Why Do I Have Hot Water but No Heating?

If your hot water is working perfectly but the radiators stay cold, it is a strong clue that the boiler itself is running but something in the heating side of the system is not doing its job. The good news is that many of the first checks are simple and safe for homeowners to try.

How your heating can fail while hot water still works

Modern boilers often give priority to hot water, so the boiler will happily heat your taps while a fault leaves the radiators cold. This usually means a problem with controls, valves or circulation rather than the gas supply or burner.

The likely causes depend on whether you have a combi boiler or a system/regular boiler with a hot water cylinder. Knowing which you have helps narrow things down, but many checks are the same for both types.

Most common causes when the heating is not working

Several typical faults can stop your radiators heating up even though the boiler still produces hot water. Some are quick fixes, while others need a qualified heating engineer.

• Room thermostat or programmer issues that mean the boiler never gets the signal to heat the radiators

• Motorised valve faults on system boilers that prevent hot water flowing to the heating circuit

• Circulation pump problems that stop hot water moving through the radiators

• Airlocks or low pressure that limit the flow of water in the system

• Diverter valve stuck in hot water position on combi boilers

Thermostat and programmer not calling for heat

If your thermostat or timer is not set correctly, the boiler may never be told to switch the heating on. This is especially common after power cuts, battery failures or clock changes.

As a quick test, turn the room thermostat up to maximum and make sure the heating is set to "on" or "continuous" on the programmer. If the boiler still does not fire for heating, there may be a wiring or control fault that needs attention.

Motorised valve stuck on hot water only

On system and regular boilers, a motorised valve directs hot water either to the cylinder, the radiators or both. If the valve sticks or the motor fails, it may only send heat to the cylinder.

You might hear the boiler start when hot water is called but not when the heating is timed to be on. The valve itself is usually near the hot water cylinder and pipework, and any repairs or replacement should be handled by an engineer.

Circulation pump struggling or failed

The circulation pump pushes hot water around the heating circuit. If it is seized, on the wrong speed or heavily sludged up, your radiators will stay cold or only warm slightly.

Warning signs include a very hot pump body, loud humming, or the boiler cycling on and off quickly because the heat is not moving away. Pump work involves electrics and water, so it is not one to tackle yourself.

Airlocks and system pressure problems

Air trapped in the pipework can block circulation and cause cold radiators. In sealed systems, low pressure can have a similar effect, as there is not enough water for the pump to move around.

If the top of your radiators is cold while the bottom is warm, or you hear gurgling noises, air in the system is quite likely. Combined with a low pressure gauge reading, this points to a problem best checked sooner rather than later.

Combi boiler diverter valve stuck on hot water

In combi boilers, a diverter valve sends hot water either to your taps or to the radiators. If it gets stuck in the hot water position, you will get lovely showers but no heating at all.

You may notice the boiler only fires when a hot tap is opened, and not when the heating is switched on. Diverter valves often need cleaning, servicing or replacement by a Gas Safe engineer.

Safe checks you can try before calling an engineer

There are a few sensible checks you can carry out yourself without tools or specialist knowledge. If at any point you feel unsure, stop and speak to a professional.

• Check thermostat batteries and settings: Replace batteries if the display is faint or blank. Ensure the temperature is set higher than the current room temperature.

• Confirm the programmer is on heating: Make sure the heating channel is set to "on" or "timed" and that the current time and day are correct.

• Look at the pressure gauge: On sealed systems, this is usually on the front of the boiler. Most systems work best around 1.0 to 1.5 bar when cold. If it is very low, check your manual for topping up instructions or call an engineer.

• Bleed one radiator to test for air: With the heating off and the radiator cool, use a radiator key on the bleed valve and release air until water appears, then close gently. Have a cloth ready for drips.

• Reset the boiler once only: Follow your manufacturer’s instructions and do not repeatedly reset a boiler that keeps locking out, as this can be unsafe.

If these steps do not restore your heating, or if anything looks or sounds unusual, it is best to stop further attempts and get expert help.

When to call a heating engineer

Some symptoms point to issues that need a qualified engineer rather than DIY checks. Ignoring them can worsen the fault or even create safety risks.

You should arrange a professional inspection if you notice any of the following:

• Several or all radiators stay completely cold despite your checks

• The boiler makes unusual banging, whistling or grinding noises

• The boiler frequently locks out or needs resetting again and again

• The pressure keeps dropping, even after topping up

These can indicate deeper problems such as pump failure, blockage, serious air ingress or leaks that need tracing and repair.

How annual servicing helps prevent heating problems

A regular boiler service is not just about ticking a safety box. It is also a chance to catch early signs that the heating side of the system is struggling.

During a service, an engineer can listen for subtle pump noises, check filters and strainers for signs of sludge, and look at indicators on system filters that show partial blockages. They can also test motorised valves and controls to ensure they respond properly.

Picking up these issues early reduces the risk of waking up to a cold house with only the shower working. It also keeps your system running more efficiently, saving wear on components and helping to keep bills under control.

Next steps and where to get more help

If you have worked through the basic checks and your heating still will not come on, it is time to book a professional repair visit. For more detailed help with specific radiator issues, you can also read our radiators not heating guide for further troubleshooting tips and explanations.

To get your home warm again safely and efficiently, contact Westhill Boiler Services on 01224548803 to book a boiler repair appointment. A qualified engineer can diagnose the cause, fix the fault properly and advise on any improvements to keep both your heating and hot water running reliably.