Hot water but no heating? What to check first

If you have hot water but no heating, the boiler itself is usually fine, because it is clearly firing for the taps. The problem is normally with whatever tells the boiler to heat the radiators: the thermostat or programmer, the system pressure, or the valve that sends hot water to the radiators instead of the taps.

Work through the checks below in order. The first few are free, safe and fix a surprising number of calls Lee takes in October when people switch the heating on for the first time.

Why would I have hot water but no heating?

Your boiler heats two things: water for the taps and water for the radiators. Something has to decide which one gets the heat, and something has to ask for heat in the first place. When the taps work and the radiators stay cold, the fault is almost always on the heating side of that decision.

Likely cause	Can you check it?
Programmer set to hot water only, or heating timer off	Yes
Room thermostat set too low, flat batteries, lost signal	Yes
Low system pressure or air in the radiators	Yes, within limits
Diverter valve stuck (combi boilers)	No, engineer job
Motorised zone valve stuck (system and regular boilers)	No, engineer job
Heating pump failed or seized	No, engineer job

Check the controls first

1. Look at the programmer. Many programmers have separate settings for heating and hot water. Make sure heating is set to on, auto or a timed period that covers right now, not off. A power cut can reset the clock, so check the time and day too.
2. Turn up the room thermostat. Set it a few degrees above the current room temperature and listen for a click or watch for the boiler to fire. If it was set to a frost or holiday setting over the summer, it will not ask for heat until the house is very cold.
3. Replace the thermostat batteries if it is wireless or has a blank or faint display.
4. Check smart controls. If you use Hive, Nest, tado or similar, open the app and make sure the heating schedule is on, there is no holiday mode, and the receiver near the boiler is powered and connected.
5. Check the radiator valves. If only one or two radiators are cold, the thermostatic radiator valves may have stuck shut after a summer of not moving. Turn them fully open and back a few times.

Check the boiler pressure

Look at the pressure gauge on the boiler. With the heating cold it should read roughly 1 to 1.5 bar. Low pressure often stops a boiler altogether, but a system that has lost water can also leave air in the radiators so they stay cold even when the boiler runs.

If the reading is below 1 bar, follow our guide to repressurising a combi boiler. If the pressure keeps falling, there is a leak somewhere, and losing pressure needs finding rather than topping up forever.

If radiators are warm at the bottom and cold at the top, they need bleeding. With the heating off, open the bleed valve with a radiator key until water appears, close it, then check the boiler pressure again.

Is it frost or cold weather?

In freezing weather, a frozen condensate pipe usually stops both heating and hot water, so it is less likely to be the cause if your taps still run hot. If the boiler is showing a fault and it is below zero outside, check the pipe anyway using our frozen condensate pipe guide.

Stuck valves and failed pumps

If the controls are calling for heat, the pressure is fine and the radiators are still cold, the fault is likely to be one of these:

• Diverter valve (combi boilers). Inside a combi, a diverter valve switches the hot water between the taps and the radiators. If it sticks in the hot water position, you get exactly this fault. It sits inside the boiler casing, so it is an engineer job.
• Motorised valve (system and regular boilers). Homes with a hot water cylinder usually have one or more motorised valves near the cylinder or boiler. If the heating valve sticks shut or its motor fails, the radiators get nothing. These are often found in the airing cupboard.
• Heating pump. The pump pushes water around the radiators. A failed pump can sometimes still allow hot water to the cylinder in older gravity systems, but leave the radiators cold. A humming pump that feels very hot, or one that is completely silent, is worth reporting when you call.
• Wiring or the wiring centre. On system and regular boilers, the controls, valves and pump all connect through a wiring centre. A loose connection or failed component here can stop the heating signal reaching the boiler.

Please do not open the boiler casing or take the cover off a motorised valve or wiring centre. These are live electrical parts, and the boiler is a gas appliance. Only a Gas Safe registered engineer should work inside it.

When to call an engineer

Call us if:

• The controls are calling for heat and the radiators stay cold
• The pressure keeps dropping after you top it up
• The boiler shows a fault code you cannot clear with a single reset
• You can hear the pump humming, grinding or making no sound at all
• You can see water around a valve, the pump or the boiler
• You smell gas. Leave the property and call the National Gas Emergency Service on 0800 111 999

A diverter valve, motorised valve or pump is a routine central heating repair. Labour is £120 an hour including VAT (£100 + VAT) Monday to Friday, 8am to 5pm, and £180 an hour including VAT (£150 + VAT) at other times. Full details are on our prices page.

If the opposite is happening and your radiators are warm but the taps run cold, see no hot water but heating works.

Before booking anything, call Lee on 020 8050 5913. If he can talk you through a controls or pressure fix on the phone, there is no charge. If it needs a visit, we cover homes across South and South West London and north Surrey, and you can also book a heating repair online.