Sequence Of Operation - Carrier 58CTA Installation And Service Instructions Manual

15. Reinstall vent connector on furnace vent elbow. Securely
fasten vent connector to vent elbow with 2 field-supplied,
corrosion-resistant, sheet metal screws located 180° apart.
16. Replace blower access door only, if it was removed.
17. Set thermostat above room temperature and check furnace for
proper operation.
18. Verify blower airflow and speed changes between heating and
cooling.
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in fire, personal
injury, or death.
Never use a match or other open flame to check for gas leaks.
Use a soap-and-water solution.
19. Check for gas leaks.
20. Replace outer access door.
Step 3—Sequence of Operation
NOTE: Furnace control must be grounded for proper operation or
control will lockout. Control is grounded through green wire
connected to gas valve and burner bracket screw.
Using the schematic diagram in Fig. 56, follow the sequence of
operation through the different modes. Read and follow the wiring
diagram very carefully.
NOTE: If a power interruption occurs during a call for heat
(W/W1 or W/W1-and-W2), the control run the blower for the
selected blower off-delay period two seconds after power is
restored and if the thermostat is still calling for gas heating. The
LED light 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 the blower door interlock switch ILK to the furnace
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. 22 or 31 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. 32.) 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 pro-
grammed 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 switch-
ing 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 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 fur-
nace control CPU will begin a 15-second prepurge
period, and continue to run the inducer motor IDM at
high-speed.
(2.) 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 pres-
sure 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. After 5 seconds, the igniter HSI is
de-energized and a 2-second 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 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.
e. Blower-ON Delay-If the burner flame is proven the
blower-ON delay for low-heat and high-heat are as follows:
Low-Heat-45 seconds after the gas valve GV-M is ener-
gized the blower motor (BLWM) is energized at LO HEAT
speed.
High-Heat-25 seconds after the gas valve GV-M is ener-
gized the BLWM is energized at HI HEAT speed. Simul-
taneously, the electronic air cleaner (EAC-1) terminal is
energized and remains energized as long as the BLWM is
energized.
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