Hot water cylinder issue - boiling sound?

Noticed the hot water cylinder making a boiling sound this evening. Only way to stop it was to turn off the wall mounted switch that has a wire connected to the top of the cylinder. If this is an immersion heater then is this a supplement to the gas boiler? Confused!!

Apologies for my ignorance but CH systems are a mystery to me and I've no idea what type this is. But we have a gas boiler in the kitchen that provides heating and hot water, and the cylinder is in an airing cupboard on 1st floor. System, and house is ten years old.

There is a Sunvic thermostat on the side of the tank 1/3 way up, set to 50º. Could this have failed and caused the issue?

Any help appreciated!

Your switch is for the immersion electric heater. Thats three times more expensive for heating water than the gas!

The thermostat inside seems to have failed! You need a heating engineer or some electricians.

But as you seem to have no idea of what your gas boiler does then I would suggest you get it serviced and an explanation of the controls and how to use it.

Tony

Thanks. I know what a gas boiler does, just didn't know exactly what type it was.

I'm not entirely dim, although my post might sound that way. I've replaced the programmer and fitted a wireless thermostat in the past (with help from this resource). So I'll purchase a replacement thermostat to resolve this issue. But we'll be leaving the immersion (we never knew we had! off from now on

As I knew the boiler provided hot water it never occurred to me that the the cylinder would have an immersion heater as well, now I do!

the thermostat on the side of the the tank would even if it failed, never cause the cylinder to boil.

the thermostat inside the element could, however its not likely, cylinder can often make a kettle like sound when heating via the electric element, this is quite normal.