What thermostat does what?

[Willsy]

Hi, if my picture uploading has worked you should see 4 pics below of my cylinder on an open vented heating system.

Just wondering which thermostat controls heating and which controls water.

If the pictures haven't come out then there's the thermostat under the cap at the top of the cylinder and there's the one that's tied to the side of the tank, cut through the jacket.

I also have no idea what a mid-point actuator does (also in pictures) and how to use it.

If someone could shed some light I'd be very happy!

 

[Riley]

Pic 2 = immersion heater, has its own thermostat built in
Pic 3 = hot water stat
Pic 4 = 3 port valve dictating hot water on, heating on or both

 

[Ben-gee]

The thermostat on side of cylinder controls the water temperature when heated by the boiler. The one under the cap controls it when heated by the immersion.
Leave the mid position actuator alone, it is controlled by the programmer and you you don't need to touch it - It diverts heat from the boiler to either the hot water or the central heating or both together.
The temperature of the central heating is most likely controlled by a roomstat somewhere else (hallway?).

 

[Ben-gee]

Riley beat me to it!

 

[Willsy]

Thanks for all the replies so far. I have a much better understanding of it but...just so I can understand this...

if you set the boiler thermostat to 65C and the immersion to 60C then the boiler would heat until 65C water was achieved and maintain that temperature and the immersion heater would not need to do anything?

and if the boiler was set to 60C and the immersion was set to 65C then the boiler would heat to 60C and the immersion would continue heating until 65C was reached?

Does the immersion remain on 100% of the time while the boiler only heats when it is timed to? So, if the boiler was set to heat for 1 hour at 6pm and the water temp dipped to 58C at 9am, would the immersion bring it back up to temperature?

Just trying to understand why 2 separate thermostats are needed!

 

[Ben-gee]

Generally speaking the immersion should be off, think of it as a back up if boiler fails - much cheaper to heat by gas.

 

[Riley]

Immersion is back up only don’t run both

 

[Willsy]

Generally speaking the immersion should be off, think of it as a back up if boiler fails - much cheaper to heat by gas.

Immersion is back up only don’t run both

OK, perfect - that makes loads of sense now. Appreciate all the answers. Thanks very much!