No Hot Water? Diagnose It by System Type
'No hot water' is three different problems wearing one symptom, and the diagnosis starts with what heats yours: a cylinder with an immersion, a cylinder heated by a boiler coil, or an instantaneous source. Identify your system below, run its checks, and you'll know whether you need a plumber, a heating engineer, or a reset button.
Cylinder + immersion (electric heating)
If your hot water comes from a cylinder heated by an immersion element: check the isolator switch is on and the timer (if fitted) is actually calling. A tripped RCD or breaker that re-trips points at a failed element leaking to earth — stop resetting and book the replacement. Water that's warm-not-hot suggests a failed thermostat or one of two elements down. Scalding water is a stat failed closed: switch off and treat as urgent. All of this is plumber territory, fixed same-visit with stocked parts.
Cylinder heated by a boiler coil
Hot water via a coil from the boiler adds candidates: the motorised valve that diverts heating water to the cylinder (a very common failure — heating works, hot water doesn't, or vice versa), the cylinder stat not calling, or the boiler itself. If radiators heat but the cylinder doesn't, suspect the valve or cylinder stat — wet-side, diagnosable, plumber-appropriate. If the boiler shows a fault code or nothing heats at all, that's the boiler: a Gas Safe engineer's job, and we'll route you to our heating partner rather than pretend otherwise.
Sudden vs gradual tells you the fault's age
Overnight failure is a component: element, stat, valve, or a tripped device. Gradual decline — water cooler than it was, running out faster, reheating slower — is scale: a jacketed element or furred coil transferring heat badly, plus possible stratification faults. Gradual decline in hard-water areas is nearly always the scale story, and it's also your electricity bill: a scaled element burns more to deliver less.
One tap or all, and the airlock case
No hot water at one outlet only, with others fine, is distribution not heating: a failed cartridge blending it away, a blocked hot-side filter, or an airlock in that branch. Airlocks — common after draining down — announce themselves with spluttering and a dead hot tap, and often clear by back-filling pressure from a mixer or the classic mains-to-hot tap connection. Persistent airlocks in the same branch indicate a pipework layout issue worth fixing properly.
| Symptom | Likely fault | Who fixes |
|---|---|---|
| Immersion trips repeatedly | Element leaking to earth | Plumber — replace element |
| Heating works, hot water doesn't | Motorised valve / cylinder stat | Plumber |
| Boiler fault code / nothing heats | Boiler fault | Gas Safe engineer (heating partner) |
| One hot tap dead, splutters | Airlock or blocked filter | Plumber / careful DIY |
| Gradually cooler over months | Scale on element or coil | Plumber — descale/replace |
If the symptoms don't quite match
Faults read textbook-clean in guides and messier in airing cupboards: if your symptoms straddle two patterns above or match none cleanly, that ambiguity is itself useful diagnostic information — tell the engineer precisely that. Intermittent faults deserve a log (time, conditions, what preceded it) because patterns invisible in one visit leap out of a week's notes. And any symptom involving escaping water you can't isolate upgrades itself to the emergency line.
Frequently asked questions
Why do I run out of hot water faster than before?
Scale displacing volume and insulating the heat source, a failed lower element on twin-element cylinders, or a stat set low. On boiler-coil systems, a furred coil reheats slowly, so you drain faster than it recovers.
Is no hot water an emergency?
Uncomfortable but rarely dangerous — with two exceptions: scalding output (stat failed closed — switch off now) and discharge from an unvented cylinder's relief pipework, which needs urgent G3-qualified attention.
Can I reset a tripped immersion?
Once, reasonably. If it trips again, the element is breaking down and each reset stresses the circuit — isolate it and book the replacement rather than gambling.
Who do I call — plumber or heating engineer?
Cylinder, immersion, valves, stats, airlocks: plumber. Boiler faults, gas anything: Gas Safe heating engineer. Describing the triage result above on the phone gets you the right trade first time.
Sorting it for good
The recurring theme across our guides holds here too: fix causes, not symptoms. A repair that addresses why the fault happened — the pressure that's too high, the scale that keeps returning, the valve that should have been replaced a decade ago — is the one you don't repeat next year. When you book through our network, tell the engineer you want the cause addressed, not just the symptom silenced; it occasionally costs a little more on the day and reliably costs less over the years.