No hot water? Here’s what it usually means
No hot water with working heating usually means a failed diverter valve (combi boilers), a failed immersion heater or thermostat (cylinder systems), or a stuck motorised valve. If heating is also down, start with the boiler itself — pressure, error codes, power.
Likely causes
- Combi boiler: diverter valve
- The valve that switches between heating and hot water sticks or fails — heating fine, taps cold. A common, well-understood repair.
- Cylinder system: immersion heater or 'stat
- If you have a hot water tank, the immersion element or its thermostat may have failed. The trip on your consumer unit is a telltale.
- Motorised zone valve
- On S-plan/Y-plan systems a stuck valve stops hot water circulating to the cylinder — sometimes announced by a humming valve body.
- Airlock after works
- If water was recently off (works in the street, a repair), an airlock can stop hot taps running. Sputtering taps are the giveaway.
What not to do
- Don't open the boiler casing (gas work is Gas Safe-only)
- Don't keep resetting a tripping immersion circuit — it's tripping for a reason
- Don't ignore a cylinder that's suddenly scalding — a failed 'stat can overheat it; turn the immersion off
When to request help
Book promptly rather than urgently for taps-only issues; treat it as urgent if you also have no heating in winter, or if anything electrical is tripping.
Related questions
Who do I need — a plumber or a Gas Safe engineer?
Cylinder, immersion and valve work: a plumber (or electrician for the immersion circuit). Anything inside a gas boiler: a Gas Safe registered engineer. Describe the symptom in your request and we route it to the right credentials automatically.
Reviewed by the London Heroes Operations Team. General guidance, not a substitute for an on-site professional assessment — and never for the gas emergency line (0800 111 999) if you smell gas.