No hot water but heating works? Causes and fixes

If you have no hot water but the heating works, the boiler itself is firing, so the fault is in the part that sends heat to your taps. On a combi that is usually the diverter valve, the flow sensor or low mains flow. On a system or regular boiler with a cylinder, start with the programmer settings, then the cylinder thermostat and the motorised valve.

The good news is that a few of these are settings you can check in a couple of minutes. First, work out which type of boiler you have. No cylinder anywhere in the house means a combi. A large hot water tank in an airing cupboard means a system or regular boiler. Our guide to combi vs system vs regular boilers explains the difference.

Combi boiler: no hot water but heating works

A combi heats water only when you open a hot tap. Inside, a sensor spots the water moving and a valve switches the heat from the radiators to the tap. If either part fails, the heating carries on normally while the taps run cold.

Diverter valve

The diverter valve switches the boiler’s heat between the radiators and the hot water. If it sticks in the heating position, radiators stay hot and taps stay cold. A common clue is radiators warming up when you run a hot tap in summer. A diverter valve is inside the boiler, so it is a job for an engineer.

Flow sensor

The flow sensor, sometimes a small turbine, tells the boiler a hot tap is open. If it is clogged with limescale or debris, the boiler never knows you want hot water and does not fire for the tap. London’s hard water makes this more common. Again, it sits inside the casing.

Low mains flow

A combi needs a minimum flow of water before it will fire. If your mains pressure has dropped, a stopcock is half closed, or a tap aerator or shower head is blocked with scale, the flow may be too low to trigger it. Our post on low water pressure covers the causes.

Other combi causes

Limescale in the plate heat exchanger can give lukewarm water rather than none. The hot water temperature may simply have been turned down on the boiler controls. And if the boiler shows a fault or low pressure warning, deal with that first.

What to check on a combi

1. Look at the display for a fault code or warning light.
2. Check the pressure gauge. It should read about 1 to 1.5 bar with the heating cold. If it is low, see how to repressurise a combi boiler.
3. Check the hot water temperature setting on the boiler is turned up.
4. Try several hot taps. If one tap works and another does not, the problem is that tap or its aerator, not the boiler.
5. Open the hot tap fully. A trickle may be below the flow the boiler needs to start.
6. Check the cold mains flow at the kitchen tap. If it is weak, check the stopcock is fully open.
7. Reset the boiler once, using the reset button. If it fails again, stop and call us.

System or regular boiler: no hot water but heating works

With a cylinder, the boiler heats a coil inside the tank, controlled by a programmer, a thermostat on the cylinder and a motorised valve that opens the route to the tank.

Programmer or timer settings

This is the most common cause and the easiest to fix. Check the hot water channel is set to on, auto or timed, not off. After a power cut, the clock may have reset and be running at the wrong time, so the hot water comes on in the middle of the night. Try the boost or advance button for an hour and see if the cylinder warms.

Cylinder thermostat

A small dial strapped to the side of the cylinder tells the system when the water is hot enough. If it has been knocked down or has failed, the system thinks the water is already hot. Check it is set to around the temperature recommended in your system’s instructions. Stored hot water needs to be kept hot enough to control legionella, so do not set it low to save energy without advice.

Motorised valve

A 2-port or 3-port motorised valve, usually near the cylinder or boiler, opens to send hot water to the cylinder. If its motor or actuator fails, the valve stays shut to the hot water side while the heating carries on. Some have a manual lever, but leave it to an engineer to check, as the head is wired in.

Immersion heater as a backup

Most cylinders have an electric immersion heater. Switching it on for an hour or two is a good way to get hot water while you wait for a repair. If the immersion does not work either, see our guide to an immersion heater not working.

What to check on a system or regular boiler

1. Check the hot water channel on the programmer is on or timed.
2. Check the clock shows the right time and day.
3. Press boost and wait an hour.
4. Check the cylinder thermostat has not been turned down.
5. Feel the pipe going into the cylinder after boost. If it stays cold while the radiators are hot, the motorised valve is the likely culprit.
6. Use the immersion heater as a backup.

When to call an engineer

• The boiler shows a fault code or keeps locking out
• The diverter valve, flow sensor or motorised valve is the likely cause
• Settings are correct and there is still no hot water
• Water is leaking from the cylinder, or the unvented cylinder’s relief pipe is discharging
• You smell gas. Leave the property and call 0800 111 999

If your heating works but hot water does not, or the other way round (see hot water but no heating), call Lee on 020 8050 5913. He will often pin it down over the phone, and if he can talk you through it there is no charge. For a visit, book a boiler repair.