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 periodÐThe inducer motor is turned on IMMEDI- ATELY operating at maximum speed, closing low- and high-pressure switches. Prepurge begins 25 sec after high- pressure switch closes.
2.Blower onÐThe blower motor is turned on IMMEDIATELY and slowly increases to maximum speed as soon as a call for heat is received. No blower calibration occurs.
3.Electronic Air CleanerÐThe EAC-1 terminal does not operate in emergency heat mode.
4.HumidifierÐThe HUM terminal is energized IMMEDI-
ATELY.
5.Ignitor warm upÐThe HSI is energized for a 17 sec warm-up period after prepurge period is completed.
6.Ignition sequenceÐAfter HSI warm-up period has com- pleted, the gas valve is energized, 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: Emergency heat mode only operates in high heat.
7.Flame sensingÐWhen burner flame is sensed, control center holds gas valve open. If burner flame is not sensed, control center de-energizes gas valve and ignition sequence is re- peated.
NOTE: Ignition sequence repeats 3 additional times before lock- out 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. Fault codes will not flash in emergency heat mode.
8.Blower off delayÐWhen 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. In addition, blower stops immediately.
9.Post purgeÐPost purge does NOT occur. The inducer stops immediately.
Step 5ÐCooling Mode
When thermostat calls for cooling, the R-G and R-Y/Y2 circuits close.
1.Cooling unitÐThe cooling unit starts when thermostat R-Y signal is received.
2.Blower onÐThe control center starts blower immediately when it receives an R-Y/Y2 and R-G signal. The blower starts at approximately 400-500 RPM. After 20 sec, the blower 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 selected cooling airflow.
NOTE: In cooling mode, the microprocessor adjusts blower RPM to operate at 400 CFM per ton as selected on the A/C setup switches. Airflow will be reduced to 340 or 315 CFM per ton, depending on board style (See Fig. 1), when a dehumidification demand exists. See Air Conditioning Setup Switches section in Installation, Start-Up and Operating Instructions for details. There is also a chart on schematic in Appendix A.
NOTE: If Y/Y2 thermostat lead is not connected to furnace control center, blower motor operates in continuous fan speed and indoor coil freeze-up may occur.
3.Electronic Air CleanerÐThe EAC-1 terminal is energized whenever blower operates.
4.Cooling unitÐThe cooling unit stops when thermostat R-Y signal is terminated.
5.Blower off delayÐWhen thermostat is satisfied, the R-Y/Y2 and R-G signals are terminated, and blower remains operating for 90 sec. On newer style control boards (See Fig. 1), the blower airflow will drop by 21 percent during the off-delay period when the DE jumper is connected. The DE jumper is only removed to enable the DEHUM input for use with a thermidistat or humidistat for dehumidification purposes.
Step 6ÐHeat Pump Mode
When furnace is operating in heat pump heating mode, R-Y/Y2 and R-G circuits are closed energizing heat pump, and blower operates at cooling speed. When heat pump defrost is required, R-W/W1 circuits close starting gas heat cycle, and blower adjusts to low-heat speed.
1.Prepurge periodÐThe 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, 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 used to determine required RPM necessary to operate inducer in high-heat mode.
2.HumidifierÐThe HUM terminal is energized whenever in- ducer prepurge period is completed.
3.Ignitor warm upÐAfter prepurge period, HSI is energized for 17 sec.
4.Ignition sequenceÐAfter HSI warm-up period is completed, the gas valve is energized, permitting gas flow to the burners where it is ignited. After 5 sec, the HSI is de-energized and a 2-sec flame-sensing period begins.
5.Flame sensingÐWhen burner flame is sensed, control center holds gas valve open.
If burner flame is not sensed, control center de-energizes gas valve and ignition sequence is repeated.
6.Blower off periodÐTen sec after gas valve is energized, the blower stops for 25 sec to allow heat exchangers to warm up.
7.Blower on delayÐAfter blower off period, blower starts.
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 CleanerÐThe EAC-1 terminal is energized whenever blower operates.
9.Inducer speed reductionÐIf cycle starts in low heat, inducer speed reduces slightly after the 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.
10.Call for heat terminatedÐWhen the call for heat is satisfied, the R-W/W1 signal is terminated, de-energizing gas valve (stopping gas flow to burners) and HUM terminal is de- energized.
a.R-W/W1 signal terminated with R-Y/Y2 and R-G still presentÐThe blower changes its speed to cooling RPM.
