Carrier 58UXT Series Operating Instructions Manual Download Page 13

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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 ignitor HSI, and gas valve GV.

1. Adaptive Heating Mode—Single-Stage Thermostat with

2-Stage Heating
(See Fig. 15 for thermostat connections.)

NOTE:

In response to thermostat call for heat R to W/W1, the

control selects high-stage gas heating only with HIGH HEAT
ONLY switch (SW1) set to ON regardless of the LOW HEAT
switch (SW2) setting. With the HIGH HEAT ONLY switch set to
OFF, LOW HEAT switch selects either low-stage gas heating only
mode when set to ON, or the adaptive heating mode when set to
OFF in response to a thermostat call for heat R to W/W1. (See Fig.
18 for description of switch settings or Fig. 19 for control circuit
diagram.)

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-gas-
heat 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-gas heat.

If power is interrupted, the stored history is erased. When this
happens, the control CPU selects low-gas heat for up to 16 minutes
and then switches to high-gas 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 low speed or high speed as appropri-
ate.

a. Inducer prepurge period—As inducer motor IDM comes up

to low speed or high speed, the low-heat pressure switch
contacts LPS (or LPS and HPS) close to begin a 15 sec
prepurge period.

b. Ignitor warm-up—At end of 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 (and high-heat pressure switch relay
HPSR for high-stage operation) close to energize the low-
and high-heat gas valve solenoid(s) GV and the humidifier
terminal HUM. 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 high-heat pressure switch HPS fails to close on call
for high-stage gas heat and the low-heat pressure switch
LPS closes, the furnace operates at low-heat gas flow rate
until the high-heat HPS pressure switch closes.

d. Flame-proving—When burner flame is proved at flame-

proving sensor electrode FSE, the control CPU begins
blower on delay period and continues to hold gas valve GV
open. If burner flame is not proved within 2 sec, the control
CPU closes gas valve GV, and the control CPU repeats the
ignition sequence for up to 3 more trials-for-ignition before
going to ignition lockout. LOCKOUT IS RESET AUTO-
MATICALLY after 3 hr, or by momentarily interrupting
115-v power to furnace, or by interrupting 24-v power at
SEC1 or SEC2 to the control CPU (not at W/W1, G, R,
etc.). Opening thermostat R-W circuit will not reset an
ignition lockout.
If flame is proved when flame should not be present, the
control CPU locks out of gas heating mode and operates
inducer motor IDM on high speed until flame is no longer
proved.

e. Blower on delay—If burner flame is proven, 45 sec after

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 EAC terminals
EAC-1 and EAC-2 are energized with 115v and remain
energized as long as blower motor BLWM is energized.

f. Switching from low- to high-gas heat—If furnace control

CPU switches from low-gas heat to high-gas heat, the
control CPU switches inducer motor IDM speed from low
to high. The high-heat pressure switch relay HPSR closes.
When 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
switches speed for high-gas heat 5 sec after the control CPU
switches from low-gas heat to high-gas heat.

g. 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.

h. Blower off delay—When thermostat is satisfied, the R-W

circuit is opened, de-energizing gas valve GV, stopping gas
flow to burners, and de-energizing humidifier terminals
HUM and C

. The inducer motor IDM remains energized

for a 5 sec post-purge period. The blower motor BLWM
and EAC terminals EAC-1 and EAC-2 remain energized
for 90, 135, 180, or 225 sec (depending on selection at
blower off delay switches SW-3 and SW-4). The furnace
control CPU is factory set for a 135-sec blower off delay.

Fig. 17—Control Center

A01370

PR2

COM

PR1

I-

COO

HI-GAS

-HEA

LO-GAS

-HEA

ARK