Carrier 58TUA Installation, Start-Up, And Operating Instructions Manual Download Page 12

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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.

2. Non-Adaptive Heating Mode—Two-Stage Thermostat and

2-Stage Heating

(See Fig. 10 for thermostat connections).

NOTE:

The low-heat-only switch SW-2 in ON position selects

low-heat-only operation mode in response to closing the thermo-
stat R-W1 circuit. When high-heat-only switch SW-1 is in OFF
position, closing the thermostat R to W1 and W2 circuits is
required for high-gas-heat operation. When high-heat-only switch
SW-1 is on, it always causes high-gas-heat operation when R-W1
circuit is closed, regardless of the setting of the low-heat-only
switch SW-2 and regardless of whether R-W2 circuit is closed or
open.

When the wall thermostat "calls for heat", R-W/W1 circuit
closes for low-gas heat or R to W1-and-W2 circuits close for
high-gas heat. The furnace control performs a self check,
verifies 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 appropriate.

The start-up and shutdown functions and delays described in
item 1 above apply to 2-stage heating mode as well, except for
switching from low- to high-gas heat and vice versa.

a. Switching from low- to high-gas heat—If thermostat

R-W/W1 circuit for low-gas heat is closed and R-W2
circuit for high-gas- heat closes, the control CPU switches
the inducer motor IDM speed from low to high. The
high-heat pressure switch relay HPSR closes. When in-
ducer motor IDM provides sufficient pressure to close
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 R-W2 circuit
closes.

b. Switching from high- to low-gas heat—If thermostat R-W2

circuit for high-gas heat opens and R to W/W1 circuit for
low-gas heat remains closed, the control CPU will switch
inducer motor IDM speed from high to low. The high-heat
pressure switch relay HPSR will open to de-energize
high-heat gas valve solenoid GV. When inducer motor
IDM reduces pressure sufficiently, the high-heat pressure
switch HPS will open. The low-heat gas valve solenoid GV
remains energized as long as low-heat pressure switch LPS
remains closed. The blower motor BLWM switches speed
for low-gas heat 5 sec after R-W2 circuit opens.

Fig. 11—Control Center

A93348

PR2

COM

PR1

EAC-2

EAC-1

I-

COO

HI-GAS

-HEAT

LO-GAS