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Hot water but no heating? What to check before you call
If the taps run hot but the radiators stay cold, the boiler itself is working. That narrows it down, and it’s often something you can check in five minutes.
- 6checks you can do yourself
- 1 to 2bar on the gauge, with the heating on
- £120repair visit if it’s a fault
6quick checks
Hot water but no heating is one of the most common calls we get once the heating goes back on in autumn. The taps are fine, but the radiators are stone cold.
The good news is that hot water at the taps tells us the boiler is lighting and heating water. So the problem is usually in how the heating is being asked for, or in the part that sends the heat to the radiators.
Work through the checks below first. Plenty of the time, it’s a setting.
Check these first
None of these need any tools.
Is the heating being asked for?
Check the timer or programmer is in a heating on period, then turn the room thermostat up a few degrees above the room temperature. You should hear the boiler fire up within a minute or two. On a smart thermostat, check the app, and check the batteries if it’s wireless.
Is the heating turned on at the boiler?
Many combis have separate heating and hot water dials, or a summer mode that turns the heating off. Make sure the heating side is on and turned up.
What does the pressure gauge say?
Most systems sit between 1 and 2 bar with the heating on. Low pressure usually stops hot water too, but it’s worth a look. Our boiler pressure guide shows you how to read it.
Is it every radiator, or just one?
If only one or two are cold, it’s likely trapped air or a stuck radiator valve rather than the boiler. If they’re all cold, keep reading.
Radiator check
What it could be
If the checks don’t fix it, these are the usual suspects.
On a combi boiler: the diverter valve
A combi has a valve inside that switches its heat between the taps and the radiators. If it sticks in the hot water position, you get exactly this: hot taps, cold radiators. It’s a repair job inside the boiler.
With a hot water cylinder: the zone valve
System and regular boilers use motorised valves to send heat to the radiators or the cylinder. If the heating valve’s motor or switch fails, the cylinder still heats but the radiators don’t.
Other causes
- A faulty pump not moving water round the radiators
- A faulty room thermostat or wiring
- An airlock, often after work on the system
- Sludge, if radiators are cold at the bottom
Fault finding
Leave the casing on
Anything inside the boiler casing is work for a Gas Safe registered engineer. The checks on this page are all outside it.
Symptom, likely cause and who fixes it
| Likely cause | Can I fix it? | |
|---|---|---|
| Boiler doesn’t fire when the thermostat is turned up | Thermostat, timer, batteries or wiring | Check settings and batteries first |
| Boiler fires, taps hot, radiators cold (combi) | Diverter valve | No, book a repair |
| Cylinder heats, radiators cold | Heating zone valve or pump | No, book a repair |
| Radiators cold at the top | Trapped air | Yes, bleed them |
| Radiators cold at the bottom | Sludge in the system | No, it needs cleaning |
| One radiator cold | Stuck radiator valve | Often, see our TRV guide |
What should I do next?
- A setting was wrongYou’re sorted
No visit needed.
- Cold at the top of radiatorsBleed them
Our guide shows you how.
- Still no heating after the checksBook a repair visit
£120 including VAT, before any parts.
- Cold at the bottom of radiatorsPower flushing
Sludge needs flushing out.
Common questions
Why do I have hot water but no heating on my combi boiler?
If the settings and thermostat are right, the most common cause on a combi is the diverter valve. It switches the boiler’s heat between the taps and the radiators, and if it sticks you get hot water but no heating.
Can a diverter valve be repaired?
Sometimes the motor or a part of the valve can be replaced, sometimes it’s the whole valve. The engineer will find out which on the repair visit and quote for the parts before fitting anything.
Is it safe to use the hot water while the heating isn’t working?
Yes. If the boiler is heating the water normally and there’s no fault showing, it’s fine to keep using the hot water until the repair.
Will bleeding the radiators fix it?
Only if the radiators are warm at the bottom and cold at the top, which means trapped air. If every radiator is cold all over, bleeding won’t help.
My radiators only get lukewarm. Why?
The heating flow temperature on the boiler may be set low, the system may need balancing, or there may be sludge. Turn the heating dial up a little first and see if it makes a difference.
Is this covered by Home Care?
Breakdown repairs to the boiler, such as a diverter valve, are covered on the Boiler and Complete plans, with parts and labour included. Controls, radiators and valves are covered on Complete and Home Care System. Faults that were there before you joined aren’t covered.
Related pages
Sources: Glow-worm, hot water but no heating. Checked 1 November 2026.
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Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We’ll tell you what we think, and if the answer is to leave things as they are, we’ll say that too.
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