Unvented Cylinder Fault Repair: Not Heating, Tripping, Weak Flow

Which unvented cylinder fault do you have?

The symptom narrows the search quickly, so match what you see before reading further. This page covers the heating and control side. Water at the tundish belongs to the valve and expansion branch, set out under TPRV and expansion vessel repair.

• Cold taps, immersion or boiler calling: supply and settings, then thermostat, cut-out, motorised valve and element.
• Works for a while, then trips: a thermostat or valve that is not switching off.
• Hot but weak at the taps: the inlet strainer or a partly closed valve, not the cylinder.
• Water or steam from the discharge pipe: stop, isolate the heat source, keep clear.

Unvented cylinder not heating: the cheapest cause first

A programmer that has slipped to heating only, an immersion timer that is off, a tripped fuse or a switched-off isolator all leave a cold cylinder with nothing broken. A cylinder that looks dead can have a perfectly good element, with the real fault a timing setting or a blown fuse upstream.

After supply and settings come the controls. On an indirect cylinder, a motorised two-port valve that has not opened keeps hot boiler water out of the coil, and a tripped overheat thermostat shuts that valve. On a direct cylinder, a combined thermostat and cut-out that has tripped or failed isolates the element.

The element comes last. With the supply isolated and proved dead, an open circuit across its terminals means it has burnt out, and a reading from a terminal to earth means water has got into the sheath. The expected reading and how it is worked out are on hot water cylinder diagnosis. Replacing an element needs the cylinder drained below the boss.

If the cylinder heats but never reaches full temperature, or runs cold after a short draw, suspect a partly working element in a two-element cylinder or scale over the element. Hot water cylinder not heating up lays out what you can check without opening anything.

Why does the cut-out keep tripping?

A thermal cut-out stops the cylinder overheating, and resetting it fixes nothing while the cause remains. Even one trip means the water got hotter than the thermostat should allow, so note it and have it checked. A trip that returns is a clear fault, and the usual causes are a thermostat that no longer switches off, a motorised valve stuck open so the boiler keeps feeding the coil, or an element scaled up and running hot locally.

Unvented cylinders must carry safety devices beyond the ordinary thermostat under Part G of the Building Regulations, and the cut-out is one of them. It is never bypassed, and it is not reset again and again until it holds.

Note how often it trips and whether the taps run very hot when it does. The pattern points to the part. Unvented cylinder tripped cut-out covers the symptom in detail.

Weak flow from the hot taps

Weak hot water on an unvented cylinder is often not the cylinder. The strainer in the inlet group collects grit and scale and chokes the flow, usually after work on the water main. A partly closed stopcock or isolation valve, scale around old stopcock nuts or a blocked filter does the same.

Compare hot with cold at the same outlet. If both are weak, the supply is the problem. If cold is strong and hot is weak, the restriction sits between the cylinder inlet and the outlet. Low pressure from taps on an unvented cylinder goes through the order.

Does a Megaflo-type cylinder fail differently?

Branded unvented cylinders, including Megaflo-type units, fail in the same short list of ways: a tripped cut-out, a dead element or thermostat, a motorised valve that is not opening, or a fault in the inlet group. What differs is the parts. The data label gives the model, capacity and whether it is direct or indirect, and the manufacturer's installation manual gives the settings and the reset arrangements for that range. Use the rating on the label or in the manual for your own unit, not a figure from a website.

The cheap checks come first here too, as in Megaflo cylinder no hot water checks, and Megaflo error and cut-out covers what the symptoms mean.

Repair or replace, and where this page stops

Almost every fault above is a part. The cylinder itself is the question only when the shell, the coil or a weld is leaking, or the parts needed would cost more than the cylinder is worth given its age. The reasoning is in repair or replace a hot water cylinder, and a shell or coil leak leads to like-for-like cylinder replacement.

Heating and control faults on an unvented cylinder sit alongside a regular unvented cylinder service check, where the safety side is tested as well.