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Immersion heaters and hot-water cylinders

A well-insulated cylinder keeps water hot for hours, so an immersion heater rarely needs to run all day. A timer, the right thermostat setting and good insulation do most of the work.

Updated 2 October 2026 · energy.org.uk editorial team

Check

What to check first

  • Is the immersion on all the time? Energy Saving Trust suggests a timer so it heats water for an hour or two before you usually need it.
  • The thermostat setting. Set the cylinder thermostat to 60°C. The Health and Safety Executive advises storing hot water at 60°C or above to control legionella; going higher wastes energy and raises the scald risk.
  • Cylinder insulation. Energy Saving Trust says a jacket should be at least 80 mm thick. Adding an 80 mm jacket to a cylinder with only thin foam can pay for itself within a year.
  • Your tariff. On Economy 7 or a similar tariff, heat water in the cheap hours.
  • Pipes. Insulating the hot pipes near the cylinder cuts heat loss on the way to the taps.

What it costs

Heating 100 litres by 40°C takes about 4.65 kWh of heat. An immersion heater turns all its electricity into heat, a gas boiler a little less, and a heat pump can make more than two units of heat for each unit of electricity. The hot water calculator compares them at current prices, and adds your cylinder's daily heat loss if you enter it from its label.

Do

Practical next steps

  1. Fit a timer if the immersion has none, and set it for the time before you need hot water.
  2. Check the thermostat is at 60°C.
  3. Check the cylinder jacket and pipe lagging.
  4. Compare heat sources in the hot water calculator.

Take care

Important limitations

  • Do not turn the cylinder down below 60°C to save money: it increases the legionella risk.
  • Immersion heaters and cylinders are wired into the fixed electrics; use a qualified electrician for any changes.

Related tools and guides

Sources

Official sources checked 2 October 2026.