No hot water (combi boiler): quick checks
Quick summary
If your combi has power and no fault showing, check hot tap flow, system pressure, and whether the boiler responds to DHW demand.
Safety first
- Smell gas or suspect a leak: leave the area and call 0800 111 999.
- CO alarm sounding or anyone unwell: get to fresh air and get urgent help.
- Do not remove boiler covers or attempt internal repairs.
Full checklist: Safety guidance
What to check (in order)
- Confirm you have cold water at other taps (rules out a mains outage).
- Run a hot tap fully for 30–60 seconds and listen: does the boiler respond?
- Check the boiler display for a fault code; note it down.
- Check system pressure on the gauge/display (follow your manual).
- Check programmer/timer settings if relevant.
- If there’s a reset button and a clear non-danger fault, reset once and observe.
What the result means
- Boiler responds to hot demand: could be flow/temperature setting or DHW-side control component (model dependent).
- No response: demand detection or control fault likely.
- Low pressure: some appliances inhibit operation until restored.
- Fault code: use manual/model lookup to narrow subsystem.
What you can safely do
- Check whether cold water works elsewhere in the property.
- Run one hot tap fully for 30–60 seconds and listen for boiler response.
- Note any fault code shown on the display.
- Check system pressure and timer settings before resetting once only.
When to call a professional
- No response to hot demand and/or repeated lockouts.
- Pressure keeps dropping after topping up.
- Any gas smell, CO alarm, overheating signs.
Engineer notes
Control-chain first: confirm DHW demand detection, then consider flow sensor/switch, DHW NTC, diverter position (if applicable), plate HX scaling, and PCB input behaviour. Record repeatability and any code history.
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