On 12/20/2011 4:34 PM, Tuuk wrote:
Ok,

I changed the main fan board, and still the exact same condition happens.

Even though the thermost is off, even disconnected, as soon as I turn
the main toggle switch for my furnace, both main blower fan and exhaust
fans immediately begin and nothing else happens.

I just unplugged on leg from the high limit switch, then one leg from
the low limit switch then both together

Same, nothing.
Any suggestions?

Thanks
73

Normally, the blower motor and inducer blower should not
start at the same time. I was thinking maybe you had the
T-stat fan set to "on" and it was running when you called
for heat, or maybe it was running due to a "power off
during heat cycle" code. But in that case, the blower
would run for appx 90 seconds, go off, and then the inducer
fan would come on. This is usually a code "12" on a
furnace with winky blinky error lights.

But there is another thing that can cause what you are
seeing, and that would be an open solid state hi-limit
control. If one of those goes open, the IFC "integrated
furnace control" which is the board, will cut the 24 volts
to the gas valve circuit and then energize the blower
blower, and the induced draft blower. These will stay
on until the SS hi-limit closes, and then the heat
cycle will restart. So you may well have an open
hi-limit switch. Test the two connections with an
ohm meter. If the furnace is cold, the hi-limit
should show continuity. If not, it's kaput. These
are usually in series with the other limits, so
check all of them. Some of the ones in series will
have little buttons you can press to reset.
A SS hi-limit does not have a reset button. It
should close back up on it's own after the furnace
cools down. But they can go kaput and stay open.
Some may have thermal fuse links also..

If the hi-limit shows open with an ohm meter,
jump the two wires going to it with a jumper.
If that was the culprit, the furnace should start it's
normal heat cycle, close the pressure switch, and
start the igniter.
You would then need to replace the hi-limit switch.
Don't use the jumper in normal service. EVER!
That is just for testing.

But! You would also want to make sure there is no
problem that is causing high exchanger temps before
calling it a day.
Those can be a dirty filter, failing fan motor,
bad cap to motor, motor running too low a speed,
dirty blower wheel, over firing of the burners
due to excess manifold pressure, or orifice size.
So at this point, I think I would be eyeballing
the limits and the SS hi-limit in particular.
That usually looks like a phenolic wafer board with
two terminals that is often inside the case behind
the IFC.
And don't be afraid to call a tech if you feel you
are getting in over your head. I'd hate to see
you burn your house down due to some overlooked
problem that is causing excess temps.

BTW, you should always diagnose a problem in an
orderly military manner. Don't buy parts until
they are proven bad. Throwing money at a problem
and hoping something sticks can often get expensive
and not cure the problem.
Same thing with cars... :/

Anyway, this gives you a bit more to ponder..