On Tue, 10 Dec 2013 16:54:54 -0600, philo wrote:

Not likely to be anything wrong with the thermostat.
May be as simple as the furnace switch is turned off.
It could also be the ignitor if the furnace starts to
energize but goes no further..

Ignitor?

I need a quick lesson on the parts of a furnace ... and what
they do, before I can diagnose or fix anything!

At first, I thought "this" was what you call the "igniter":
http://farm3.staticflickr.com/2812/1...8e651e36_o.gif

But, this youtube video calls that a "3-wire pilot assembly":
http://www.youtube.com/watch?v=JFoVcYpq1p8

Following the red "high tension lead", that video calls this
black box the "spark igniter" (is what you meant?):
http://farm6.staticflickr.com/5493/1...04b85a9f_o.gif

This is clearly what that video calls the "gas valve":
http://farm8.staticflickr.com/7358/1...fbde6523_o.gif

This isn't in the video, but, it's clearly some sort of fusible
link and something called a "lockout timer":
http://farm8.staticflickr.com/7323/1...56224304_o.gif

The brown board with the fusible link attached has two wires that
go upward to this thing (is it a sensor?), which, unfortunately,
wasn't described in the video:
http://farm3.staticflickr.com/2814/1...0de1c8bd_o.gif

This compartment was held shut with a 1/4" sheet-metal screw:
http://farm6.staticflickr.com/5503/1...351134b4_o.gif

Below that compartment was this set of terminals (I recognize
those wire colors as the same as those of the thermostat):
http://farm3.staticflickr.com/2849/1...e7793ca9_o.gif

To the right of the terminals, this 3Amp fuse tested good:
http://farm4.staticflickr.com/3759/1...c03444e1_o.gif

Inside the box was what the video named a "fan control board":
http://farm8.staticflickr.com/7443/1...bf7cb9e1_o.gif

And, I observed the basic stuff, such as this door safety switch
(which I taped tightly to keep in the on position):
http://farm4.staticflickr.com/3789/1...1f88de9a_o.gif

This is the on/off switch for the gas, which remains on:
http://farm8.staticflickr.com/7295/1...4cc8b179_o.gif

The 16x25x1 3M filter was surprisingly clean:
http://farm3.staticflickr.com/2814/1...7ba0ef50_o.gif

Googling, I found there usually is an on/off switch on the
side of the furnace where the power comes in. Mine doesn't
seem to have that switch; the power wires go right inside
the furnace from outside without a switch.

So now that I've identified "most" of the parts (I'm not sure what
the purpose of the lockout timer and that sensor thing are), it's
time for me to figure out what/how to test and debug the thing...