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Unvented Hot Water Cylinders:
A Fault-Finding Guide for Homeowners

This fault-finding guide has been specially designed for homeowners to determine any faults that may occur in a Hot Water Cylinder.

Important: Please be aware that any work that is undertaken on an Unvented Cylinder MUST always be performed by a G3 qualified engineer. An Unvented Hot Water Cylinder is an extremely dangerous device if handled by someone who is not trained. It includes the use of mains pressure equipment and you must be a qualified engineer to handle the unit.

For a fault finding guide geared towards engineers, please see the following link: A Fault-Finding Guide for Engineers

What is an unvented hot water cylinder and what is it used for?

A hot water cylinder is a storage system that heats water in your home and allows you to have access to hot water instantly whenever needed. An unvented system is fed directly from the cold-water mains and then heated by either an immersion heater (for a direct system) or gas from a boiler (for an indirect system). One of the benefits of an unvented system versus a vented system is that there is no need for a cistern tank, meaning you have more flexibility when it comes to space in your property.

Please use the below checklist as a fault-finding guide for an unvented hot water cylinder.


The first thing to do is check the immersion heaters directly.

Take off the immersion cap and press the reset button in both immersions. If they are still not getting any heat, check to see if there is a remote timer that tripped out or is not functioning correctly.

  • If they are all working correctly this means, there is likely to be a more complicated issue with the immersion. In this case, any adjustments MUST be made by a qualified engineer.

Secondly, check the circuit breakers on the Fuse Box.

Is the circuit breaker for the hot water cylinder in the ON position as required? Depending on the model of cylinder some may have 2 circuit breakers; one is the main immersion heater and the other would be the boost immersion heater.
  • If the circuit breaker is in the OFF position, please turn it to the ON position.
  • If the circuit breaker does not stay in the ON position, do not hold it or try to force it to stay on the ON position. Leave it in the OFF position and contact a qualified engineer as there may be an electrical fault, such as a faulty immersion heater.

Thirdly, is the supply RCD (Residual Current Device) protected?

In addition to the circuit breaker which is supplying the cylinder may be RCD protected. If this is the case, then try to turn it on. As before with the circuit breaker if the RCD does not stay in the ON position then do not try to force it or hold it in the ON position. Instead turn off all the circuit breakers protected by the RCD and turn the RCD back on. Now try to switch the RCD to the ON position. If it still will not stay in the ON position contact a qualified engineer.

  • If it does stay in the ON position, then next try to switch each circuit breaker on one at a time to identify which circuit breaker is tripping the RCD. This problem can be a result of either a fault on the cylinder or it could be a fault on another circuit.

If the fault is with the cylinder or the fuse box, then contact a qualified engineer.


An indirect tank is heated by a coil indirectly from a boiler. What controls this process is the dual control thermostat on the cylinder, the motorised zone valve and the boiler output control thermostat. These items work in tandem together to heat your indirect cylinder.

Firstly, check the boiler control output thermostat.

  • Check that this has not been turned down, or even turned OFF. In addition, if you have this at a too low temperature, there will not be enough hot water to satisfy the tank.

Secondly, test the control thermostat.

  • Next, check the cylinder directly and manually check the dial on the dual control stat. This will make a clicking noise when it has stopped calling for water and you will hear the boiler starting up.

Thirdly, check the Motorised Zone Valve.

  • If the boiler is not firing up it means there is a problem with the motorised zone valve. Please contact a qualified heating engineer at this stage as ONLY a qualified heating engineer can check the motorised zone valve at it is fully wired in.


If you are unable to fix you Hot Water Cylinder and are in need of a replacement, we offer a wide variety to fit your needs.

See the full range here: Click here!

Any further questions?

Our dedicated technical sales support team is on-hand to offer you advice and expertise.

Call us today on 01386 760066 or email at

We recommend contacting your local G3 trained engineer. A good starting place is

Please note: Advanced Water Company have no relationship with the Gas Safe Register and do not personally recommend any engineers.