How EXACTLY do immesion heater tank thermostats work ?

[RogerJ]

John Ward, the presenter) didn't get it EXACTLY right either He was corrected more than once in the comments....

 

[johntheo]

Where I find this stat interesting is where a single stat is used in a top mounted immersion where normally sink & bath lives are switched manually depending on the volume of HW required, one terminal on each heating element are permanently linked to each other and the thermostat switches the common neutral.
the thermostat rod length is matched to the longest (bath) heating element, on a standard HW cylinder with 11" and 23" elements the stat length will be 22/23".

I wonder what effect just fitting a 11" stat would have. Sink selected no problem. Bath selected and assuming a cold cylinder then the temperature will rise uniformly until 60C is reached and the stat switches Off, if you then start using HW the cold water will displace the HW until the contents of the cylinder are almost exhausted, temp falls to (by say 5C, hysteresis) to 55C, stat switches ON, cylinder reheats. The only? advantage I see with using a 22" stat is that it may switch on once a smaller vol of water has been used, some of the stat will be at 60C and some at say 15C (mains temp) so perhaps this is enough to switch on the stat.

 

[johntheo]

Actually, think I got a bit mixed up, just think the stat pocket is really only 11" or so, no mystery.

 

[johntheo]

Just to clear up the above point, all top mounted twin immersions with a common stat means that the stat pocket & stat will never be longer than the short element, both elements will work fine, and the stat will cut out at its setpoint temperature. If the stat was the same length as the longer element, and if this (bath) element is selected it will cut out at its setpoint temperature and cut in and start reheating the cylinder once the incoming cold water is in contact with a fairly small length of the stat.
If the shorter (sink) element is selected then the stat just might cut out (if at all) but at a very high temperature as "1/2" the stat will be ~ at mains water temperature and the other "1/2" might be at 100C if a vented cylinder.

I did a few simple tests on a new 11" stat that I found lying around, the stat, when fully immersed in water switched out at 62C and switched in at 50C, with 4.5" immersed, it cut out at 73C and switched in at 67C. Stat nominally set to 60C for both tests.

 

[RogerJ]

All very interesting but..... The point of the thread was to discover if anyone knew for certain that the thin steel rod inside the stat's brass tube is Invar.. I think we're 90% certain it is.. That's all..

 

[EricMark]

Must admit I don't know exactly how it all works, the willis immersion heater is clearly different from direct, and going in from side of tank is also different from going in from top, but it seems unlikely that all thermostats work in the same way.