GF - B 3
Induced Draft Motors
a learning routine to learn the optimum operating
speed. The adaptive learning routine begins on the
first call for heat anytime that the POWER has been
CYCLED or the integrated furnace control has done
a RESET.
Vent pressure switch must be open prior to
motor starting
On this first call for first stage heat, the IFC checks
the two limit circuits and the first stage pressure switch
to be certain it is in the open position. The switch
must be open before the IFC starts the variable speed
vent motor. PS-2 is not checked at this time.
If PS-1 is closed prior to the induced draft blower
running, the IFC will send a three flash fault code.
The induced draft blower motor will remain OFF. If
PS-1 should open and there is still a call for heat, the
IFC will stop sending the three flash fault code and
will then start the first stage learning routine.
Learning routine
The IFC ramps up the vent motor to its first stage
default transition speed of 2600 RPM. If PS-1 is not
closed, the IFC will increase the vent motor’s RPM
in 300 RPM steps, until PS-1 closes or first stage
vent motor’s max. RPM of 4400 is reached. If PS-1
does not close when maximum RPM is reached, the
IFC will stop the vent motor and send a three flash
fault code. Thirty seconds later the IFC will again
start the vent motor and stop sending the three flash
fault code. This off period is programmed to provide
time for any water that may have accumulated in the
vent system to drain out.
When PS-1 does close, the IFC will start the hot sur-
face ignitor warm up cycle. Near the end of the igni-
tor warm up cycle, the IFC turns on the first stage
gas valve and checks for ignition. One and one-half
minutes after flame is detected by the IFC, the IFC
will reduce the RPM of the vent motor until PS-1 tra-
nsitions open. The IFC then increases the vent mo-
tor RPM by three hundred RPM and stores this learned
RPM in its memory and uses it for the next one hun-
dred and fifty times when the first stage is called for.
The one and one-half minute time delay after flame
has been detected allows the heat exchanger to heat
up to its operating temperature and the density of
the products of combustion to stabilize. This learn-
ing cycle is done so the IFC control can determine
and then set up the vent motor’s minimum speed
needed for this installation to obtain the efficiency
rating of the furnace.
Second stage learning routine
The second stage adaptive learning routine begins
on the first call for second stage heat anytime that
the POWER has been CYCLED or the IFC has done
a reset. One this first call for second stage heat, the
learning routine for second stage heat will begin.
During the learning routine, the IFC checks PS-2 to
see if it is open. If PS-2 is closed, the IFC will send
a three flash fault code and remain in first stage op-
eration.
If PS-2 is open, it will ramp up the vent motor to its
second stage default transition speed of 3600 RPM.
If PS-2 is not closed at this time, the IFC will increase
the vent motor’s RPM in 300 RPM steps until PS-2
closes or the second stage vent motor max RPM of
5200 is reached.
If PS-2 does not close when second stage max RPM
is reached, the IFC will slow the vent motor back
down to its first stage learned RPM and send a three
flash fault code. The IFC will wait ten minutes and
again try to get PS-2 to close. When PS-2 does
close, the IFC will reduce the vent motor RPM in
steps until PS-2 transitions open. The IFC then in-
creases the vent motor RPM by two hundred RPM
and stores this learned RPM in the IFC memory and
uses this learned RPM for the next fifty calls for second
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