Immersion Heaters & Electric Hot Water: Temperature, Timers, Efficiency

Pipe Assassin Technical Team — G3 certified, WRAS approved — 10+ years in UK plumbing & water hygiene

Set your immersion thermostat to 60°C, heat the water on a timer matched to when you actually use it (overnight if you're on an off-peak tariff), and make sure the cylinder is well insulated. That combination keeps electric hot water safe from Legionella and as cheap to run as it reasonably gets.

As an electric-boiler and immersion specialist (we don't do gas), this is bread-and-butter work for us. Here's how to get the most out of an electric hot water system.

How an immersion heater works

An immersion heater is essentially a large electric element — like a giant kettle element — fitted into a hot water cylinder, controlled by its own thermostat. Many cylinders have two: a top element for a quick top-up and a bottom element to heat the full tank. It heats the stored water directly, independently of any boiler.

What temperature to set

Key Fact

60°C is the number. High enough to kill Legionella, low enough to avoid excessive standing heat loss and limescale. Don't set it lower to save money — that's a false economy that creates a bacterial risk. Control scald risk at the tap with a TMV, not at the cylinder.

Timers and off-peak tariffs

The single biggest lever on running cost is when you heat the water, not the temperature. A modern, well-insulated cylinder keeps water hot for many hours.

Use a timer / programmer to heat a tankful before you need it, rather than leaving the immersion on 24/7.
On Economy 7 or similar, heat overnight on the cheap rate using the lower (full-tank) element; use the top element only as a daytime boost if you run short.
Match the heat to demand — a household of two doesn't need a full tank reheated twice a day.

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Cutting the running cost safely

1
Insulate the cylinder
A modern factory-foamed cylinder or a good jacket on an older one slashes standing heat loss — the cheapest upgrade available.
2
Lag the pipework
Insulate the first metre or two of pipe off the cylinder, and any pipe in cold spaces.
3
Right-size the heating window
Use the timer to heat only what you'll use, and only on the cheap rate if you have one.
4
Fit TMVs, keep storage at 60°C
Never trade safety for savings — blend down at the tap instead of cooling the tank.

When it stops working

No hot water from an electric system is almost always a tripped circuit, a failed element, or a failed thermostat. Work through our no hot water troubleshooting checklist first. If the element or thermostat has failed it's a straightforward replacement for a plumber — and if you're thinking about a fuller upgrade, see our electric boiler installation service.

Ninja Tip

If your cylinder is unvented (a sealed, pressurised system rather than one fed from a loft tank), the immersion and safety controls are part of a G3 system that must be serviced annually by a competent person — read G3 & unvented cylinders.

Frequently Asked Questions

What temperature should an immersion heater be set to?

Set the immersion thermostat to 60°C. That's hot enough to kill Legionella in the stored water but not so high that it wastes energy or accelerates scaling. Use a TMV at the taps to deliver water safely at 38-46°C.

Is it cheaper to leave an immersion heater on all the time?

No. A well-insulated cylinder holds heat for hours, so heating on a timer to match your usage is cheaper than leaving the immersion on constantly. If you're on an Economy 7 or similar off-peak tariff, heat the water overnight on the cheap rate.

Why has my electric hot water stopped working?

The usual causes are a tripped circuit, a failed immersion element, or a failed thermostat. Check the consumer unit first, then the immersion's own thermostat/cut-out. See our no-hot-water troubleshooting checklist, and call a plumber if the element needs replacing.

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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