No hot water (combi boiler): quick checks
Quick answer
No hot water from a combi boiler is often linked to low system pressure, a control fault, or the boiler not detecting demand when you open a tap. Start by confirming cold water still works, then check pressure and listen for the boiler responding when you run a hot tap. If it doesn’t respond, shows a fault code, or pressure keeps dropping, it’s time to call a gas-safe engineer.
Safety first
- If you smell gas or suspect a leak, leave the property and call the National Gas Emergency Service on 0800 111 999.
- If a carbon monoxide (CO) alarm sounds, or anyone feels unwell, get everyone to fresh air and seek urgent help.
- Never remove boiler covers or attempt any internal repair — this is gas and electrical work for a Gas Safe registered engineer only.
Full checklist: Safety guidance
What you may notice
- No hot water anywhere in the property — may indicate a control or ignition fault, or low system pressure.
- A fault code on the boiler display — can help narrow down the cause, though the exact meaning depends on your make and model.
- The boiler stays silent when you open a hot tap — may suggest the boiler isn’t detecting demand for hot water.
- Lukewarm rather than fully cold water — can suggest a flow or temperature setting issue rather than a full shutdown.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Confirm cold water works at other taps. This rules out a mains water supply issue, which is a different problem entirely.
- Run a hot tap fully for 30–60 seconds and listen. You’re checking whether the boiler fires up in response. Silence may suggest a demand-detection or control fault.
- Check the boiler display for a fault code and note it down. This is useful information for an engineer, even if you can’t interpret it yourself — codes vary by manufacturer.
- Check the system pressure gauge or display. Many combi boilers won’t operate properly if pressure has dropped too low. Compare against your manual’s recommended range.
- Check your programmer or timer settings. Confirm hot water isn’t accidentally switched off or scheduled incorrectly.
What you can do safely
- Checking whether cold water works elsewhere in the property.
- Running one hot tap and listening for the boiler responding.
- Noting down any fault code shown on the display.
- Checking system pressure against your manual’s guidance.
- If your manual describes a reset button and there are no danger signs, resetting once and observing what happens.
When to call a professional
- The boiler doesn’t respond to hot water demand at all.
- It resets but then locks out again repeatedly.
- Pressure keeps dropping even after you’ve topped it up.
- There’s any gas smell, CO alarm, or sign of overheating.
What to tell the engineer
- Whether the boiler responded at all when you ran a hot tap.
- Any fault code shown, and whether it’s happened before.
- The pressure reading, both cold and after trying the hot tap.
- Whether you’ve already reset the boiler, and how many times.
- Roughly when the problem started and whether it’s constant or comes and goes.
FAQs
Why has my combi boiler stopped producing hot water but heating still works?
This pattern can point to a fault specific to the hot water side of the boiler, such as a flow sensor or diverter valve issue on some models. An engineer can confirm this once they’ve inspected your particular boiler.
Is it safe to reset my boiler more than once?
Most manufacturers advise resetting only once and observing the result. Repeated resets without addressing the underlying cause can mask a fault that needs proper attention — if it keeps locking out, stop resetting and call an engineer.
Can low pressure alone cause no hot water?
On many systems, yes — pressure that’s too low can prevent the boiler operating at all. Topping up may help, but if pressure keeps dropping, there’s likely an underlying leak or fault that needs investigating.
How quickly should I get an engineer out if I have no hot water?
If there’s no safety concern, it’s reasonable to try the checks above first. If the boiler won’t respond, keeps locking out, or you’re not confident, most households treat total loss of hot water as something to book fairly promptly, especially in cold weather.
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