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PROCEDURE 1—SELF-TEST MODE
The control center goes through a brief self-test whenever 115-v or 24-v power is interrupted. The self-test takes approximately 2 sec to complete.
After power is restored, red (microprocessor) LED briefly comes on. Then green LED comes on for 1 sec, followed by 1 sec where both yellow
and green LEDs are on. During this time, the microprocessor is checking itself.
PROCEDURE 2—HEATING MODE
When thermostat calls for heat, R-W/W1 circuit closes.
1. Prepurge period—The inducer motor is turned on and slowly comes up to speed. When low-pressure switch closes, inducer motor RPM
is noted by microprocessor, and a 25 sec prepurge period begins. The RPM is used to evaluate vent system resistance. This evaluation is
then used to determine required RPM necessary to operate inducer in low-heat mode.
NOTE: The heat cycle can start in either high or low heat. If a high-heat cycle is initiated, the inducer continues increasing its speed after
low-pressure switch closes. When high-pressure switch closes, inducer motor RPM is noted by microprocessor before the 25 sec prepurge period
begins. The RPM is used to evaluate vent system resistance. This evaluation is then used to determine required RPM necessary to operate inducer
in high-heat mode.
2. Humidifier (HUM)—The HUM terminal is energized after the inducer prepurge period is completed.
3. Ignitor warm up—At end of prepurge period, the hot surface ignitor (HSI) is energized for a 17-sec HSI warm-up period.
4. Ignition sequence—After HSI ignitor warm-up period is completed, the gas valve opens, permitting gas flow to burners where it is ignited.
After 5 sec, the HSI is de-energized, and a 2-sec flame-sensing period begins.
NOTE: The initial heat mode after 115-v or 24-v power interruption will be LOW HEAT. Low heat remains energized for 16 minutes before high
heat is initiated, providing thermostat is still calling for heat.
After initial cycle, the microprocessor evaluates the length of low- and high-heat operating times and calculates optimum length of low and high
heat for next heat cycle. This accommodates the heat load requirement seen as a result of thermostat operating time.
5. Flame sensing—When burner flame is sensed, the control center holds gas valve open and begins blower on delay period.
NOTE: Ignition sequence repeats 3 additional times before a lockout occurs. Lockout automatically resets after 3 hr, or can be manually reset
by turning 115-v or 24-v power off (not at thermostat) for 3 sec minimum, then turning on again.
6. Inducer speed operation—If cycle starts in low heat, inducer speed reduces slightly after flame sense. If cycle starts in high heat, inducer
speed increases 15 sec after flame sense. The reduction in speed in low heat is to optimize combustion for maximum efficiency.
7. Blower on delay—The blower starts 60 sec after flame sense if cycle started in low heat or 35 sec after flame sense if cycle started in high
heat.
NOTE: The blower starts at approximately 400-500 RPM. After 20 sec, the motor is turned off for 1/10 of a sec where a coast down calibration
is done to evaluate resistance of the conditioned air duct system. The microprocessor then determines blower RPM required to provide proper
airflow for heating mode.
8. Electronic Air Cleaner—The EAC-1 terminal is energized whenever the blower operates.
9. Blower off delay—When thermostat is satisfied, the R-W/W1 signal is terminated, de-energizing gas valve (stopping gas flow to burners),
and HUM terminal is de-energized.
The blower reduces its speed to low-heat RPM. The blower and EAC remain operating 90, 135, 180, or 225 sec (depending on blower off
time selection). The furnace is factory set for a 90-sec blower off delay.
10. Post purge—The inducer continues operating for 15 sec after gas valve is de-energized.
PROCEDURE 3—HEATING MODE-TWO STAGE
The control center provides 2-stage heating using a single-stage thermostat. The control center maximizes comfort while optimizing efficiency to
meet the demands of conditioned area when a thermostat R-W/W1 signal is received.
If thermostat control over furnace staging is desired, a 2-stage thermostat can be used. When control center receives a thermostat R-W/W1 and
R-W2 signal, high heat is energized and when a R-W/W1 signal alone is received, low heat is energized. This method overrides microprocessor
control of high or low heat.
NOTE: When using 2-stage thermostat operation with R-W/W1 and R-W2 signals, setup switch SW-2 MUST be in ON position.
The heat cycle operates as stated in Heating Mode section.
To allow for greater comfort, 2-stage thermostat control is recommended when zone systems are used.
PROCEDURE 4—EMERGENCY HEAT MODE
NOTE: The furnace should not be operated in emergency heat mode for extended periods of time. Operation is only recommended to provide
heat until replacement components can be obtained or fault resolved.
In this mode, the microprocessor is bypassed and motors operate at full speed with high-heat operation. The heat exchangers, motors, and
electronics can be overstressed and may reduce the life of components if operated for an extended period.
NOTE: No safeties are bypassed when using emergency heat mode.
Emergency heat mode can be selected using setup switch SW-4. SW-4 should be used when a fault condition exists or difficult to resolve problems
occur. This allows heating until fault can be corrected.
In emergency heat mode, the normal heat mode outlined in Heating Mode section is not followed. The following sequence will occur:
When thermostat calls for heat, the R-W/W-1 circuits close.
1. Prepurge period—The inducer motor is turned on IMMEDIATELY operating at maximum speed, closing low- and high-pressure switches.
Prepurge begins 25 sec after high-pressure switch closes.
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