Stormin Mormon wrote:
Since it does heat if the temp is way up, sounds like a thermostat
problem. Please let us know what you find.

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
.


okay... sorry it took me so long to get this reply out. Thanks to
everyone who contributed to the explanations! I just have to say that
I'm really glad that manuals exist and are easily downloadable on the
internet! The problem was indeed in the thermostat. It turns out
some of the settings in the thermostat (Totaline P474-0100) were not
right (and the previous owners of the house must have just suffered
with it). The settings in question were the 'electric heater fan'
option, which was mis-set to ON. This was causing the blower in the
[gas] furnace to blow cool air through the house. The other setting
was the 'dT' setting (or the 'wiggle factor' setting) which gives a
'buffer' to the temperature set point. It was set to 5 degrees
(wow!), I set it to 1 degree. This may make more sense if I explain
how the thermostat works... For this thermostat, the 'call for heat'
is set up in two stages:
(say my house is at 66 degrees and the temp I've set the thermostat to
turn the heat on at is 71 degrees)
stage 1) when the indoor temp reaches the setpoint (71 degrees) then
'stage 1' has been reached. A red LED turns on on the thermostat
indicating that it's reached the set point, and if the 'electric heater
fan' option is set to ON, the blower is turned on.
stage 2) if the temperature gets to the setpoint - wiggle factor - 2
more degrees then the furnace is told to turn on.

So, what I was getting was the blower turning on at 71 degrees because
the thermostat thought my furnace was electric. This horrible breeze
would circulate through the house until the thermostat reached 64
degrees (71 degrees - 5 degrees of wiggle - 2 degrees), but since the
house was sitting at 68, the thermostat would never turn on the
burners! The result for me was exactly what I was experiencing. It
appeared that for small differences in temperature (between the heat
setpoint and the house temperature) I was just getting this cool
breeze, but if I boosted the heat setpoint up (say to 90) then the
stage 2 criteria were satisfied and the thermostat would send an
appropriate 'call for heat' to the furnace...

On top of this, the thermostat was measuring the wrong temp to begin
with, so I went ahead and recalibrated the temperature sensor... What a
difference!

So now my furnace turns on at exactly 3 degrees below the heat set
point since I can't set the 'wiggle factor' to 0 and 2 extra degrees
are hardcoded into the thermostat, but I no longer get the cool breeze.
This is MUCH more acceptable! Tthe previous owners must have always
just compensated for this problem! I'm really glad that the manuals
were online! Thanks again for everyone's suggestions!