Bryant 355AAV Installation Instructions Manual page 46

Fig. 49 - - Inducer Housing Drain Tube
SINGLE- -STAGE THERMOSTAT AND TWO- -
STAGE HEATING (ADAPTIVE MODE)
See Fig. 32 or 58 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. 37.) 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.
DESIRED
SETUP SWITCH (SW1---7 AND SW1---8)
HEATING
MODE
BLOWER OFF
SW1--- 7
DELAY (SEC)
90 OFF
120 ON
150 OFF
180 ON
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.
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POSITION
SW1--- 8
OFF
OFF
ON
ON
Fig. 50 - - Filling Condensate Trap
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
deenergize 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 GV, the gas valve opens. The
gas valve GV permits gas flow to the burners where it is
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