I recently moved to a new home and I'm a bit confused about the heating system. I seem to have an immersion heater (which uses electricity?), and also something in the kitchen (that uses gas?). I had assumed the one in the kitchen was just for tap water on-demand, but it seems to randomly fire up when no taps are running. Can you please explain how this all fits together? Can you please also explain why the switch I have highlighted appears to be turned off, but everything seems to be working? Thank you.

My current guess is that the main tank is heated up with gas, but has an immersion coil as a backup (which is currently turned off). Am I way off base?

Main tank: Main tank

Better view of where the switch goes Better view of where the switch goes

Kitchen: Kitchen

  • how do you know that the switch is turned off?
    – jsotola
    Apr 25, 2021 at 22:46
  • @jsotola sorry, I should have put that in the question. If I flick it, the tank starts to make a sound like when you boil a kettle. Guessing it's the heating element
    – rjdown
    Apr 25, 2021 at 23:10

1 Answer 1


The unit in your kitchen is the boiler and looks like a very simple gas model with a simple temperature control. There should somewhere be a time clock to control it. The plumbing near the tank looks like you have radiators for room heating so there should also be a room thermostat somewhere.

The water in the tank is normally heated by the boiler. The two pipes that go around to the back of your tank will be fitted to a coil of pipe inside the tank, and the boiler circulates hot water through these to heat the water in the tank. The electric immersion heater is there because it can heat the water faster than the gas boiler, however it does cost more to run. Typical uses for this are:

  • In an emergency if the gas boiler has broken down
  • If you need a lot more hot water than normal e.g. several guests staying all wanting hot showers in quick succession.

In your photo there is a second switch on the same wall nearer the camera. This is probably the master switch for the system, it looks like it is on in the photo and should normally be on. The immersion heater switch you should only turn on when you want the extra heating power (and are willing to pay for it). If you hare having some work done, you could think about moving the switch outside the cupboard and changing to one with an indicator light so that you don't forget to turn it off once the extra need is over.

The timing of the boiler running should be mainly controlled by a time clock somewhere, but it will only run if there is need, that is if the cylinder thermostat (white cuboid on the front of the cylinder) indicates that the water in the tank is colder than it should be, or if the room thermostat indicates that the radiators should be on.

  • That was very clear, thank you
    – rjdown
    Apr 26, 2021 at 13:54

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