On Jan 26, 1:29*pm, Steve Barker wrote:
On 1/25/2011 10:59 AM, Mark wrote:

* *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.

I don't have the answers to your other questions but in my older (oil
fired hot air) furnace with a massive cast iron heat exchanger, *when
the burner shuts off, the air circulation blower remains on until the
heat exchanger is cooled down.

So most of the stored heat is recovered and NOT lost to the chimney.

Mark

That's the way any furnace works. *They never shut the blower off with
the exchanger hot.

--
Steve Barker
remove the "not" from my address to email

I uderstand that. But at whatever temp they do shut off, eg 85,
there is still some
residual heat left in it that goes into the basement, garage, up the
chimney, etc. On an
old 80% furnace, 5 mins after it shuts off, it's not unusual for it to
still feel warm
to the touch. You can't cool off all the heat exchanger mass in just
the extra min
or so the blower runs after the burner shuts off. Hence, it would
seem to me you'd
want to run it at least the same number of cycles an hour that you'd
run a newer
furnace. In the grand scheme of things, maybe it doesn't amount to
much energy,
but why the difference in cycles an hour then?