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A combi boiler heats water on demand, so when it stops, you notice straight away. Before you book an engineer, it's worth running through a few quick checks. Many hot water problems turn out to be something simple, like low pressure, a setting that's changed or a frozen pipe.
Quick answer
- No heating or hot water: check the power, gas supply, boiler pressure and any fault code.
- Heating works but no hot water: check the hot water settings. If they're right, the diverter valve is the most likely cause, which needs an engineer.
- Freezing outside: the condensate pipe may have frozen.
6 checks to try first
1. Is the boiler getting power?
Look for lights or a display on the boiler. If it's blank, check the switch on the wall next to it (usually a fused spur) and your fuse box for a tripped switch.
2. Is gas getting to the boiler?
If you have a gas hob, see if it lights. If you have a prepayment meter, check you have credit, as the gas supply stops when it runs out. If nothing gas-powered works, contact your energy supplier.
3. Is the pressure too low?
Most combis shut down if the pressure drops below about 1 bar. Check the gauge and, if it's low, top it up to about 1 to 1.5 bar. Our boiler pressure guide shows how.
4. Have the settings changed?
Check hot water hasn't been switched off or set to holiday mode, and that the hot water temperature hasn't been turned right down. A power cut can also reset the clock or programmer.
5. Is there a fault code?
Look the code up in your boiler manual (most are online if you've lost yours). Pressing reset once is fine, but if the code comes back, don't keep resetting: the boiler is telling you something needs fixing.
6. Has the condensate pipe frozen?
If it's been below freezing and you hear gurgling, the condensate pipe outside may be blocked with ice. Our guide to a frozen condensate pipe shows how to thaw it safely.
Heating works but no hot water
If your radiators heat up but the taps run cold, and you've checked the settings, the most likely cause is the diverter valve. This is the part inside a combi that switches hot water between the radiators and your taps. If it sticks, the boiler can keep heating the radiators but never sends hot water to the taps.
Diverter valve faults are common, especially in boilers that are 10 years old or more. It's an internal part, so it needs a Gas Safe registered engineer.
Other possible causes include a faulty sensor, or a build-up of scale or sludge inside the boiler that restricts the flow of water. These also need an engineer.
Hot water but no heating
If it's the other way round, start with the controls. Make sure the heating is switched on and not in summer mode, the timer is calling for heat, and the thermostat is set higher than the room temperature. Wireless thermostats can stop working when their batteries run low. If only some radiators are cold, see our guide to radiators not heating up.
When to call an engineer
Call a Gas Safe registered engineer if:
- you've been through the checks and still have no hot water
- a fault code keeps coming back after a reset
- the heating works but the hot water doesn't
- the boiler makes unusual banging or whistling noises
If your boiler is getting old and keeps breaking down, a repair may only buy you a little time. Our guide to how long a boiler lasts explains when replacing makes more sense.
If you smell gas: open doors and windows, don't use light switches or naked flames, turn off the gas at the meter if you can safely reach it, leave the property and call the National Gas Emergency Service on 0800 111 999.
No hot water questions
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Get my fixed priceReviewed by our technical team. All BoxHeat articles are written by experienced heating professionals and reviewed by our team of Gas Safe registered engineers to ensure accuracy, safety, and compliance with current UK building regulations and manufacturer guidelines.
