Hot water but no heating? Causes and what to check

This guide is for a boiler that makes hot water but won't heat the radiators. No hot water either? Then the boiler has stopped, so see our guide to a boiler that won't turn on. If the checks below don't help, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

What should I check first?

Check the heating settings first: the thermostat, the timer and the boiler's own controls. A setting that isn't asking for heat is the most common reason for hot water but no heating. Then check the pressure and any fault code. Try the heating again after each step.

1. The boiler controls. Many boilers have a "summer" or "hot water only" setting. Make sure the heating is switched on at the boiler, and the heating dial isn't turned right down.
2. The timer or programmer. This tells the heating when to come on. Check the heating channel is set to "on" or "auto", not "off". Check the clock shows the right time. It can drift after a power cut or when the clocks change.
3. The thermostat. Set it a few degrees above the room temperature. If it runs on batteries, change them. A dead screen means it can't send a signal.
4. A smart or wireless thermostat. These talk to a small receiver box, usually near the boiler. Check its light shows it's linked. Check your app and home internet are working. The manual shows how to link them again.
5. The radiator valves. TRVs are the numbered dials on the side of radiators. TRV means thermostatic radiator valve. If they're all turned to 0 or the frost symbol, the radiators stay cold.
6. The pressure. With the heating off, the gauge should usually read about 1 to 1.5 bar. If it's low, see our guide to a boiler losing pressure.
7. A fault code. Write down any code or flashing light, and the boiler make and model. Try the reset button once, as the manual shows.

What do the signs tell you?

What you notice often points to the cause. If the boiler fires but the radiators stay cold, the hot water isn't getting round the pipes. This table sums up the most common signs.

Why won't my heating come on after the summer?

After months without use, a part can stick. The pump and the heating valves are the parts most likely to stick over the summer. Air can also build up in the radiators, and the pressure may have dropped. So the first cold day is often when faults show up.

• The pump. This pushes hot water from the boiler round your radiators. If it sticks, the boiler may heat up and then shut down, while the radiators stay cold.
• A valve. A heating valve can stick shut after a long rest. So can the pin inside a TRV. Take the dial off a TRV and see if the small pin moves when you press it. If it stays down, a plumber can free or replace it.
• Air. Radiators cold at the top usually have air in them. See how to bleed a radiator in our guide to cold radiators.
• Pressure. Bleeding radiators lets out water too, so check the gauge afterwards. Our guide to a boiler losing pressure shows how to top it up.

A useful tip: turn the heating on for a short while in early autumn, before you need it. Then any fault shows up on a mild day, not the first freezing night.

Why do I have hot water but no heating on a combi boiler?

On a combi boiler, a diverter valve stuck on hot water is a common cause. This valve inside the boiler sends hot water either to your taps or to your radiators. If it sticks on the taps side, the hot water works, but the heating never does.

A combi boiler heats water straight from the mains when you run a tap, so there's no tank. On most combis, the pump is inside the boiler case too. A failed pump or diverter valve needs a Gas Safe engineer. Don't take the case off to look.

Why does it happen with a hot water cylinder?

If you have a hot water cylinder, a tank usually in the airing cupboard, look at the motorised valve and the programmer. A motorised valve stuck shut is a common cause on these systems. It can let hot water reach the cylinder, but not the radiators.

• The motorised valve, also called a zone valve. It sits on the pipes, often near the cylinder. A small motor in its head opens it to send hot water to the radiators, the cylinder or both. If it sticks, the heating stays cold.
• The programmer. On these systems it has separate heating and hot water settings. Check the heating setting is on as well as the hot water.
• The pump. On a system or regular boiler, the pump is often outside the boiler, near the cylinder. If it's stuck or broken, the radiators stay cold.

Don't take the cover off a motorised valve or pump. They have mains wiring inside. If you have the reverse problem, heating but no hot water, see our guide to heating but no hot water.

Is it the thermostat or the boiler?

A simple test is to turn the thermostat right up. If the boiler never fires for the heating, the thermostat, timer or a setting is the likely cause. If it fires but the radiators stay cold, the problem is the pump, a valve or the boiler.

Many thermostats click when you turn them up, or show a flame symbol. That means they're asking for heat. If you see that, but the boiler doesn't respond, the wiring or the receiver may be at fault. That needs someone safe with electrics and boilers. We're both Gas Safe engineers and electricians, so one engineer can check both.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the checks above haven't helped, or the boiler locks out again after one reset.A stuck pump, valve or diverter valve won't fix itself.

• the settings, thermostat and pressure are all fine, but there's still no heating
• the boiler fires, but the radiators and pipes stay cold
• you hear a hum or buzz from the pump, but no heat comes through
• the boiler locks out again after a reset, or the same fault code comes back
• you smell burning, or see soot or black marks round the boiler

The engineer first runs a thorough check of the boiler and the heating system. That finds the fault and the parts needed. We repair and replace central heating pumps, zone valves and TRVs. See our boiler repair page for the faults we fix. We repair all makes.

A yearly boiler service helps stop many of these faults. The engineer checks the boiler is safe and spots worn parts before they fail in winter.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Frozen condensate pipe: how to thaw it safely.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Cold radiators: what cold at the top or bottom means.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler won't turn on: the checks to try first, and what a pilot light is.
• All common boiler problems.

Still no heating? Book a boiler repair and we'll find the cause.