"Bill" wrote in message
ups.com...
I have a furnace that keeps locking out for an hour at a time when it
returns to normal until the next lock-out occcurs. My furnace
contractor has replaced the control board twice, the gas-valve once,
and put in three new flame sensors. They now are wondering if my LP gas
could be at the root of the problem because the flame detectors have a
green tinge to them.

I have two questions:

1) The ignition failures that lead to the lock-outs do not involve any
flame. That is, they occur before ignition; the gas just doesn't seem
to flow and the ignitor is definitely on. If the flame sensor was the
problem, wouldn't the failure occur after the flame was present, that
is it would not detect the flame even though one was actually present?
Isn't the flame detector's only function to detect for absence of flame
when gas is flowing into the burners and to shut off the gas at that
point?



Most all gas furnaces cycle in this manor, the stat calls for heat, the
inducer motor starts and satisfies a pressure switch. After a short time, 15
seconds perhaps the igniter starts. Now on a hot surface igniter, it needs
to heat for a few seconds before the gas valve opens, where a spark ignition
will fire at the same time the gas valve opens. The gas valve will open and
stay open for a couple of seconds before the control board even looks for
flame. After a couple seconds for an ignition trial, the board will "look"
for flame sense and keep the valve open, or if no flame is sensed go into
lockout, or retry.

If your furnace locks out on flame sense failure and no flame has been
present, you have other problems other than the flame sensor itself. It
could be many, board, gas pressure, valve, even wiring could be a problem.
Greg