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Hot water cylinders

A montage images showing a hot water tank and a close up of a hot water tank thermostat

Our hot water cylinder guide explains how to save energy and money if you have one of these in your home.

Last updated in August 2022

If your home has a hot water cylinder (as opposed to a combi boiler), fitting it with a thermostat and making sure it is properly insulated are simple and cost-effective ways to save energy.


Cylinder thermostats are well worth having because they mean you don’t spend money heating water to a higher temperature than you need to.

Usually installed about a third of the way up the cylinder, the thermostat measures the temperature of the water inside and if this falls below a set level, the boiler will come on. Once the water reaches this temperature, the thermostat will turn the boiler off again, ensuring that water is not being heated unnecessarily and no energy is wasted.

The usual setting for a domestic hot water cylinder thermostat is between 60ºC and 65ºC – high enough to kill off harmful bacteria such as Legionella. If you set it much higher than this the water that comes out of your taps will be too hot; there’ll be a risk of scalding and you’ll have to add cold water which is simply a waste of the energy used to heat the water that extra 5 degrees or so.

Note that the thermostat doesn’t control the speed at which water in your tank is heated up. This is governed by the design of the heating system.

All cylinder thermostats should be installed following the manufacturer’s instructions and should comply with the Institution of Engineering & Technology wiring regulations.

Tank insulation

Many houses in the UK have un-insulated hot water tanks which allow heat to escape. Fitting a hot water cylinder jacket will insulate the tank and the reduced rate of heat loss will help maintain the water temperature in the tank for longer. Heat loss from the tank can be reduced by over 75%. This could save you up to £50 per year (based on energy prices at October 2023).

Even if you already have a hot water cylinder jacket, you may want to consider replacing it if it is less than 75mm thick or doesn’t completely cover the tank all the way around.

Cylinder jackets typically come in two standard sizes: 900mm x 450mm and 1,050mm x 450mm and they cost around £20. We recommend you buy one that displays the British Standard Kitemark. It will pay for itself in about a year.

Fitting the jacket is simply a case of wrapping it around the hot water cylinder and securing it with string or similar – a DIY job for most people.

For extra savings, lag your hot water pipes too

Foam insulation for pipes, known as lagging, is very cheap, and like fitting the tank jacket, it’s a DIY job if your pipes are accessible. Simply measure the diameter and length of your pipes and purchase the correct size and quantity of lagging from a hardware store. If your pipes are difficult to reach then you may need professional help, which will incur some cost.

The cost savings from lagging your boiler and pipework are relatively small compared to other energy efficiency measures, but the materials are cheap, and few improvements pay for themselves as quickly.

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