Can an electric boiler just heat water?
Yes, an electric boiler can just heat water. An electric flow boiler heats water in a small primary circuit and pumps it through a coil inside an indirect hot water cylinder, which warms the stored water. Heatrae Sadia's product literature for the Amptec, for example, describes domestic hot water from the boiler "in conjunction with an indirect cylinder", and EHC markets the Slimjim for heating and hot water with a separate cylinder.
The catch is that the boiler adds cost without making the water any cheaper to heat. The electricity still ends up as heat in the cylinder, just by a longer route with a pump and extra pipework. That is why most homes that only need hot water from electricity use an immersion heater instead, and keep an electric boiler for when they also want radiators. Our guide to the electric boiler covers that heating side.
Your electric hot water only options
There are five main ways to heat water electrically in a UK home. They differ in how much water they store, how fast they deliver it and what they cost to fit.
| Option | How it works | Best for |
|---|---|---|
| Electric flow boiler plus indirect cylinder | Boiler heats a coil inside the cylinder | Homes that also want electric radiators now or later |
| Immersion heater in a cylinder | Element sits directly in the stored water | Most homes with a cylinder, especially on off-peak tariffs |
| Point-of-use heater (small store) | A 5 to 30 litre unit under or over one sink | A cloakroom, kitchen sink or outbuilding far from the main supply |
| Instantaneous point-of-use heater | Heats water as it flows, no store | Hand basins where only a small flow is needed |
| Heat pump cylinder | A small heat pump on top of the cylinder takes heat from the air | Lowest running cost where there is space for one |
An electric combi with a built-in cylinder is another route if you also need heating. See our guide to the electric combi boiler for what that can deliver.
Immersion heaters
A standard immersion heater is a 3 kW element screwed into the cylinder with its own thermostat. Many cylinders have two: one near the bottom that heats the whole tank on an off-peak rate overnight, and one near the top that boosts a smaller amount during the day. Vented cylinders are fed from a cold water tank in the loft, while unvented cylinders run at mains pressure and give stronger showers.
Point-of-use heaters
Small instantaneous units are limited by their power. Product data for single-phase models shows flows of about 1.5 litres a minute at 2.8 kW up to just under 5 litres a minute at around 9 kW, which is fine for a basin but slow for a bath. Small stored units, typically 10 to 15 litres under a sink, deliver a better flow until the store runs out.
Is an immersion heater cheaper?
Yes, an immersion heater is usually cheaper than an electric boiler for hot water only, mainly because it costs much less to fit. Both convert electricity into heat at close to 100% efficiency, so heating the same water by the same amount uses the same kWh either way.
Where the running cost differs, it tends to favour the immersion heater. An electric boiler adds a pump that uses electricity and a primary pipe circuit that loses some heat. The things that change your bill far more than the choice between them are:
- Your tariff: heating the whole cylinder on a cheaper off-peak rate, such as Economy 7, cuts the cost per litre.
- The thermostat and timer: heating the cylinder once or twice a day, not all day, avoids keeping hot water you do not use.
- Cylinder insulation: an older cylinder with a thin jacket loses heat all day. A modern factory-insulated cylinder loses much less, and its heat loss figure is on the label.
If you have both a gas boiler and an immersion heater, heating water with gas is normally about a third of the cost of using the immersion, because gas is so much cheaper per kWh. Keep the immersion as a backup for when the boiler breaks down.
Running costs on price cap rates
Hot water running costs come down to how much water you heat and by how much. It takes about 4.19 kJ to warm 1 litre of water by 1 degree, so the sums are simple. The figures below use Ofgem's average Direct Debit electricity rate of 26.32p per kWh for 1 October to 31 December 2026.
| Task | Energy used | Approximate cost at 26.32p/kWh |
|---|---|---|
| Heat a 150 litre cylinder from 10°C to 60°C | About 8.7 kWh | About £2.30 |
| Heat a 120 litre cylinder from 10°C to 60°C | About 7.0 kWh | About £1.84 |
| Run a 3 kW immersion for one hour | 3 kWh | About 79p |
| Heat 5 litres at a basin by 30°C | About 0.17 kWh | About 5p |
On an off-peak rate the same energy costs less, so multiply the kWh by your own off-peak unit rate to compare. Add a little for standing heat loss from any stored water. A cylinder rarely starts from completely cold each day, so a typical daily top-up uses less than the full-tank figure.
What is an electric water boiler?
An electric water boiler can mean two different things, so check which one you are looking at. In home heating, it means an electric boiler or immersion system that heats water for your taps and shower, which is what this guide covers.
In shops and catalogues, the same words often describe catering water boilers and urns, or wall-mounted boiling water units for offices and kitchens. These heat a small amount of water close to 100°C for drinks. They are not designed to supply showers or basins and are not a replacement for a cylinder or boiler.
Which option suits your home?
| Situation | Usually the best fit |
|---|---|
| Flat with no gas and an existing cylinder | Immersion heater, ideally dual, on an off-peak tariff |
| No gas, planning to add radiators later | Electric flow boiler with an indirect cylinder |
| Gas combi home wanting a backup | Electric shower for one bathroom |
| Single sink far from the hot water supply | Point-of-use store under the sink |
| Space for a cylinder and wanting low running costs | Heat pump cylinder |
If your hot water comes from a gas system boiler or heat only boiler and has stopped working, the immersion heater can cover you until a Gas Safe registered engineer fixes the boiler. If the taps run cold while the heating works, our guide to no hot water when the heating works covers the likely causes.
Installation and safety
Most electric hot water work needs a qualified tradesperson. Replacing an immersion element means draining the cylinder, and new circuits for a boiler, heater or shower are notifiable electrical work. Unvented cylinders over 15 litres must be installed by someone qualified under Building Regulations Part G3, because their safety valves protect against over-pressure and overheating.
Stored hot water should reach 60°C to control legionella, which is what HSE guidance for landlords and workplaces recommends, but water at that temperature can scald in seconds. Fit a thermostatic mixing valve on baths and basins used by children or older people, and never cap or block a cylinder's discharge pipe.
What to do next
- Check if your cylinder already has one or two immersion heaters, and if they still work, before paying for a new system.
- Look at your tariff. If it has an off-peak period, set the immersion timer to heat the cylinder then.
- For a new installation, ask for quotes that name the cylinder size, heat loss figure and any electrical upgrade.