No hot water? Here’s what it usually means
Guidance last checked on against UK safety regulation and the sources behind our service pages.
No hot water with the heating still working usually means one failed part. Combi boiler: a stuck diverter valve. Cylinder system: a failed immersion heater or thermostat, or a stuck motorised valve. If heating is also down, start with the boiler itself instead — pressure, error codes, power.
No hot water: the questions people ask
Why does my combi boiler give heating but no hot water?
In a combi boiler, one valve switches the flow between the radiators and the hot taps, and when it sticks or fails the heating carries on working while every tap runs cold — or sometimes lukewarm at best. It is a well-understood, routine part to replace inside a gas boiler, so it is a job for a Gas Safe registered engineer rather than a general plumbing job.
Why is there no hot water from my cylinder?
If the house has a hot water cylinder rather than only a combi boiler, the electric immersion heater or its thermostat inside the cylinder may have failed, cutting hot water while the central heating carries on as normal. A tripped switch on the consumer unit that will not reset, or a switch that resets and trips again immediately, is the usual telltale sign worth checking first.
Why is only the hot water affected on my S-plan or Y-plan system?
On the S-plan or Y-plan systems common in larger London houses, a motorised valve directs hot water to the cylinder separately from the radiators, and a stuck valve can cut that flow while the rest of the heating carries on normally. A faint humming or buzzing from the valve body near the cylinder, rather than a clean click as it opens, is often the giveaway.
Why did my hot water stop after the water supply was off?
If the water supply to the house was recently interrupted — street works, a repair, a stopcock closed and reopened — air can get trapped in the hot pipework and stop hot taps running, even though the boiler and cylinder are both working. Sputtering, spitting taps that run cold in bursts rather than a steady dribble are the giveaway that this is air, not a failed part.
Related symptoms
What to check, in order
1. Check whether the heating still works
Turn the heating up and feel a radiator. If it warms while every hot tap runs cold, the fault is on the hot water side alone. If the heating is dead too, start with the boiler itself instead: pressure, error codes, power.
2. Rule out trapped air after a water supply cut
If the water was recently off for street works, a repair or a closed stopcock, open a hot tap and watch it. Sputtering, spitting bursts that run cold point to air trapped in the pipework, not a failed part.
3. Find out whether you have a combi or a cylinder
Look for a hot water cylinder, often in an airing cupboard. If there is none and your boiler is a combi, heating without hot water points to a stuck diverter valve inside the boiler, a job for a Gas Safe registered engineer.
4. Check the immersion switch on a cylinder system
If an immersion heater heats your cylinder, check the consumer unit for a tripped immersion switch and reset it once. If it will not reset, or trips again straight away, leave it off: that points to a failed heater or thermostat, and is urgent.
5. Listen at the hot water valve on an S-plan system
On an S-plan, switch the hot water on at the programmer and listen at its valve near the cylinder. A click is normal; a hum or buzz without one, or silence, points to the valve or its power. On a Y-plan, silence proves nothing.
6. Book a visit and describe what you found
Diverter valves inside gas boilers are Gas Safe work; cylinder, immersion and motorised valve faults are not. Book urgently for an electrical trip, a suddenly scalding cylinder, a hissing or discharging unvented cylinder, or no heating in winter; otherwise promptly. Describe what you checked.
What not to do
- Don't open the boiler casing — what's behind it is gas work, which the law allows only someone competent to do, and any engineer you pay to do it must be Gas Safe registered
- 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
- Don't touch the pressure relief or discharge valves on an unvented (pressurised) cylinder — those are its safety devices, and a discharging or hissing unvented cylinder is an engineer's job today, not a DIY adjustment
- If you smell gas, or a carbon monoxide alarm sounds: don't smoke, light a flame or touch light switches, open the doors and windows, turn the gas off at the meter if you can reach it safely and it isn't in a cellar or basement, get everyone out, and call the National Gas Emergency Service free on 0800 111 999 before anything else — before calling us
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, if anything electrical is tripping, if the cylinder is suddenly scalding, or if an unvented cylinder is hissing or discharging.
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Related questions
Who do I need — a plumber or a Gas Safe engineer?
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.