Carrier 58TUA Installation, Start-Up, And Operating Instructions Manual Download Page 11

Page: 11 / 24

This furnace is equipped with a manual reset limit switch in
the gas control area. The switch will open and shut off power
to the gas valve if a flame rollout or overheating condition
occurs in the gas control area. DO NOT bypass the switch.
Correct inadequate combustion-air supply, component fail-
ure, or restricted flue gas passageway before resetting the
switch.

Before operating furnace, check each manual reset switch for
continuity. If necessary, press and release button to reset switch.

SEQUENCE OF OPERATION

Using schematic diagram in Fig. 13, follow the sequence of
operation through the different modes. Read and follow diagram
very carefully.

NOTE:

If power interruption occurs during "call for heat" (W/W1

or W/W1 and W2) and if the thermostat is still calling for gas
heating, the control will start a 90-sec blower only on period 2 sec
after power is restored. The red LED will flash code 12 during
90-sec period, after which the LED will be on continuously as long
as no faults are detected. After 90-sec period, furnace will respond
to the thermostat normally.

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. 9 for thermostat connections.)

NOTE:

With high-heat-only switch SW-1 off, low-heat-only

switch SW-2 selects either the low-heat-only operation mode when
on (see item 2 below), or adaptive heating mode when off in
response to a call for heat. (See Fig. 11.) When high-heat-only
switch (SW-1) is in ON position, it always causes high-gas-heat
operation when R-W/W1 circuit is closed, regardless of setting of
low-heat-only switch (SW-2).

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 appropriate.

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 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 C

. 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 high-heat gas valve solenoid GV is also energized after
normally closed high-heat pressure switch relay HPSR
closes and after inducer motor IDM goes to high speed and
provides sufficient pressure to close high-heat pressure
switch HPS. HPSR is open while furnace is powered in
standby mode. If high-heat pressure switch HPS fails to
close and low-heat pressure switch LPS closes, furnace
operates at low-heat gas flow rate until high-heat 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

Table 6—Setup Switch Description

SETUP

SWITCH NO.

NORMAL

POSITION

DESCRIPTION

OF USE

SW-1

Only High-Gas Heat

OFF (Staged Gas

Heat)

Turn switch on to obtain only high-gas-heat operation on any call
for heat regardless of whether R-W/W1, or R-W/W1, -W2 is
closed. SW-1 overrides SW-2.

SW-2

Low-Gas Heat (Adaptive Mode)

OFF (Single-Stage Thermostat)

Turn switch off for installations with single-stage thermostats; con-
trol selects low-gas-heat or high-gas-heat operation based on pre-
vious cycles. Turn switch on for installations with 2-stage thermo-
stats to permit only low-gas-heat operation in response to closing
R-W/W1. High-gas heat is supplied only when R to W/W1 and W2
are closed.

SW-3 and SW4

ON, OFF

Switches control gas heating mode blower off delay. (See Table
7.)

Table 7—Blower Off Delay Setup

Switch (SW) Position

DESIRED HEATING

MODE BLOWER OFF

DELAY (SEC)

SETUP SWITCH

SW-3

SW-4

OFF