On Tue, 13 Nov 2012 18:06:11 -0500, "TomR" wrote:



Again, I don't know if a gas oven and a gas furnace have a similar setup,
but I thought I'd pass this on in case it helps.

My furnace looks for inducer negative pressure and ignitor glow before
the gas valve opens. Then good flame sensor sensing keeps it open if
flame is present.
I've replaced an inducer pressure switch for no purpose at all. You
can hear the switch close, and if the ignitor glows, it's okay.
Mine is timed for about 30 seconds before the ignitor glows.
I don't know how "glow" is detected by the circuitry, but I do know no
glow, no gas valve open, because I've replaced an ignitor that didn't
glow. Gas valve never opened.. But maybe there's other failure modes.
And I know a bad/dirty/flaky flame sensor can shut the gas valve.
Seen that happen too. Always immediately after flame ignition in my
case.
Pretty sure the gas valve has 2 steps. Low volume during the flame
sensor check, then high volume and main blower kicks on.
Been about 5 years since I've had a problem and actually looked at it.
This is all start-up ciruit logic, and I have no idea about logic in
effect during running operation.
AFAIK my roll-out sensors have never been tested, so I don't know if
they work.
A good manual for the controls should tell you all this. I also have
schematics in the furnace case, but don't know how to read them.
I'm an electricity know-nothing.
My furnace manual came with about 5 different possible control
setups, so you have to get that right first.
There's control board logic circuitry that can get messed up just like
an automobile ECU.
If you're guessing and confused, a pro will fix you up.