Why do I have hot water but no heating?

Hot water working while heating does not is a very specific symptom, and it narrows the possibilities considerably. It means the boiler is capable of firing and the fault is somewhere in the heating side's controls or the valve that directs the heat.

Safety

What you'll need

• The programmer's instructions if available
• Spare batteries for a wireless thermostat
• A bleed key

Step by step

1. Check the programmer is calling for heating.It is a separate channel from hot water. Confirm the heating channel is on now, not just that hot water is working, and check the clock is showing the right time and the right half of the day.
2. Override the schedule to test.Set the programmer to constant or use the advance button so heating is definitely being called for while you test the rest.
3. Turn the room thermostat well up.Set it several degrees above the current room temperature and listen. Many thermostats click audibly when they call. No response at all is significant.
4. Change the thermostat batteries.A wireless thermostat with weak batteries loses its link to the receiver and simply stops calling. It is the commonest cause of this exact symptom and takes two minutes.
5. Check the system pressure.On a sealed system, a gauge below about 0.5 bar will lock the boiler out of heating. Repressurise to about 1.2 bar cold if it is low.
6. Check every TRV is open.If every radiator's valve happens to be closed or seized shut, the boiler has nowhere to send heat. Open them all fully as a test.
7. Feel the pipes at the boiler.With heating called for, the flow pipe to the radiators should get hot. If the boiler fires and only the hot water side warms, the diverter or zone valve is the suspect.
8. Listen for a zone valve.On a system with a cylinder, motorised zone valves are usually near the cylinder. A valve that hums but does not move, or a lever that is stuck, is a common failure.
9. Check the boiler for a fault code.Note any code on the display exactly, and check it against the manual. Do not open the casing.
10. Stop and call an engineer.If the controls all check out and the boiler still will not heat the radiators, the fault is in the boiler or its valves. Tell the engineer everything you have already eliminated — it shortens the visit.

Tips

• Change the thermostat batteries before anything else. It is free, takes two minutes, and is the single most common cause.
• Write down what you have checked before phoning. An engineer who knows the controls are proven can go straight to the valve.
• Note the exact fault code if there is one. Different codes point at completely different components.

Common mistakes

• Assuming hot water working means the boiler is fine — A combi's diverter valve directs heat either to the hot water plate or to the radiators. It can work perfectly on one side and be stuck on the other, which is exactly this symptom.
• Opening the boiler casing to investigate — It is a gas appliance. Removing the casing is Gas Safe work, it invalidates any warranty, and there is nothing behind it a householder may adjust.

If it doesn't work

Questions people ask