No Hot Water but Heating Works?
When the radiators heat up but the taps run cold, the boiler is working; it is the part that sends heat to your hot water that has failed or been switched off. On a combi that is usually the diverter valve. On a system or regular boiler with a cylinder, it is usually the programmer, the cylinder thermostat or a motorised valve. Here is how to narrow it down.
Start with your boiler type
| Boiler | Most likely causes | Check first |
|---|---|---|
| Combi | Diverter valve, low mains flow, plate heat exchanger, flow sensor | Pressure gauge, and whether any hot tap works at all |
| System or regular (with a cylinder) | Programmer hot water setting, cylinder thermostat, motorised valve | Programmer and cylinder thermostat settings |
Not sure which you have? If there is a hot water cylinder in an airing cupboard, it is a system or regular boiler. If not, it is almost certainly a combi; our guide to combi vs system boilers explains the difference.
On a combi: the diverter valve
A combi has one heat source and a valve that sends the heat either to the radiators or to the hot water. When you open a hot tap, the diverter valve should switch to hot water. If it sticks in the heating position, the radiators stay hot and the taps run cold. It is one of the most common combi repairs, and it is a job for an engineer, because it means opening the boiler. Before you call, check the pressure gauge is in the normal range, and try more than one hot tap: if only one tap is cold, the problem is that tap or its pipework, not the boiler.
On a combi: flow and the hot water side
A combi needs a minimum flow of water through it before it fires for hot water. A partly closed stopcock, a blocked tap filter or very low mains pressure can stop it detecting the flow. A plate heat exchanger blocked with debris gives lukewarm water that never gets properly hot. The flow sensor that tells the boiler a tap is open can also fail. An engineer checks each of these in turn.
On a cylinder system: the controls
Most of the time the cause is simpler than it looks. Check the programmer has hot water switched on and the times are right, especially after a power cut or a clock change. Check the cylinder thermostat, a small dial strapped to the side of the cylinder, has not been turned down. If both are fine, the motorised valve that directs heat to the cylinder may have stuck or its actuator failed, which is a common repair on regular and system boilers. If you have an immersion heater, switching it on will get you hot water while you wait for an engineer.
Lukewarm rather than cold
If the hot water comes through warm but never properly hot, the cause is usually different from a total failure. On a combi, a plate heat exchanger part-blocked with debris, a tap filter restricting flow, or the hot water temperature set low on the boiler are the common causes. On a cylinder system, check the cylinder thermostat setting, and remember stored water can run out after several showers in a row; a cylinder that takes far longer than it used to to reheat may have a coil furred with scale or sludge.
When to call an engineer
If the settings are right and the pressure is normal, it is a component fault. Diverter valves, motorised valves, actuators and cylinder thermostats are routine repairs, usually done in one visit once the part is confirmed. Book a boiler repair and the engineer will tell you the cost before doing the work. If the problem is the opposite, hot water but cold radiators, our guide to a radiator not heating up covers that case.
Common questions
Why do I have heating but no hot water on my combi?
Most often the diverter valve has stuck in the heating position. Low mains flow, a blocked plate heat exchanger or a failed flow sensor can also cause it. Check the pressure gauge and try more than one hot tap, then call an engineer.
Can I fix a diverter valve myself?
No. It is inside the boiler, so replacing it means opening the case and working on the appliance. That should be done by a Gas Safe registered engineer.
My cylinder system has heating but no hot water. What should I check?
The programmer's hot water setting and times, the cylinder thermostat dial on the side of the cylinder, and whether the immersion works as a backup. If those are fine, the motorised valve or its actuator is the likely fault.
How long does it take to fix no hot water?
Diverter and motorised valve repairs are usually done in one visit once the fault is confirmed and the part is available. The engineer confirms the part and the cost before starting.
Updated 15 October 2026
