No Hot Water but the Heating Works: Check the Cylinder

Why is there no hot water when the heating works?

A cylinder system runs two separate demands from one boiler. The heating zone calls for heat from the radiators. The hot water zone calls for heat from the cylinder. They share a boiler and a pump but they use different programmer channels, different thermostats and, in most homes with a cylinder, different motorised valves. One can fail while the other carries on, which is why the radiators can be fine while the taps run cold.

That also explains why most of the advice you will find for this symptom is about boilers. If you have a combination boiler there is no cylinder and the fault is inside the boiler. If you have a cylinder, the boiler is often working perfectly and is simply never being told to heat the water. This page assumes a cylinder, either a vented one fed from a loft tank or an unvented one fed from the mains.

Check which heat source you rely on before anything else. An indirect cylinder is heated by the boiler through an internal coil. A direct cylinder is heated only by an electric immersion heater. Many homes have an indirect cylinder with an immersion heater as a back-up. If you only ever relied on the boiler and the immersion switch has been off for years, that is why the back-up did not rescue you.

What can I check safely before calling anyone?

Start at the programmer or room-and-cylinder controller, because a hot water channel set to off, or a timer that has drifted after a power cut, is the most common cause and costs nothing to fix. Then look at what you can see without opening anything.

• Programmer: confirm the hot water channel is on, not set to off or to a time that has already passed, and press the hot water boost if there is one.
• Cylinder thermostat dial: if there is an external dial on the cylinder or its pipework, check it has not been turned right down.
• Immersion switch: if the cylinder has an immersion heater, check the labelled double-pole switch or fused spur is on and its neon, if it has one, is lit.
• Consumer unit: look for a tripped breaker or RCD on the immersion circuit. Do not keep resetting it: if it has tripped, leave it off and have the circuit tested.
• Pipes: with the hot water calling, touch the two pipes that run from the boiler towards the cylinder briefly with the back of your hand, because they can be hot enough to scald. A flow pipe that is hot all the way to the cylinder means the boiler and valve are doing their job.
• Two-port valve: if you can see the valve head, check whether its lever or indicator moves when the hot water calls. Do not force it.

What does each result point to?

The pipe test does most of the sorting. The boiler sends hot water along a flow pipe to the cylinder coil and takes the cooled water back on a return pipe. Where that journey stops tells you which part to suspect. These pairings are the usual ones, not guarantees.

• Boiler fires for the hot water call but the flow pipe near the cylinder stays cold: the two-port valve is stuck shut, its motor has failed, or the valve is open but the pipe run is blocked.
• Boiler never fires for the hot water call but fires for heating: the cylinder thermostat is not calling, the programmer hot water output is dead, or the valve end switch is not making contact.
• Flow pipe hot and return pipe hot but the cylinder stays cold: the coil is not passing heat into the stored water. Heavy scale on the coil, or an air lock in the coil circuit, is the usual cause.
• Flow pipe hot but return pipe cold: water is not circulating through the coil, which points to a blockage or a circulation fault.
• Heating, hot water and the immersion all dead together: look for a blown fuse or tripped breaker first, because one electrical fault can take out the back-up as well.

What does an engineer check at the cylinder?

The order matters because each step rules out a whole branch. The expert first confirms that the hot water call actually leaves the programmer, then checks the cylinder thermostat with the circuit isolated, measuring whether its contacts close when the stored water is cold. A thermostat that never closes can be a fault in the stat, or in the wiring between the stat and the valve. That is a common reason a perfectly good valve sits shut.

Next comes the two-port valve, the motorised valve that opens the hot water pipe to the coil. The engineer checks that the head drives open, that the valve body behind it moves when the head does, and that the end switch inside the head closes so the boiler gets its fire signal. A head that whirs but does not move the body points to a seized valve body. A head that does not move at all points to a failed motor or no supply.

If the valve opens and the boiler fires, the check moves to the coil. The engineer reads the flow and return temperatures at the cylinder, compares them with the cylinder surface temperature, and listens for gurgling that signals trapped air. A flow pipe that is very hot with a return pipe only warm means heat is going in. A cold return means it is not.

On an immersion-only or back-up circuit the engineer tests the supply, the switch, the thermostat and the element separately. Our guide to what to test on an immersion heater sets that sequence out in the order it is done. If you want the whole sequence for a cylinder in one record, the hot water cylinder diagnosis page explains how a diagnosis moves from symptom to confirmed cause.

Is the cylinder itself the problem?

Usually not. The parts that fail most often on this symptom are cheap and sit outside the cylinder: the programmer output, the thermostat, the valve head and the fuse. A failed coil is the one exception where the cylinder carries the fault. It shows as a hot flow pipe and a cold return, or as water in the primary circuit that keeps needing topping up. Our cylinder heating coil guide covers how that presents.

Replacement becomes the sensible answer when the coil is leaking or blocked on a cylinder that is already old, or when the shell shows weeping at the seams. Where only a control part is at fault, the right outcome is to fit that part and leave the cylinder alone. If the symptom comes back after a part has been changed, the diagnosis has missed something, which is a reason for a second look rather than a bigger repair.

The answer is often mundane. A no-hot-water fault can end at a programmer timing change, or at a blown fuse that was never replaced after an earlier electrical visit. Neither involves a failed cylinder. Working out which branch you are on first saves paying to replace the wrong part.