I've got a curiousity question regarding the Honeywell VisionPro
thermostats. When you set them up, you have to select the type
of system. And you also have to set the number of cycles per
hour. They use that method instead of setting a temp differential
that turns the system on and off. That way of doing it raises
it's own questions, eg, what happens if it's only 55 outside and
it's set to 5 cycles an hour? Does it still fire up for 5 cycles of
just a minute or even less?

But here's the main question that has me stumped. They
recommend setting the cycles per hour to 5 for furnaces
of less than 90% efficiency and to 3 per hour for furnaces that
are 90+% efficiency. That seems backwards to me.
Older, 80% eff furnaces have fully open paths from combustion
air in right through to the chimney. Each time it turns off,
whatever heat remains in the furnace is lost mostly to the
chimney and I would think there would be more of it left in
the old furnace too. Think heat exchanger mass.

So, why would you want the 80% furnace to cycle 5 times
an hour, but the 95% one to cycle only 3 times?