts that aren't actually faulty.
3. Lukewarm Water That Never Gets Hot
What you see: Hot water arrives but isn't properly hot, or runs warm and then cold quickly.
The likely causes:
- Cylinder thermostat set too low (should be 60–65°C)
- Heat exchanger inside the cylinder is scaled (common in hard water areas like Orpington and Bromley)
- Failed thermostatic mixing valve at the outlet
- Cross-connection between hot and cold supplies somewhere in the property
The fix: A combined service of the cylinder and a check of the system controls usually identifies the issue quickly. Severe scaling may require descaling or eventual replacement.
4. Banging or "Kettling" Noises
What you see: Loud rumbling, banging or kettle-like noises from the cylinder when heating.
What it means: Almost always limescale build-up on the heating element or coil. Hard water across South East London and Kent makes this extremely common.
The fix: If caught early, descaling can extend the cylinder's life. Severe scaling damages internal components and may require replacement of the heat exchanger or the whole cylinder. This is exactly why annual servicing matters — I catch scale build-up long before it gets to the noisy stage.
5. Pressure Drops in Hot Water
What you see: Strong cold water pressure but weak hot water flow.
The likely causes:
- Partially closed isolation valve on the cylinder feed
- Scaled or blocked outlet pipework
- Failed pressure-reducing valve at the mains
- TMV (thermostatic mixing valve) cartridge blocked with scale
The fix: A pressure test at the cylinder identifies whether the problem is upstream or downstream. Replacement of scaled valves usually restores normal flow.
6. Visible Leaks Around the Cylinder
What you see: Water on the floor near the cylinder, on the airing cupboard floor, or stains on the ceiling below.
The likely causes:
- Failed seal on the immersion heater
- Corrosion at a connection point
- Pinhole leak in the cylinder itself (terminal — usually requires replacement)
- Failed expansion vessel diaphragm leaking water
The fix: Don't ignore any leak from an unvented cylinder. The system holds significant pressure and a small leak can become catastrophic. Isolate the supply and call a G3 engineer immediately.
7. Cylinder Tripping the Electrics
What you see: RCD or MCB tripping when the immersion heater operates.
What it means: The immersion element has failed, usually with the resistance wire shorting to the casing through scale or corrosion.
The fix: The element needs isolating, testing and replacing. While doing this it's the perfect opportunity to inspect the cylinder for early signs of internal corrosion.
How to Avoid These Problems in the First Place
Almost every issue I've listed can be caught early — or prevented entirely — by an annual G3 service. The service includes recharging the expansion vessel, testing the relief valves, checking the thermostat operation, inspecting all connections and providing a written record for your warranty.
If your cylinder hasn't been serviced in over 12 months, book one. It's far cheaper than the repair bill when something fails.
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