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Hot water cylinders

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 regulate the temperature of your domestic hot water by switching off the heat supply from your boiler once the set temperature has been reached. They are worth getting because they can save you money - around £37 on your energy bills by most estimates. 

Thermostats are usually installed about a third of the way up the hot water cylinder. They measure the temperature of the water inside and if this falls below the required temperature (usually 60-65°C), the boiler will come on. Once the water reaches the correct temperature, the thermostat will turn the boiler off again, ensuring that water is not being heated unnecessarily and no energy is wasted.

A domestic hot water cylinder thermostat should be set at 60-65ºC. This is high enough to kill off harmful bacteria such as Legionella. If you set the thermostat much higher than this the water that comes out of the taps will be too hot and there’ll be a risk of scalding. For extra safety, you can install a thermostatic mixing valve. This mixes hot and cold water to ensure the water in the taps is automatically maintained at its pre-set temperature, even if other appliances are being used or the water pressure varies.

Setting the thermostat to a higher setting doesn’t make the water heat up faster. This is governed by the design of the heating system. It will simply mean that you have to add lots of cold water to the hot, and will have wasted the energy spent in raising it to an unnecessarily high temperature.

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%, saving around £40 per year on your energy bills.

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 it does not completely cover the tank all the way around. Fitting the jacket is simply a case of wrapping it around the hot water cylinder and securing it with string or something similar. It’s a DIY job for most people. Jackets typically come in two standard sizes: 900mm x 450mm and 1,050mm x 450mm. It is recommended that you buy a cylinder jacket that displays the British Standard Kitemark. They typically cost up to £15.

Here are some approximate costs and savings (based on an 80mm thick jacket, and a gas-heated, 3-bed semi. For larger homes, or those heated by electricity or oil, the savings will be significantly higher).

 Cylinder jacketPipe insulation
Annual saving£49£15
Payback time< 6 months1-2 years
CO2 saving per year215kg60kg

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 local hardware store. If your pipes are difficult to reach then you may need professional help, which will incur some cost.

These savings seem relatively small compared to other energy efficiency measures, but few improvements pay for themselves as quickly. Moreover, if every hot water cylinder in the UK had a decent jacket, it would mean a reduction in CO2 emissions of 740,000 tonnes and a saving of around £89m in combined energy bills each year!

Images: hot water thermostat, Energy Saving Trust; tank insulation,

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