Bryant Bruant 4 Way Gas 355AAV User Manual

Page 46

A99118

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

HEATING

MODE

BLOWER OFF

DELAY (SEC)

SETUP SWITCH (SW1---7 AND SW1---8)

POSITION

SW1---7

SW1---8

OFF

120

OFF

150

OFF

180

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.

A99119

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

355A