36
If power is interrupted, the stored history is erased. When this
happens, the control CPU will initially select low--heat for up to
16 minutes and then switch to high-- heat, as long as the
thermostat continues to call for heat. Subsequent selection is
based on stored history of thermostat cycle times.
The wall thermostat “calls for heat,” closing the R to W circuit.
The furnace control CPU performs a self--check, verifies the
low--heat and high--heat pressure switch contacts LPS and HPS
are open.
A06456
Fig. 41 --- Inducer Housing Drain Tube
A06457
Fig. 42 --- Filling Condensate Trap
TABLE 10—BLOWER OFF DELAY SETUP SWITCH
POSITION
DESIRED
HEATING MODE
BLOWER OFF
DELAY (SEC)
SETUP SWITCH (SW1---7 AND
SW1---8) POSITION
S W 1 --- 7 S W 1 --- 8
90 OFF OFF
120 ON OFF
150 OFF ON
180 ON ON
1. Inducer Prepurge Period-- The furnace control CPU turns
on inducer motor IDM and slowly increases the inducer
motor speed. When the low--heat pressure switch LPS
closes, inducer motor RPM is noted by the furnace control
CPU, and a 25 -- sec. prepurge period begins. The RPM is
used to evaluate vent system resistance. This evaluation is
then used to determine the required RPM necessary to
operate the inducer motor during the low--heat prepurge
period and low-- heat mode.
NOTE: The heat cycle can start in either high--or low--heat. If a
high--heat cycle is initiated, the furnace control CPU will
deenergize the high--heat pressure switch relay HPSR to close the
NC contact and continues to increase the inducer motor speed
after the low--heat pressure switch LPS closes. When the
high--heat pressure switch closes, inducer motor RPM is noted by
the furnace control CPU before the 25-- sec prepurge period
begins. The RPM is used to evaluate vent system resistance. This
evaluation is then used to determine the required RPM necessary
to operate the inducer motor in high--heat mode.
58UVB
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