control CPU, transformer TRAN, inducer motor IDM, blower motor BLWM, hot-surface igniter HSI, and gas valve GV.
1.Two-Stage Heating (Adaptive mode) with Single- Stage Thermostat
(See Fig. 23 or 32 for thermostat connections)
NOTE: The low-heat only switch (LHT) selects either the low-heat only operation mode when ON, (see item 2. below) or the adaptive heating mode when OFF in response to a call for heat. (See Fig. 33.) When the W2 thermostat terminal is energized it will always cause high-heat operation when the R to W circuit is closed, regardless of the setting of the low-heat only switch. This furnace can operate as a two-stage furnace with a single-stage thermostat because the furnace control CPU includes a programmed adaptive sequence of controlled operation, which selects low-heat or high-heat operation. This selection is based upon the stored history of the length of previous gas-heating periods of the single-stage thermostat.
The furnace will start up in either low- or high-heat. If the furnace starts up in low-heat, the control CPU determines the low-heat on-time (from 0 to 16 minutes) which is permitted before switching to high-heat.
If the power is interrupted, the stored history is erased and the control CPU will 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 the thermostat cycle times.
The wall thermostat “calls for heat”, closing the R to W circuit. 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.
a. Inducer Prepurge Period
(1.) If the furnace control CPU selects low-heat opera- tion the inducer motor IDM comes up to speed, the low-heat pressure switch LPS closes, and the fur- nace 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 pre- purge period, and continue to run the inducer motor IDM at high-speed.
(2.) If the furnace control CPU selects high-heat opera- tion, 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.
b.Igniter Warm-Up-At the end of the prepurge period, the (Hot Surface Igniter) HSI is energized for a 17-second igniter warm-up period.
c.Trial-for-Ignition Sequence-When the igniter warm- up 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 by the HSI. Five sec after the GVR closes, a 2-sec flame proving period begins. The HSI ig- niter will remain energized until the flame is sensed or un- til the 2-sec 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 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 Ig- nition- 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.
e.Blower-ONDelay-If the burner flame is proven the blower-ON delay for low-heat and high-heat are as fol- lows:
Low-Heat-45 seconds after the gas valve GV-M is en- ergized the blower motor (BLWM) is energized at LO HEAT speed.
High-Heat-25 seconds after the gas valve GV-M is en- ergized the BLWM is energized at HI HEAT speed. Sim- ultaneously, the electronic air cleaner (EAC-1) terminal is energized and remains energized as long as the BLWM is energized.
f.Switching from Low-to High-Heat-If the furnace con- trol CPU switches from low-heat to high-heat, the fur- nace 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 pres- sure 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 fur- nace control CPU switches from low-heat to high-heat.
g.Switching from High- to Low-Heat-The furnace con- trol 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-energiz- ing the humidifier terminal HUM. The inducer motor IDM will remain energized for a 5-second post-purge period. The blower motor BLWM and air cleaner termin- al 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.
2.Two-Stage Thermostat and Two-Stage Heating (See Fig. 31 for thermostat connections)
NOTE: In this mode the LHT 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.
