Glossary · Faults, servicing and gas safety
Why do I have hot water but no heating?
Hot water but no heating means the boiler is heating the taps but the radiators stay cold. The cause is often a setting, such as the timer, thermostat or flow temperature, or low pressure or air in the system. On a combi boiler it can also be a faulty diverter valve, which needs an engineer.
Key points
- Check the simple things first: heating switched on, not held off by the timer, and the room thermostat calling for heat.
- Glow-worm says a flow temperature of 60 to 70°C makes sure the water is warm enough when it reaches the radiators.
- Glow-worm gives 1 to 2 bar with the heating on as a general guide, and says to check your manual for your model.
- On a combi boiler, Vaillant says hot showers with cold radiators can point to a faulty diverter valve.
- Work on the diverter valve or anything inside the boiler is gas work for a Gas Safe registered engineer.
What causes it
A boiler can be working for hot water and still send nothing to the radiators. Glow-worm says this is sometimes just the central heating controls being set wrongly, and sometimes a fault in the system.[1]Glow-worm, Why is my boiler giving me hot water but no heating? Vaillant adds that the boiler may be set to heat hot water only, or set to come on at an unexpected time.[3]Vaillant, Why is my radiator not working?
What you can check yourself
These are checks Glow-worm and Vaillant describe for homeowners.
| Check | What to look for |
|---|---|
| Boiler settings | Central heating switched on and not held off by the timer. Glow-worm says to look at the display. |
| Flow temperature | Glow-worm says setting it to between 60 and 70°C makes sure the water is warm enough at the radiators. See flow temperature. |
| Radiators and valves | Check all radiators and any thermostatic valves are turned up. Warm radiators with cold spots point to trapped air. |
| Room thermostat | Glow-worm says setting it to between 18 and 21°C should heat a home to a comfortable level. For a wireless one, check the batteries and position. |
| Boiler pressure | Glow-worm gives 1 to 2 bar with the heating on, and 1 to 1.5 bar with it off, as a rule of thumb. Your manual gives the figure for your model. |
If the pressure is low, Glow-worm says the system can be topped up using the filling loop, but the steps differ by boiler and you should follow your manual. It adds that low pressure is not dangerous but can be a sign of a fault such as a small leak.[1]Glow-worm, Why is my boiler giving me hot water but no heating? See low water pressure.
The diverter valve on a combi
Vaillant explains that a combi boiler has a diverter valve that sends hot water either to the radiators or to the taps and showers. When a hot tap is on, it prioritises the tap over the radiators. System and conventional boilers do not have a diverter valve because they store hot water in a cylinder.[2]Vaillant, How to fix a boiler diverter valve
Vaillant lists three signs of a faulty one: hot showers but cold radiators, only lukewarm water from the taps, and hot water that works only when the central heating is on.[2]Vaillant, How to fix a boiler diverter valve
When to call an engineer
If the checks above do not bring the heating back, call a Gas Safe registered engineer. Vaillant recommends a qualified engineer for a stuck diverter valve and says any work on a gas appliance is dangerous and can damage the boiler further if not handled carefully.[2]Vaillant, How to fix a boiler diverter valve HSE states that anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work.[4]Health and Safety Executive, Domestic gas: frequently asked questions
Vaillant also says that if several radiators are not heating, the cause may be the boiler itself, and the error code on the display should be checked against the manual.[3]Vaillant, Why is my radiator not working?
Example
Sunita in Bradford has a combi boiler. Her shower is hot but the radiators are cold. She checks that the heating is on, that the timer is not holding it off and that the thermostat is set to 20°C. The pressure gauge reads normally. Nothing changes, so she books a Gas Safe registered engineer to check the diverter valve and the rest of the boiler.
What people get wrong
Many people assume that hot water working means the boiler is fine, so the radiators must be at fault. Glow-worm’s advice is to rule out the settings, thermostat and pressure before assuming a fault, and to check whether it is one radiator or all of them.
The opposite mistake is to open the boiler to look at the valve. Vaillant’s own guide says to call a Gas Safe registered engineer.[1]Glow-worm, Why is my boiler giving me hot water but no heating?[2]Vaillant, How to fix a boiler diverter valve
The bottom line
Hot water but no heating is often a setting, a thermostat or a pressure problem you can check from the front of the boiler. On a combi boiler it can also be a diverter valve that is stuck. Once the simple checks are done, leave the diagnosis and any work inside the boiler to a Gas Safe registered engineer.
Common questions
Why does my hot water work but not my radiators?
The boiler is firing for hot water but heat is not getting to the radiators. Glow-worm says to check the heating settings, the thermostat, the radiators and valves and the boiler pressure. A combi diverter valve can also be the cause.
What is a diverter valve?
Vaillant says it is a mechanism in a combi boiler that opens or closes to send hot water either to the radiators or to the taps and showers. System and conventional boilers do not have one.
Can I free a stuck diverter valve myself?
No. Vaillant recommends a qualified heating engineer. HSE says anyone working on gas appliances in domestic premises must be a Gas Safe registered engineer, and the diverter valve is inside the boiler.
Could it be the thermostat?
It could. Glow-worm says that if the boiler settings are right and the heating still does not work, the room thermostat could be faulty. For a wireless one, check the batteries and its position.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Glow-worm. Why is my boiler giving me hot water but no heating?. Checked 11 November 2026. ↩
- Vaillant. How to fix a boiler diverter valve. Checked 11 November 2026. ↩
- Vaillant. Why is my radiator not working?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩