Unvented cylinder not producing hot water
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Unvented (pressurised) hot water cylinders operate at mains pressure and must only be serviced by a G3-qualified engineer. Do not tamper with the pressure relief valve, expansion vessel, or immersion heater on an unvented cylinder.
An unvented hot water cylinder is a brilliant convenience — no loft tank needed, mains pressure throughout, and instant hot water on demand. But when one stops heating, it can feel like a real headache. The good news is that most faults fall into a handful of predictable categories: a failed heating circuit, a broken immersion heater, or occasionally a pressure issue that's developed over time. This guide walks you through the checks you can safely do yourself, and explains when you absolutely need to call a qualified G3 engineer. Don't worry — most of these problems have straightforward fixes, but safety is paramount with pressurised systems, so professional help is often the right call.
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Most likely cause & what to check
An unvented cylinder (e.g. Megaflo, Telford, Heatrae Sadia) is a sealed system operating at mains water pressure. It has no cold water tank in the loft — it is fed directly from the mains.
Check the expansion relief valve (a brass valve, typically on the pipework near the top of the cylinder) — if it is continuously dripping, the expansion vessel has failed and pressure cannot be safely absorbed. This is a G3 engineer job.
Check whether your boiler is firing for hot water — look for fault codes. If the boiler is working but the cylinder is not heating, the coil inside the cylinder or a motorised valve may have failed.
Check the immersion heater backup: unvented cylinders usually have an immersion heater. Find the red switch and turn it on — if the water heats up after 45 minutes, the heating circuit is the problem, not the cylinder itself.
In all cases, servicing or repairing an unvented cylinder requires a G3-qualified plumber. Call and describe the symptoms — especially if the expansion valve is dripping, as this is a safety issue.
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Frequently asked questions
How do I know if my hot water cylinder is unvented?
Look in your loft — if there's no large cold water tank up there, you have an unvented cylinder. Unvented cylinders are typically cylindrical, sit under the stairs, in a cupboard, or in an airing cupboard, and are fed directly from the mains water supply. They're usually branded Megaflo, Telford, Heatrae Sadia, or similar, and have a brass relief valve on the pipework nearby.
Why is my expansion relief valve dripping water?
A continuously dripping expansion valve means the internal expansion vessel has failed and can no longer absorb the pressure created as water heats up. This is a safety issue and must be fixed by a G3-qualified engineer immediately — do not ignore it or try to adjust the valve yourself.
Can I use the immersion heater as a permanent fix if the boiler circuit fails?
The immersion heater is a backup, not a long-term solution — it's slow and expensive to run continuously. If your immersion heater works but the boiler circuit doesn't, you'll need a G3 engineer to diagnose whether it's the heating coil or a motorised valve that's failed, so it can be properly repaired.
Is it safe to bleed air from an unvented cylinder myself?
No — unvented cylinders operate at mains pressure and any work on them, including bleeding air, must be done by a G3-qualified engineer. Tampering with the system can release pressurised water or damage safety components, so always call a professional.
How long should it take an immersion heater to heat the cylinder?
A full tank typically takes 45 minutes to an hour to heat from cold via immersion heater, depending on the cylinder size and element wattage. If nothing happens after an hour, the immersion heater element itself has likely failed.
What fault codes should I look for on my boiler?
Every boiler brand displays fault codes differently — check your manual or look at the digital display or light codes on the boiler itself. Common heating faults show codes like E119 or F61 (Baxi), 12H or 24H (Worcester), or similar — write down the code and provide it to your engineer when you call.