Gravity fed system not sure how this valve works

[Rikki2]

This valve is fitted to the lower heating coil of the hot water cylinder Can anyone say what is the point of it I have been told its a thermostatic valve to control hot water temp so would have thought it may be in the wrong position ?Any explanations welcome

 

[exedon2]

Looks like a Drayton cyltrol very basic thermostatic control.
That's how they were fitted.