Steve Litt:
...
> Every temperature control system I've seen depended on hysteresis,
...

That would be a so called "bang-bang" regulator, i.e. when at bottom
value turn on heater, when at top value turn off heater, which is what
you get with bimetal mecanical thermostats.

What you use in boiler rooms is usually a computer controlled system
like plc and the like, which commonly uses some form of PID-regulator.

I think the term you are looking for is not "hysteresis", but "error band".

Regards,
/Karl Hammar