seconds after the GVR closes, a 2-second flame prov- ing period begins. The HSI igniter will remain ener- gized until the flame is sensed or until the 2-second flame proving period begins. If the furnace control CPU selects high-heat operation, the high-heat gas valve solenoid GV-HI is also energized.
d.Flame-Proving- When the burner flame is proved at the flame-proving sensor electrode FSE, the inducer motor IDM switches to low-speed unless the furnace is operating in high-heat, and the furnace control CPU begins the blower-ON delay period and continues to hold the gas valve GV-M open. If the burner flame is not proved within two seconds, the control CPU will close the gas valve GV-M, and the control CPU will repeat the ignition sequence for up to three more Tri- als-For-Ignition before going to Ignition-Lockout. Lockout will be reset automatically after three hours, or by momentarily interrupting 115 vac power to the furnace, or by interrupting 24 vac power at SEC1 or SEC2 to the furnace control CPU (not at W/W1, G, R, etc.). If flame is proved when flame should not be present, the furnace control CPU will lock out of Gas- Heating mode and operate the inducer motor IDM on high speed until flame is no longer proved.
e.Blower-On delay - If the burner flame is proven the blower-ON delays for low-heat and high-heat are as follows:
Low-heat- 45 seconds after the gas valve GV-M is opened the blower motor BLWM is turned ON at low-heat airflow.
High-heat- 25 seconds after the gas valve GV-M is opened the BLWM is turned ON at high-heat airflow. Simultaneously, the humidifier terminal HUM and electronic air cleaner terminal EAC-1 are energized and remain energized throughout the heating cycle.
f.Switching from Low- to High-Heat - If the furnace control CPU switches from low-heat to high-heat, the furnace control CPU will switch the inducer motor IDM speed from low to high. The high-heat pressure switch relay HPSR is de-energized to close the NC contact. When sufficient pressure is available the high- heat pressure switch HPS closes, and the high-heat gas valve solenoid GV-HI is energized. The blower motor BLWM will transition to high-heat airflow five seconds after the furnace control CPU switches from low-heat to high-heat.
g.Switching from High- to Low-Heat -The furnace control CPU will not switch from high-heat to low- heat while the thermostat R-to-W circuit is closed when using a single-stage thermostat.
h.Blower-Off Delay -When the thermostat is satisfied, the R to W circuit is opened, de-energizing the gas valve GV-M, stopping gas flow to the burners, and de-energizing the humidifier terminal HUM. The in- ducer motor IDM will remain energized for a
5-second post-purge period. The blower motor BLWM and air cleaner terminal EAC-1 will remain energized at low-heat airflow or transition to low-heat airflow for 90, 120, 150, or 180 seconds (depending on selection at blower-OFF delay switches). The fur- nace control CPU is factory-set for a 120-second blower-OFF delay.
2.Two-Stage Thermostat and Two-Stage Heating See Fig. 27-34 for thermostat connections.
NOTE: In this mode the low-heat only switch SW1-2 must be ON to select the low-heat only operation mode in response to closing the thermostat R-to-W1 circuit. Closing the thermostat
R-to- W1-and-W2 circuits always causes high-heat operation, regardless of the setting of the low-heat only switch.
The wall thermostat “calls for heat”, closing the R-to-W1 circuit for low-heat or closing the R-to-W1-and-W2 circuits for high-heat. The furnace control performs a self-check, verifies the low-heat and high-heat pressure switch contacts LPS and HPS are open, and starts the inducer motor IDM in high-speed.
The start up and shut down functions and delays described in item 1. above apply to the 2-stage heating mode as well, except for switching from low- to high-Heat and vice versa.
a.Switching from Low- to High-Heat - If the thermo- stat R-to-W1 circuit is closed and the R-to-W2 cir- cuit closes, the furnace control CPU will switch the inducer motor IDM speed from low to high. The high-heat pressure switch relay HPSR is de-energized to close the NC contact. When sufficient pressure is available the high-heat pressure switch HPS closes, and the high-heat gas valve solenoid GV-HI is ener- gized. The blower motor BLWM will transition to high-heat airflow five seconds after the R-to-W2 cir- cuit closes.
b.Switching from High- to Low-Heat -If the thermo- stat R-to- W2 circuit opens, and the R-to-W1 circuit remains closed, the furnace control CPU will switch the inducer motor IDM speed from high to low. The high-heat pressure switch relay HPSR is energized to open the NC contact and de-energize the high-heat gas valve solenoid GV-HI. When the inducer motor IDM reduces pressure sufficiently, the high-heat pres- sure switch HPS will open. The gas valve solenoid GV-M will remain energized as long as the low-heat pressure switch LPS remains closed. The blower motor BLWM will transition to low-heat airflow five seconds after the R-to-W2 circuit opens.
3.Cooling mode
The thermostat “calls for cooling”.
a.Single-Speed Cooling-
See Fig. 27 for thermostat connections
The thermostat closes the R-to-G-and-Y circuits. The R-to- Y circuit starts the outdoor unit, and the R-to- G-and-Y/Y2 circuits start the furnace blower motor BLWM on cooling airflow. Cooling airflow is based on the A/C selection shown in Fig. 57. The electronic air cleaner terminal EAC-1 is energized with 115 vac when the blower motor BLWM is operating.
When the thermostat is satisfied, the R-to-G-and-Y circuits are opened. The outdoor unit will stop, and the furnace blower motor BLWM will continue operating at cooling airflow for an additional 90 seconds. Jump- er Y/Y2 to DHUM to reduce the cooling off-delay to 5 seconds. (See Fig. 26.)
b.Single-Stage Thermostat and Two-Speed Cooling (Adaptive Mode) -
See Fig. 34 for thermostat connections.
This furnace can operate a two-speed cooling unit with a single-stage thermostat because the furnace control CPU includes a programmed adaptive se- quence of controlled operation, which selects low- cooling or high-cooling operation. This selection is based upon the stored history of the length of previous cooling period of the single-stage thermostat.
NOTE: The air conditioning relay disable jumper ACRDJ must be connected to enable the adaptive cooling mode in response to a call for cooling. (See Fig. 26.) When ACRDJ is in place the furnace control CPU can turn on the air conditioning relay ACR to energize the Y/Y2 terminal and switch the outdoor unit to high-cooling.
