"BillC" wrote in message
use 10 degree setbacks for best efficiency and least problems

OK, but why? Does 15 or 20 plus degrees of heat differential mean the
furnace gets too hot when running to close the gap, or what?

Heat is constantly seeking cold to achieve equilibrium. The house is warmer
than the outside, so heat is traveling out of the building. You add heat as
needed by burning fuel.

Stored heat (sensible heat) is heat that has been absorbed over time. If you
allow the air temperature to go too low, the stored heat from the
furnishings, fixtures, etc, is also given off. The time to restore it all
is making the heater work longer overall than a 10 degree setback.