Carrier 58TUA - Manual (Page 11)

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period, the furnace will respond to the thermostat normally.
The iblower door must be installed for power to be conducted
through the blower door interlock switch ILK to the furnace
control CPU, transformer TRAN, inducer motor IDM, blower
motor BLWM, hot-surface ignitor HSI, and gas valve GV.
1. Single-Stage Thermostat with 2-Stage Heating — adaptive
mode. (See Fig. 8 for thermostat connections.)
NOTE: With the high heat only switch SWl (see Fig. 10) OFF,
the Low-Heat-Only switch SW2 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. When the
high heat only switch (SWl) is in the ON position it will always
cause high gas heat operation when the R to W circuit is closed,
regardless of the setting of the low heat only switch (SW2).
This furnace can operate as a 2-stage furnace with a single-stage
thermostat because the furnace control CPU includes a pro­
grammed adaptive sequence of controlled operation, which selects
low gas heat or high gas 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 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 gas heat.
If the power is interrupted, the stored history is erased. When this
happends the control CPU will select low gas heat for up to 16
minutes and then switch to high gas heat, as long as 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 WAVl circuit.
The furnace control performs a self check, verifies the low heat
and high heat pressure switches’ contacts LPS and HPS are open,
and starts the inducer motor IDM in low speed or high speed as
appropriate.
a. Inducer prepurge period—As the inducer motor IDM
comes up to Low-Speed or High Speed, the low heat
pressure switch contacts LPS close to begin a 15 sec
prepurge period.
b. Ignitor warm-up—At the end of the prepurge period, the
hot-surface ignitor HSI is energized for a 17-sec ignitor
warm-up period.
c. Trial-for-ignition sequence—When the ignitor warm-up
period is completed the main gas valve relay contacts
MGVR-1 and -2 close to energize the low heat gas valve
solenoid GV the gas valve opens, and 24-v power is
supplied for a field-installed humidifier at terminals HUM
and COM. The low heat gas valve solenoid GV permits gas
flow to the burners where it is ignited. After 5 sec, the
ignitor HSI is de-energized and a 2-sec flame-proving
period begins.
If the furnace control CPU selects high gas heat operation, the
high heat gas valve solenoid GV is also energized after the
normally-closed high heat pressure switch relay HPSR closes
and after the inducer motor IDM goes to high speed and
provides sufficient pressure to close the high heat pressure
switch HPS. (HPSR is open while the furnace is powered in
standby mode.) 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.
d. Flame-proving—When the burner flame is proved at the
flame-proving sensor electrode FSE, the control CPU
begins the blower on delay period and continues to hold the
gas valve GV open. If the burner flame is not proved within
2 sec, the control CPU will close the gas valve GV, and the
control CPU will repeat the ignition sequence for up to 3
more trials-for-ignition before going to ignition lockout.
Lockout will be reset automatically after 3 hours, or by
momentarily interrupting 115-v power to the furnace, or by
interrupting 24-v power at SECl or SEC2 to the control
CPU (not at W/Wl, G, R, etc.). Opening the thermostat R
to W circuit will not reset an ignition lockout.
If flame is proved when flame should not be present, the
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, 45 sec
after the gas valve GV is opened the blower motor BLWM
is energized on the appropriate heating speed, low gas heat
or high gas heat speed. Simultaneously, the electronic air
cleaner terminals EAC-1 and EAC-2 are energized and
remain energized as long as the blower motor BLWM is
energized.
f. Switching from low to high gas heat—If the furnace control
CPU switches from low gas heat to high gas heat, the
control CPU will switch the inducer motor IDM speed from
low to high. The high heat pressure switch relay HPSR will
close. When the inducer motor IDM provides sufficient
pressure to close the high heat pressure switch HPS, the
high heat gas valve solenoid GV is energized. The blower
motor BLWM will switch speed for high gas heat 5 sec
after the control CPU switches from low gas heat to high
gas heat.
Switching from high to low gas heat—The control CPU will
not switch from high-gas heat to low gas heat while the
thermostat R to W circuit is closed when a single-stage
thermostat is used.
g. Blower off delay—When the thermostat is satisfied, the
R to W circuit is opened, de-energizing the gas valve GV,
stopping gas flow to the burners, and de-energizing the
humidifier terminals HUM and COM. The inducer motor
IDM will remain energized for a 5 sec post-purge period,
’rae blower motor BLWM and air cleaner terminals EAC-1
and EAC-2 will remain energized for 90, 135, 180, or 225
sec (depending on selection at blower off delay switches
SW3 and SW4). The furnace control CPU is factory-set for
a 135-sec blower off delay.
2. Two-Stage Thermostat and 2-Stage Heating mode (See Fig. 9
for thermostat connections).
NOTE: The low heat only switch SW2 in ON position selects the
low heat only operation mode in response to closing the thermostat
R to W1 circuit. When the high heat only switch SWl is in OFF
position, closing the thermostat R to W1 and W2 circuits is
required for high gas heat operation. When the high heat only
switch SWl is ON it will always cause high gas heat operation
when the R to W1 circuit is closed, regardless of the setting of the
low heat only switch SW2 and regardless of whether the R to W2
circuit is closed or open.
The wall thermostat "calls for heat", closing the R to W1
circuit for low gas heat or closing the R to Wl-and-W2
circuits for high gas heat. The furnace control performs a
self-check, verifies the low heat and high heat pressure
switches’ contacts LPS and HPS are open, and starts the
inducer motor IDM in low speed or high speed as appropriate.
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 gas heat and vice versa.
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