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This article is provided for information purposes only. You should only attempt any
procedures explained here if you are competent to perform the task and have the appropriate
tools and materials, including any relevant safety equipment.
Readers acting upon this article do so at their own risk and no responsibility is
accepted by the author or publisher for any loss or damage incurred by following the
article or failing to follow it.
If in doubt always consult a properly qualified tradesperson or other competent person.
WHAT IS A CYLINDER THERMOSTAT?
A cylinder thermostat is a safety device to regulate the temperature of water stored
in a hot water cylinder.
Cylinder thermostats are usually fitted on hot water storage cylinders (particularly unvented
hot water cylinders) to prevent the temperature of the water inside becoming higher
than the householder's requirement or exceeding safe levels for storage.
Cylinder thermostats are also used to control the fuel efficiency of a storage cylinder,
turning off the heat source when the desired temperature is reached in the cylinder
so that fuel is not wasted on heating the water when this is not required.
EXAMPLE OF A
HOW DOES A CYLINDER THERMOSTAT WORK?
Usually cylinder thermostats control the supply of electricity to the water heater
(e.g. a boiler).
The thermostat is wired into the power supply circuit for the water heater but the
circuit is only complete (therefore powering the heater) when the thermostat senses
that the water is not hot enough.
The thermostat typically senses the water temperature through a strip of metal clamped
to the side of the hot water cylinder.
The strip is made from two types of metal pressed together. The two metals have different
reactions when heated by contact with the cylinder and these differing reactions cause
the strip to either bend or straighten.
This movement of the metal strip either completes the electrical circuit to the water
heater, which turns the water heater on, or breaks the circuit, which turns the water
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