On Tue, 10 Dec 2013 18:19:44 -0600, Irreverent Maximus wrote:

http://www.xpedio.carrier.com/idc/gr...0394dp65-a.pdf
download above link. If it is corrupted, update your PDF reader.

I read the entire PDF, from front to back.
Here's a summary of what I gleaned about the furnace operation:

Summary:
1. Red wire is 24VAC hot.
2. Thermostat "calls for heat" by connecting Red to White.
3. Thermostat "calls for fan" by connecting Red to Green.

Details:
Thermostat "calls for heating" by connecting Red to White.

Power then goes from the Transformer - Fusible Link - Limit Switch -
Vent Safety Shut-off Switch - pilot (both "pick" & "hold" gas valves)
such that pilot gas flows, where it ignites.

~60 seconds after pilot, safety pilot switches its contacts and energizes
the main valve portion of the gas valve, where, about 10 seconds later,
the main gas valve opens, and the main gas is ignited by the pilot flame.

~75 seconds after the pilot lights, the fan control board activates the
fan on low speed (it uses the low speed for heating purposes).

When the thermostat "is satisfied", the connection between R & W is broken.
Gas is immediately stopped to both the pilot and main burners.
The blower continues for about 100 seconds.

If the furnace overheats, the Limit Switch opens.
If the furnace overheats in the vestibule, the Fusible Link opens.
If the furnace overheats in the vents, the Vent Safety Switch opens.

I see the fusible link on the red wire he
http://farm8.staticflickr.com/7323/1...56224304_o.gif

I'm guessing this is the limit switch:
http://farm3.staticflickr.com/2814/1...0de1c8bd_o.gif

But, I can't seem to find the "Vent Safety Switch".