I have two strap-on cylinder thermostats, as normally used for DHW cylinder but in this case applied to a thermal store. I understand (right or wrong) the Switching Differential to be the temperature difference between 'On' and 'Off'.
I've had a Danfoss ATC, with a quoted SD of 6-10K approx. However, for my installation I measured the peak& trough at about 30degr. I though it must be broke, so I replaced with a new Honeywell L641A with a stated SD of 10 approx. But it turns out this also gave me a peak-to-trough difference of about 30degr. Seems to be mounted OK, flat to the cylinder (and yes I dug out the insulation to the copper). I can understand these being a _bit_ out but not by 300%. Can anyone shed any light on this?.- I'm baffled - I can't have two broken stats?.
The boiler control circuit just has this 'stat wired in series - electrical connection between 'C' and '1' for make-on-rise & break-on-fall. The stat location is a bit over halfway up the cyl. The SD needs to such that the boiler doesn't frequently switch on/off (undesirable for varoius reasons) or the CH doesn't go cool for long periods. The supplied built-in immersion 'stat is too sensitive and switches the boiler too much, hence the strap-on stats. Actual SD of 12-15C would make the boiler run every hour - about right.
One way out of this would be an adjustable cyl stat - preferably one with the control dial connected by a wire to a cylinder sensor (to make it accessible). I haven't found one yet (and I'm dreading the price tag if I do). Or, take pot luck with the Horstman cyl stat (SD of ~3) and see where I end up there. Just seems like something simple has somehow become difficult!.
I don't quite know where I'm going with this so any thoughts would be welcome.
Egremont.
PS - I know there are better and more expensive/complicated solutions which I'm trying to avoid if I can.