Operation & Installation Manual 09/2002

still calling for gas heating. The amber LED will flash code 12 during this period, after which the LED will be ON continuous as long as no faults are detected. After this period, the furnace will respond to the thermostat normally.

The blower door must be installed for power to be conducted through blower door interlock switch ILK to furnace control CPU, transformer TRAN, inducer motor IDM, blower motor BLWM, hot surface igniter HSI, and gas valve GV.

TWO-STAGE HEATING WITH SINGLE-STAGE

THERMOSTAT (ADAPTIVE MODE)

(See Fig. 27 for thermostat connections.)

NOTE: Low-heat-only switch, SW-1, selects either the low-heatonly operation mode when ON, or adaptive heating mode when

OFF, in response to a call for heat. (See Fig. 32.)

This furnace can operate as a 2-stage furnace with a single-stage thermostat because furnace control CPU includes a programmed adaptive sequence of controlled operation, which selects low-gasheat or high-gas-heat operation. This selection is based upon the stored history of the length of previous gas heating on/off periods of the single-stage thermostat.

The furnace will start up in either low- or high-gas heat. If the furnace starts up in low-gas heat, the control CPU determines the low-gas heat on time (from 0 to 16 minutes) which is permitted before switching to high-heat.

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.

When wall thermostat

″calls for heat″, R-W/W1 circuit closes. The furnace control performs a self check, verifies the low-heat and high-heat pressure switch contacts LPS and HPS are open, and starts inducer motor IDM in high speed.

1. Inducer Prepurge Period- If the furnace control CPU selects low-heat operation the inducer motor IDM comes up to speed, the low-heat pressure switch LPS closes, and the furnace control CPU begins a 15-second prepurge period. After the low-heat pressure switch re-closes the furnace control CPU will begin a 15-second prepurge period, and continue to run the inducer motor IDM at high-speed.

If the furnace control CPU selects high-heat operation, the inducer motor IDM remains running at high-speed, and 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 furnace control CPU begins a 15-second prepurge period after the low-heat pressure switch LPS closes. If the high-heat pressure switch HPS fails to close and the low-heat pressure switch LPS closes, the furnace will operate at low-heat gas flow rate until the high-heat pressure switch closes for a maximum of 2 minutes after ignition.

2. Ignitor Warm-Up-At end of the inducer prepurge period, the

Hot Surface Ignitor HSI is energized for a 17-sec igniter warm-up period.

3. Trial-For-Ignition Sequence-period is completed the main gas valve relay contacts GVR-1 and -2 close to energize the gas valve solenoid GV-M, the gas valve opens, and 24 vac power is supplied for a field-installed humidifier at the HUM terminal. The gas valve solenoid GV-M permits gas flow to the burners where it is ignited. After 5 seconds, the igniter HSI is de-energized and a 2-second Flame-Proving period begins.

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If the furnace control CPU selects high-heat operation, the high-heat gas valve solenoid GV-HI is also energized.

4. Flame-Proving-When burner flame is proved at the flameproving sensor electrode FSE, the inducer motor IDM switches to low-speed unless running at high-speed, 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 Trials-For-Ignition before going to Ignition-Lockout. Lockout will be reset automatically after three hours, 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.

5. Blower-On Delay-If burner flame is proven the blower motor

BLWM is energized 66 sec after gas valve GV-M is opened.

Low-heat-The blower motor BLWM is energized at LO

HEAT speed.

Hi-heat-The BLWM is energized at HI HEAT speed.

Simultaneously, the electronic air cleaner terminal EAC-1 is energized and remains energized as long as the blower motor

BLWM is energized.

6. 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 deenergized 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 switch to HI HEAT speed five seconds after the furnace control CPU switches from low-heat to high-heat.

7. Switching From High- To Low- Heat-The control CPU will not switch from high-heat to low-heat while the thermostat

R-to-W circuit is closed when a single-stage thermostat is used.

8. 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 inducer motor IDM will remain energized for a 15-second post-purge period. The blower motor BLWM and air cleaner terminal EAC-1 will remain energized for 90, 120, 150, or 180 seconds (depending on selection at blower-OFF delay switches). The furnace control CPU is factory-set for a 120-second blower-OFF delay.

TWO-STAGE HEATING WITH TWO-STAGE THERMOSTAT

(NON-ADAPTIVE HEATING MODE)

(See Fig. 28 and 50A-G for thermostat connections).

NOTE: In this mode, the low-heat only switch 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-W1and-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.