On Jan 28, 11:49*am, Jeff The Drunk wrote:
On Thu, 28 Jan 2010 11:39:14 -0500, "Existential Angst"
wrote:

Based on my limited experience with furnaces and from what I was told,
this is my understanding:

When I had my 40 YO Perfection furnace with a massive heat exchanger,
the blower would be delayed at startup to allow to heat exchanger/
plenum air to heat up to avoid spewing cold air from the registers.

On the back end of the heating cycle, the blower would remain on to
both use up the residual heat in the heat exchanger/plenum and protect
the heat exchanger from over heating.

Called a "differential". Fan/limit switch can be set to turn on at
whatever temp lets say 170F, and off at any temp let's say 100F.
If you wish a little warmer air on the off cycle increase to 110F.

Closed combustion He furnaces could be controlled by a timer since the
heat exchanger is in a more controlled environment. Traditional
open/forced combustion air furnaces not so controlled so it's better
to use a fan/limit switch which probes the temp right above the heat
exchanger on the inside., =

The new furnace is an HE unit, so I can't (don't know how to) adjust
the "timer" to have the blower come on or go off at a different time,
nor do I want to.

I assume the manufacturer got it right and I have no desire to
override their wishes. ;-)

The "mechanical" fan/limit switch were much easier for the user to
control.