Homeowner How-To · 8 min read
Updated 28 May 2026
No Hot Water? An Electric Boiler & Immersion Troubleshooting Checklist
No hot water on an electric system almost always comes down to four things: the timer or programmer not calling for heat, the cylinder thermostat, the immersion element, or a tripped MCB / reset button. Work through them in order — most of the time you'll find it yourself in ten minutes.
This guide is for electric hot water systems — an immersion heater in a hot water cylinder, or an electric boiler feeding a cylinder. We are an electric-boiler and water-hygiene specialist; we are not Gas Safe, so if you have a gas combi or system boiler, call a Gas Safe engineer.
Quick checks before you panic
- Is anything else on the same circuit also dead? A whole-house power cut hides itself behind "no hot water".
- Have you recently changed the timer, the thermostat dial or the immersion switch?
- Is the hot tap actually running cold, or just lukewarm? Lukewarm points at a partially-working system.
- Has the cylinder been on long enough? After a full draw-down, allow 2-3 hours to reheat fully.
The full troubleshooting checklist
- 1
Check the timer / programmer
Make sure it's showing the correct time, in the right mode (constant or timed), and that you're actually inside a heating period. A power cut often resets these to a default that no longer matches your routine.
- 2
Check the immersion switch on the wall
There's usually a 20A double-pole switch beside the airing cupboard or cylinder. The neon should glow when it's on. If the neon is dead but the switch is on, you've already found the problem — no power getting to the immersion.
- 3
Check the consumer unit (fuse box)
Look for a tripped MCB labelled 'Immersion', 'Hot Water' or 'Boiler'. Reset it once. If it trips again immediately, stop — there's a fault and you need an electrician or plumber.
- 4
Check the cylinder thermostat
Most are strapped to the side of the cylinder under the insulation. Set point should be 60°C. If the dial has been turned right down (often to save money) the immersion will never reach satisfaction. Turn it back up.
- 5
Press the immersion reset button
On top of the immersion thermostat, under a small plastic cover. Press once. If it clicks back in and stays in, you're back in business. If it pops again within a few minutes, the element or thermostat is failed — get it replaced.
- 6
Listen and feel
An active immersion is silent, but the cylinder itself will be warm at the top within 20-30 minutes. A cold cylinder after an hour of 'on' time means power isn't reaching the element, or the element is open-circuit.
Common symptom → likely cause
| Symptom | Most likely cause | Fix |
|---|---|---|
| No hot water at all, cylinder stone cold | Tripped MCB, blown spur fuse, failed element | Reset MCB once; if it trips again, call a plumber |
| Hot water for a short time then runs cold | Thermostat set too low, or only off-peak coming on | Turn thermostat up to 60°C; check timer setup |
| Lukewarm water that never gets hot | Thermostat drifting low, or partially scaled element | Test thermostat at 60°C; if no change, replace element |
| Hot water boiling / scalding from tap | Failed thermostat (stuck on) | Switch off at the wall immediately — this is a safety fault |
| Immersion reset button keeps tripping | High-limit fault: element shorted or thermostat failed | Stop resetting it. Replace immersion. |
A safety note on reset buttons
Electric boiler specifics
A modern electric boiler — Heatrae Sadia, Elnur, Comet, Mattira and similar — has its own diagnostic display. Note the fault code before you call anyone; it saves diagnostic time. The most common causes of "no hot water" on an electric boiler feeding a cylinder are:
- The boiler is in CH-only (central heating) mode and not calling for hot water.
- The DHW thermostat or 2-port valve has failed and the cylinder isn't getting flow.
- A heating element inside the boiler has failed (one of two or three).
- A low-pressure lockout on a sealed system — top up to ~1 bar at the filling loop.
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When to stop troubleshooting and call us
Pull the plug on DIY and pick up the phone if any of the following are true:
- The MCB trips immediately every time you reset it.
- You can smell scorching or see scorch marks at the immersion switch or spur.
- Water is leaking from the top of the cylinder where the immersion screws in.
- Hot water is coming out genuinely scalding (over 70°C) — the thermostat has failed open.
- The cylinder is unvented (sealed and pressurised) and you suspect a fault — those are G3-regulated and must be touched by a qualified person.
Ninja Tip
Frequently Asked Questions
Why is there no hot water from my electric boiler or cylinder?
On an electric system there are only a handful of likely culprits: the timer or programmer hasn't called for heat, the cylinder thermostat has failed, the immersion element has burnt out, or the MCB at the consumer unit has tripped. Work through them in that order.
Where is the reset button on an immersion heater?
On the top of the immersion's thermostat assembly, under a small plastic cover. It's a tiny button you press in with a screwdriver. It only resets the high-limit (thermal cut-out) — if it pops again straight after, the element or thermostat needs replacing, don't keep resetting it.
How long should an immersion take to heat a cylinder?
A 3 kW immersion will heat a typical 150-litre cylinder of cold water to 60°C in about 2-3 hours. Top-up reheats take 30-60 minutes. If yours is running for hours and the water is still lukewarm, the thermostat is set too low or the element is partially scaled and failing.
Should I call a plumber or an electrician for no hot water?
Either can usually help, but a plumber who works on electric systems will be quicker — they'll diagnose the thermostat, immersion and cylinder in one visit, and replace any of them on the spot. Pure electricians often won't touch the cylinder itself.
Sources & further reading
Guidance only. This article is general information for UK readers, not a substitute for a site-specific assessment by a competent person. Regulations and best practice change — always check the current official guidance and, for compliance work (Legionella risk, unvented cylinders, water regulations), use a suitably qualified professional. Pipe Assassin is an electric-boiler and water-hygiene specialist and is not Gas Safe registered; we do not carry out gas work.
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