Hot Water Only Works When the Heating Is On? Here Is Why

First, rule out the settings

Set the programmer so the hot water channel is on constant and the heating channel is off. Turn the cylinder thermostat, the dial strapped to the side of the cylinder, up to around 60. Then listen. Within a minute or two of the hot water being called, you should hear a motorised valve move in the airing cupboard or near the cylinder and the boiler fire.

If nothing happens at all, check the programmer is actually switching the hot water. Many programmers show a small light or symbol when a channel is on. Wireless controls can lose their pairing with the receiver next to the boiler, and flat batteries in a wall unit cause the same thing. If the controls say hot water is on and the boiler still does not fire until the heating is switched on too, you have a genuine fault rather than a setting.

Why the heating can heat the cylinder but the hot water cannot

On a system with a cylinder, the boiler only fires when something tells it there is a demand. That signal does not come directly from the programmer. It comes through the motorised valves: when a valve has physically opened, a small switch inside its head closes and passes the demand on to the boiler and pump. That design stops the boiler firing against closed valves.

If the hot water valve is open but its switch has failed, the boiler never gets the hot water demand. Nothing heats. Then, when the heating is switched on, the heating valve opens, its switch works, and the boiler fires. If the hot water valve body is stuck open, or its manual lever has been latched open, boiler water now flows through the cylinder coil as well as the radiators. The cylinder heats, but only because the heating is running. That is the most common explanation for this exact symptom. It is the mirror image of no hot water but heating works, where the hot water side of the same valves fails the other way.

Two port valves: a failed switch or a latched lever

Many homes with a cylinder have two separate two port valves, one for heating and one for hot water, usually on the pipework near the cylinder or boiler. Each has a head with a manual lever on the side. Look at the hot water one. If the lever is pushed across and latched in the manual open position, which people sometimes do to get hot water during an earlier fault, the valve will stay open permanently and the symptom follows.

If the lever is in the auto position, the valve head may still be the problem. The motor can fail, the switch inside can fail, or the brass spindle can seize. A head that feels warm and hums without moving, or one that never clicks when the hot water is called, has usually failed. The head can often be changed without draining the system, which makes it one of the cheaper fixes on the hot water side.

Do not leave a valve lever latched in manual as a long term fix, and do not bridge wires in the wiring centre to force the boiler on. It can leave the boiler firing with no control over the cylinder temperature.

Three port valves and wiring faults

Other homes have a single three port valve that sends boiler water to the heating, the cylinder or both. Its motor, its internal switch and its wiring all have to agree for the boiler to fire on a hot water demand. A failing motor, a worn switch or a loose connection in the wiring centre can leave the valve and the boiler responding properly only when the heating is also calling.

These faults take testing rather than guessing. An engineer will check what each wire is doing with the hot water on and the heating off, and work back from the boiler to the valve, the cylinder thermostat and the programmer. It is the same chain described on our page for a hot water tank not heating, approached from the opposite direction.

The cylinder thermostat is not calling

The cylinder thermostat tells the system when the stored water needs heating. If it has failed, or been set below the temperature of the water already in the cylinder, it never asks for heat. With the heating on, the cylinder can still pick up heat through a valve that is not fully closing, which again makes it look as though the hot water only works with the heating.

Turn the dial right up and listen for a click. No click at any setting, or a click with no response from the valve or the boiler, points at the thermostat or its wiring. Some older systems have no cylinder thermostat at all, which is worth knowing because it changes the diagnosis.

Older gravity systems work differently

Some older houses still have a gravity hot water system, where the cylinder is heated by natural circulation from the boiler and only the heating is pumped. On those, the cylinder heats whenever the boiler runs, and the programmer is often set so the heating cannot run without the hot water. That is the design, not a fault. But the hot water should still heat on its own. If it does not, the same checks apply, and a gravity system is often a good candidate for upgrading the controls.

On a combi boiler

A combi has no cylinder and no hot water channel. It fires whenever a hot tap is opened and a flow sensor detects water moving. If you only get warm water while the heating is running, the boiler is almost certainly not detecting the tap. The heat you are getting is only because the boiler is already hot from the heating.

The usual causes are a failed flow sensor, a blocked filter on the cold water inlet, or a diverter valve that is not moving to the hot water position. They are boiler repairs for a Gas Safe engineer, and the full picture is on our page on no hot water from a combi boiler.

When to call us

Call when the settings are right and the boiler still will not fire for hot water on its own, when a valve head hums or does not move, or when you have found a lever latched in manual and want the fault behind it fixed properly. It helps to tell us whether you have a combi or a cylinder, whether there are one, two or three valves near the cylinder, and what happens when you switch the hot water on by itself.

We have 20 engineers across London and hot water systems are our core work, so a motorised valve, a cylinder thermostat or a wiring centre fault is usually found and fixed on one visit. Our rate is on the pricing page and agreed before we attend.