Hot Water Cylinder Not Heating Up: Which Heat Source Failed?

What would cause a hot water tank not to heat up?

A cylinder heats from one of two sources, an electric immersion element or a coil fed by the boiler, and some have both. A failure falls into one of three groups: the heat source has stopped (a burnt-out element, a coil with no flow), the control that switches it has stopped (the thermostat on the cylinder, a programmer output, a motorised valve), or the power or fuel behind it has stopped (a fuse, a breaker, an isolated boiler).

The important point is to work out which heat sources your cylinder has, because the same symptom means different things. If only one of two sources has failed, the cylinder may heat slowly or only partly, and the fault is easy to miss because the other source keeps it lukewarm. A cylinder that never rises above warm often has one source working and one dead.

For a no-heat symptom with working radiators, start with the hot water circuit checks. This page assumes you have already confirmed the controls are on and that the cylinder is the thing not warming.

How do I tell which heat source is the problem?

Run each source alone so the other cannot hide the fault. If the cylinder has an immersion, switch the boiler hot water channel off and the immersion on for a few hours, using the usual switch only. Then do the reverse. The source that raises the temperature is working. The one that does not is the suspect.

Use the back of your hand on the side of the cylinder, top and bottom, and the pipe leaving the top. A boiler coil heats from the lower middle upwards. A top-entry immersion heats the water above the element first, while a longer element reaches near the base. A cylinder that is warm at the top and cold below, when the element is a short top-entry one, is behaving normally. If the element is a full-length one and the base stays cold, it has failed partway or is badly scaled.

• Immersion on, boiler off, cylinder heats: the element and its thermostat work. The boiler side or its controls are at fault.
• Immersion on, boiler off, no heat: the element, thermostat, high-limit cut-out or supply is at fault.
• Boiler on, immersion off, cylinder heats slowly: the coil is delivering but poorly, which suggests scale on the coil or a restricted flow.
• Boiler on, no heat, and the flow pipe stays cold: the two-port valve or the call for hot water has failed.
• Both sources heat but the water cools quickly: the cylinder may be under-sized for the household, or the insulation may be poor.

How do I test a cylinder thermostat?

A cylinder thermostat is a switch that opens when the stored water reaches the set temperature and closes when it cools. Testing it means measuring whether its contacts close on a cold cylinder, with the circuit isolated and proved dead. That is a job for an engineer or electrician, not something to do by feel with the cover off.

There are two common types. A strap-on thermostat clips to the cylinder side or pipework and controls the boiler call. A rod thermostat sits in a pocket inside the immersion boss and controls the element. Their failures differ. A strap-on can lose contact with the cylinder wall and read wrongly, which stops the boiler heating. A rod thermostat can stick closed, which overheats the water, or stick open, which means no heat at all.

What you can do safely is check the dial. If someone has turned it to its lowest setting after cleaning or a holiday, the cylinder will never call. If the dial is where it should be and the cylinder is cold, the result still points to a faulty thermostat, element or supply, not to a replacement cylinder.

Is it a failed element or a stuck valve?

The two look alike from the taps, since both give cold water, but they show up differently at the cylinder. A failed element leaves the immersion circuit with power present and no temperature rise. A stuck two-port valve leaves the boiler side with a cold flow pipe and a boiler that never fires, or fires with no hot water going anywhere.

So the valve check is whether the boiler runs when the hot water calls and whether the flow pipe near the cylinder warms. The element check is whether it draws current and whether the cylinder warms on immersion alone. An engineer measures the element resistance with power isolated. A three-kilowatt element on a 230 volt supply should read in the region of 17 to 18 ohms, an open circuit meaning it has burnt out. Our guide on a stuck two-port valve covers what the head and body should do.

A third possibility is an air lock or scaled coil that stops heat transfer without any control fault. That shows as a hot flow pipe and a cold or warm return. It sits between the valve and the cylinder, and our heating coil guide explains how it presents.

Repair or replace?

Most of these faults are repairs. A thermostat, an element, a valve head or a fuse is replaced and the cylinder stays. The exception is a coil that has failed or blocked beyond cleaning, or a cylinder so scaled and old that further parts are likely to fail soon, which turns into a replacement discussion once the cause is confirmed.

Replacement is not the starting point. A cylinder that is cold because a thermostat has failed is not a failing cylinder, and replacing it would cure nothing the thermostat did not. If you are unsure which of the heat sources has failed, a hot water cylinder diagnosis sets out the order in which each is tested, so the fault is named before any part is chosen.