Two-Stage Heating With Single-Stage Thermostat (Adaptive Mode); Two-Stage Heating With Two-Stage Thermostat (Non-Adaptive Heating Mode) - Carrier 58MTB Installation, Start-Up, And Operating Instructions Manual

TWO-STAGE HEATING WITH SINGLE-STAGE
THERMOSTAT (ADAPTIVE MODE)
(See Fig. 31 for thermostat connections.)
NOTE: Low-heat-only switch, SW-1, selects either the low-heat-
only operation mode when ON, or adaptive heating mode when
OFF, in response to a call for heat. (See Fig. 37.)
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-heat.
If power is interrupted, the stored history is erased. When this
happens, the control CPU will initially 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 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 high speed.
1. Inducer Prepurge Period- 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 furnace control CPU
will begin a 15-second prepurge period, and continue to run
the inducer motor IDM at high-speed.
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 pressure
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.
2. Igniter Warm-Up-At end of the inducer prepurge period, the
Hot Surface Igniter HSI is energized for a 17-sec igniter
warm-up period.
3. Trial-For-Ignition Sequence-When the igniter warm-up pe-
riod is completed the main gas valve relay contacts GVR close
to energize the gas valve GV, the gas valve opens. The gas
valve GV permits gas flow to the burners where it is ignited by
the Hot Surface Igniter HSI. Five seconds after the GVR
closes, a 2-second flame period begins. The HSI igniter will
remain energized until the flame is sensed or until the
2-second flame proving period begins.
4. Flame-Proving-When 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.
5. Blower-On Delay-If burner flame is proven the blower motor
BLWM is energized 66 sec after gas valve GV-M is opened.
Low-heat-The blower motor BLWM is energized at LO
HEAT speed.
Hi-heat-The BLWM is energized at HI HEAT speed.
Simultaneously, the electronic air cleaner terminal EAC-1 is
energized and remains energized as long as the blower motor
BLWM is energized.
6. Switching From Low- To High- Heat- If the furnace control
CPU switches from low-heat to high-heat, the furnace control
CPU will switch the inducer motor IDM speed from low to
high. 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 blower
motor BLWM will switch to HI HEAT speed five seconds
after the furnace control CPU switches from low-heat to
high-heat.
7. Switching From High- To Low- Heat-The control CPU will
not switch from high-heat to low-heat while the thermostat
R-to-W circuit is closed when a single-stage thermostat is
used.
8. Blower-Off delay- When the thermostat is satisfied, the R to
W circuit is opened, de-energizing the gas valve GV-M,
stopping gas flow to the burners, and de-energizing the
humidifier terminal HUM. The inducer motor IDM will
remain energized for a 15-second post-purge period. The
blower motor BLWM and air cleaner terminal EAC-1 will
remain energized for 90, 120, 150, or 180 seconds (depending
on selection at blower-OFF delay switches). The furnace
control CPU is factory-set for a 120-second blower-OFF
delay.
TWO-STAGE HEATING WITH TWO-STAGE THERMOSTAT
(NON-ADAPTIVE HEATING MODE)
(See Fig. 32 and 50A-G for thermostat connections).
NOTE: In this mode, the low-heat only switch must be ON to
select the low-heat only operation mode in response to closing the
thermostat R-to-W1 circuit. Closing the thermostat R-to-W1-
and-W2 circuits always causes high-heat operation, regardless of
the setting of the low-heat-only switch.
The wall thermostat "calls for heat", closing the R to W1 circuit for
low-heat or closing the R to W1 and-W2 circuits for high-heat. 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.
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-heat and vice versa.
1. Switching From Low- To High- Heat-If the thermostat R to
W1 circuit is closed and the R to W2 circuit closes, the furnace
control CPU will switch the inducer motor IDM speed from
low to high. The high-heat pressure switch relay HPSR is
de-energized to close the NC contact. When sufficient pres-
sure is available the high-heat pressure switch HPS closes, and
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