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

Fig. 37—Inducer Housing Drain Tube
Fig. 38—Filling Condensate Trap
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.
SINGLE-STAGE THERMOSTAT AND TWO-STAGE
HEATING (ADAPTIVE MODE)
See Fig. 21 or 46 for thermostat connections
NOTE: Low-heat-only switch, SW1-2, 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. 25.) 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 furnace control CPU includes a programmed
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 furnace control CPU determines the
low-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.
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The wall thermostat "calls for heat", closing the R to W circuit.
The furnace control CPU performs a self-check, verifies the
low-heat and high-heat pressure switch contacts LPS and HPS are
open.
1. Inducer Prepurge Period-The furnace control CPU turns on
inducer motor IDM and slowly increases the inducer motor
speed. When the low-heat pressure switch LPS closes, inducer
motor RPM is noted by the furnace control CPU, and a 25-sec.
prepurge period begins. The RPM is used to evaluate vent
system resistance. This evaluation is then used to determine
the required RPM necessary to operate the inducer motor
during the low-heat prepurge period and low-heat mode.
NOTE: The heat cycle can start in either high-or low-heat. If a
high-heat cycle is initiated, the furnace control CPU will de-
energize the high-heat pressure switch relay HPSR to close the NC
contact and continues to increase the inducer motor speed after the
low-heat pressure switch LPS closes. When the high-heat pressure
switch closes, inducer motor RPM is noted by the furnace control
CPU before the 25-sec prepurge period begins. The RPM is used
to evaluate vent system resistance. This evaluation is then used to
determine the required RPM necessary to operate the inducer
motor in high-heat mode.
2. Igniter Warm-Up-At end of the 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.
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If the furnace control CPU selects high-heat operation, the
high-heat gas valve solenoid GV-HI is also energized.
4. Flame-Proving-When burner flame is proved at the flame-
proving sensor electrode FSE, 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
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