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Electric water heater thermostat buying guide

Guide written by:
John, DIYer & IT developer, Brighton

John, DIYer & IT developer, Brighton

59 guides

Thermostats are used to regulate the temperature of a storage water heater by switching the heating element on and off. Whether you go for a stem, bulb or digital model, thermostats are essentially safety devices that protect you from scalding and stop your hot water cylinder from malfunctioning. Read on for our tips.

Important features

  • Stem thermostat
  • Bulb thermostat
  • Digital thermostat
  • Smart and safety thermostats

What is a thermostat?


A thermostat is a device that regulates the temperature of any type of equipment that produces heat. Thermostats are not only found on storage water heaters, but also on units such as electric radiators, boilers, and so on.

Regulating the temperature of a hot water cylinder is a cyclical process that halts the production of heat as soon as the temperature of the water reaches a set point. When the water starts to cool, the thermostat will then fire up the heating element again in order to maintain your set temperature.

Thermostat: a safety and money-saving device


Thermostats are essential for any type of heating device as they ensure the safety of your equipment and help you to save money: they are considered safety devices as they reduce the risk of scalding (along with various other features found at the tap and in the hot water cylinder); they save you money by measuring and adjusting the temperature only as needed. 


Role of a thermostat in an electric water heater

Thermostats are used to regulate the temperature of the water in the cylinder. Once the water in the cylinder has reached a set temperature, the thermostat switches off the power supply to the heating element. If the temperature of the water heater cylinder decreases – due to hot water being drawn or simply through natural cooling – the thermostat will trigger the heating element until the water has reached the set point once more.

Thermostats are important in terms of your comfort, your wallet and your safety. Maintaining a water temperature of 60°C within your hot water tank will satisfy all your hot water needs while limiting your energy consumption. This will also help to avoid the risk of scalding and protect the hot water cylinder from limescale.

Different types of thermostats

There are three different types of thermostat which function using different types of technology.

Stem thermostats



Reliable and inexpensive, this type of thermostat is plugged in.  Its long stem expands as the temperature rises.

If the thermostat overheats, it is necessary to reset the thermostat manually. However, this overheating is sometimes caused by a heating element failure; it is therefore strongly advised to check both to determine the origin of the malfunction.

Bulb thermostat


Also known as a capillary thermostat, bulb thermostats are as reliable as stem thermostat and are more precise.

The bulb is filled with heat-reactive liquid which increases in volume and allows the thermostat to trigger or stop the production of heat by the heating element.

Digital thermostats

Digital thermostats offer even more precision and a wider range of possible temperature set points.

These thermostats contain a thermistor which uses electrical resistance to measure temperature variations. The resistance value of the thermistors changes according to the temperature.

Safety features and smart thermostats



Smart thermostat

Smart thermostats are used to regulate the production of hot water in the storage cylinder according to the weather and your habits.

Safety thermostat

This is a safety device that prevents the thermostat from overheating and malfunctioning.

Choosing a replacement thermostat

Thermostats are usually marked with a reference number and replacing them should simply be a case of finding the same model (depending on the brand and type of your storage water heater).

The diameter of the probe should also be measured (in millimetres) so that it can fit into the bracket.
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Guide written by:

John, DIYer & IT developer, Brighton 59 guides écrits

John, DIYer & IT developer, Brighton

Since I was a child, I was always interested in manual and technical works. Always fascinated by woodworking, I took advantage of my first flat as a playground. On the cards: electricity (of course, safety first!) and some partition walls; but also decorating with the help of the missus, made-to-measure furniture and little tricks to optimise the space, all the while remaining as original as possible. When the little one arrived, I started building bits and pieces for him!

Lacking space, I have not got a permanent workshop and certain tools I dream about but are not part of my collection. Not to worry, I already know a lot about DIY and I have a high-tech profile that I hope will guide you in your decisions!

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