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. 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 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.
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
period is completed the main gas valve relay contacts GVR
close to energize the gas valve solenoid 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.
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 two seconds, the control CPU will close the gas
valve GV-M, and the furnace 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
Fig. 52—Inducer Housing Drain Tube
A99118
Fig. 53—Filling Condensate Trap
A99119
—38—
Summary of Contents for CONDENSING GAS FURNACE 355MAV
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