John Williamson wrote
Onetap wrote
Mathew Newton wrote

How much simpler do you need it? At first I thought you simply didn't understand, which is fair enough, but now I'm
beginning to think you don't even *want* to understand.

Yeah, we know what it's meant to do; the question is, how does it do it? If you heat the element, it will open
earlier because it's hotter, but how does that help you get a more accurate measurement of the air temperature?

The aim of the design isn't to get an accurate measurement of the air temperature *at the time of the measurement*,
but to anticipate what the air temperature will reach *after the heating is turned off*. As the radiators and the
water they contain as used in most domestic
heating systems hold a fair amount of heat which will be released
after the boiler is turned off, the heat source (boiler) needs to be
turned off *before* the desired air temperature is reached, otherwise
the set temperature will be exceeded. This is why the thermostat has either software anticipation or a hardware heater
built in.

The trouble with that line is that even systems like say
electrical fan heating still see a big improvement in the
hysterises with a bi metallic strip thermostat with a heater.

The real reason that happens is because the heater just turns off the
heating before it would otherwise be turned off by the bi metallic strip.

In an ideal world, the preheat on the thermostat will be calibrated for each installation,

No need, it works fine even when there is no inertia in the heating system.

but most of the time, the normal value as installed is "close enough".

In reality, heating the bi metallic strip ALWAYS reduces the hysteresis.

Blown air heating systems need a different control regime,

Nope, they still get a reduced hysteresis with a heated bi metallic strip thermostat.

preferably a proportional control system,

That just reduces the hysteresis even more by a different approach.

You can do that with a hot water system too.

which takes account of the heat lost by the building,

Thats never constant.

so that the heating runs at a reduced power level at all times, in a steady state.

Yes, thats why the proportional system works even better again.