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of the mid pressure switch is completed. Also, a new cali-
bration sequence will be executed on the next heat cycle.
Provided the mid pressure switch is open, the furnace
control will continue increasing the inducer RPM in dis-
crete steps. The furnace control will first verify that the in-
ducer drive is operating at the correct speed. The inducer
speed will be increased slightly and the control will again
first verify that the inducer drive is operating at the new
speed and then wait for the mid pressure switch to close.
This cycle repeats until the mid pressure switch is closed.
If the RPM demand is at the maximum value for the mid
pressure switch and the pressure switch still is not closed,
a fault code "67" will be set at level 1 and displayed simul-
taneously with "H" or "h" (as appropriate) at the seven-
segment displays. The control will then enter the high
pressure switch calibration sequence without a valid mid
pressure switch RPM. Calibration will still proceed but
without verifying the mid pressure switch and the proving
and pre-purge sequences will continue as long as all other
requirements are met. Firing rates between 50% and 65%
will fix the firing rate at 70% until a successful calibration
of the mid pressure switch is completed. Also, a new cali-
bration sequence will be executed on the next heat cycle.
Upon sensing the mid pressure switch closed, the RPM
(demand) at which the inducer was running when the mid
switch closed will be added to a predetermined value and
the result will be stored in the furnace control memory as
the cold mid pressure switch RPM for the 50% to 65%
cold gas heat rate.
During mid pressure switch cold calibration, if the high
pressure switch should close before the mid pressure
switch closes, fault "67" will be issued and the RPM at
which the high pressure switch closed will be added to a
pre-determined adder and this sum will be used as the in-
ducer rpm for the 100% cold gas heat rate. Mid pressure
switch cold calibration will be bypassed, high pressure
switch cold calibration will be considered complete and
operation will proceed to pre-purge. The fault "67" shall
remain present and displayed alternately with "H" or "h"
as appropriate until the heat call is gone or until the pres-
sure switch closes on a subsequent calibration attempt. A
calibration cycle will be forced on the next heat cycle.
If the low pressure switch should open during mid-pres-
sure switch cold calibration sequence, the control will sus-
pend the calibration sequence and wait 30 seconds for the
low pressure switch to re-close. If the low pressure switch
re-closes within this 30 second period, cold calibration re-
sumes as normal. If the low pressure switch does not re-
close within 30 seconds, the control will exit the calibration
sequence and enter a five-minute wait period during which
the fault level for the open low pressure switch fault "45"
is set to 1. After the five-minute wait, the calibration se-
quence is restarted. If the low pressure switch will not
close during the new cold calibration sequence, the open
low pressure switch fault "45" will then be set to level 2.
High Pressure Switch Cold Calibration
Since the 65% heat inducer RPM is determined or the mid
64
calibration sequence has been skipped for reasons stated,
the inducer speed is then increased from the value at
which the mid pressure switch closed. The furnace control
will first verify that the inducer drive is operating at the cor-
rect speed. After that, the control waits for the high pres-
sure switch to close. If the high pressure switch does not
close, the inducer RPM (demand) is then increased
slightly and the control will again first verify that the in-
ducer drive is operating at the new speed and then waits
for the high pressure switch to close. If the RPM demand
is at the maximum value for the high pressure switch and
the pressure switch still is not closed, fault code "57" will
be set at level 2 and displayed at the seven-segment dis-
plays and the control will shut off the inducer and enter a
five minute wait period before attempting a new calibration
sequence. The fault will remain at level 2 until the end of
the lockout period even if the heat call is removed.
Upon sensing the high pressure switch closed, the RPM
(demand) at which the inducer was running when the high
switch closed will be added to a predetermined value and
the result will be stored in the furnace control memory as
the cold high pressure switch RPM for the for the 70% to
100% cold gas heat rates.
If the low pressure switch should open during high pres-
sure switch cold calibration sequence, the control will sus-
pend the calibration sequence and wait 30 seconds for the
low pressure switch to re-close. If the low pressure switch
re-closes within this 30 second period, cold calibration re-
sumes as normal. If the low pressure switch does not re-
close within 30 seconds, the control will exit the calibration
sequence and enter a five-minute wait period during which
the fault level for the open low pressure fault "45" switch
is set to 1. After the five-minute wait, the calibration se-
quence is restarted. If the low pressure switch will not
close during the new cold calibration sequence, the open
low pressure switch fault "45" will then be set to level 2.
Pressure Switch Proving:
Upon entry into the pressure switch proving state, if the
furnace is in a calibration cycle, the inducer will continue
to run. If the system is not in a calibration cycle, with in-
ducer off, if both the low and high pressure switches are
not open, the IFC will display fault code "44" and/or "55"
(as appropriate) on the dual seven-segment displays
(S.S.D.s) and set the fault active. If either pressure switch
is closed when it should be open, the IFC will wait indefi-
nitely and will not proceed with the heat call until both
pressure switch circuits are determined to be open. This
fault will continue to be set and displayed until both pres-
sure switches are open even if the heat call is removed.
If the mid pressure switch is not open during pressure
switch proving, the fault code "65" will be set and dis-
played simultaneously with the "H" or "h" (as appropri-
ate) and the proving and pre-purge sequences will
continue as long as all other requirements are met. How-
ever, furnace operation will fix any heating rate which is
between 50% and 65% to 70% regardless of the demand
of the thermostat until a successful calibration of the mid
pressure switch is completed. Also, a new calibration se-
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