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Thermostat section, EXCEPT that the communicating control will
send modulating rate command signals through the communication
bus rather than energizing the 24-v thermostat terminals.
SINGLE-STAGE THERMOSTAT AND MODULATING
HEATING (ADAPTIVE MODE)
Note: Minimum-heat only switch SW1-2 selects the minimum-heat
only operation mode when ON. Intermediate-heat only switch SW4-
2 selects intermediate-heat only operation mode when ON. If both
switches are ON the furnace control will default to intermediate-heat.
If either or both switches are ON the furnace control will operate at
two-stages only as referenced in the Two Stage Thermostat section
below. If both switches are OFF the furnace control will operate in
the adaptive heating mode in response to a call for heat. When the
W2 thermostat terminal is energized it will always cause maximum-
heat operation as long as the R to W circuit is closed, regardless of
the setting of the minimum-heat or intermediate-heat only switches.
This furnace can operate as a modulating furnace with a single-stage
thermostat because the furnace control CPU includes a programmed
adaptive sequence of controlled operation, which selects a modulated
rate between minimum- and maximum-heat. 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 intermediate-, or maximum-heat.
The furnace will transition and operate at minimum-heat or the
calculated modulating rate after starting and operating for 45 seconds
at intermediate-heat. The furnace control CPU defaults to minimum-
heat on the first thermostat cycle and calculates the modulating rate
that the furnace should run at for 19 minutes on subsequent heat
cycles.
If the power is interrupted, the stored modulating rate is erased and
the furnace control will select intermediate-heat for 45 seconds,
minimum-heat for 19 minutes and then switch to maximum-heat, as
long as the thermostat continues to call for heat. The furnace control
CPU then uses this information to calculate the modulating rate the
furnace will operate at on the next heating cycle. If the calculated
modulating rate is between 40% - 99% then the furnace control CPU
will operate for 45 seconds at intermediate-heat, and then operate at
the calculated modulating rate for up to 19 minutes and then switch
to maximum-heat as long as the thermostat continues to call for heat.
If the calculated modulating rate is 100% then the furnace control
will only operate at maximum-heat until the thermostat is satisfied.
The wall thermostat "calls for heat", closing the R to W circuit. The
furnace control CPU performs a self-check, verifies the low, medium,
and high pressure switch contacts LPS, MPS, and HPS are open,
then de-energizes the PSR relay to close the NC contact.
Inducer Prepurge Period - The furnace control CPU turns on
1.
inducer motor IDM and slowly increases the inducer motor
speed. After the low pressure switch LPS closes the furnace
control CPU continues to increase the inducer motor speed until
the medium pressure switch MPS closes. When the medium
pressure switch MPS closes, inducer motor RPM is noted by
the furnace control CPU, and a 25-second 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 prepurge, the first
45 seconds of intermediate-heat mode, or any modulating rate
that the furnace will transition to after the blower on-delay is
completed.
Note: The heat cycle can start in either intermediate- or maximum-
heat. If a maximum-heat cycle is initiated, the furnace control CPU
will continue to increase the inducer motor speed after the medium
pressure switch MPS closes. When the medium pressure switch
closes, inducer motor RPM is noted by the furnace control CPU, and
a 25-second 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 maximum-
heat pre-purge, and maximum-heat mode. The high pressure switch
HPS should be closed before ignition but the furnace control ignores
this input until after ignition occurs.
Igniter Warm-Up - At the end of the prepurge period, the
2.
Hot-Surface Igniter HSI is energized for a 17-second igniter
warm-up period.
Trial-For-Ignition Sequence - When the igniter warm-up
3.
period is completed the main gas valve relay contact GVR
closes to energize the gas valve solenoid GV (pin PL17-5). The
gas valve solenoid GV permits gas flow to the burners where it
is ignited. Five seconds after the GVR closes, a 2-second
Flame-Proving period begins. The HSI igniter will remain
energized until flame is sensed or until the 2-second flame
proving period begins.
Flame-Proving - When the burner flame is proved at the
4.
flame-proving sensor electrode FSE, the furnace control CPU
begins the blower-ON delay period and continues to hold the
gas valve GV open. If the burner flame is not proved within
two seconds, the furnace control CPU will close the gas valve
GV, 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 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 at full speed until flame is no longer
proved.
Inducer Speed Change – If the cycle starts in intermediate-
5.
heat, the furnace control CPU reduces the inducer speed slightly
after flame sense. If the cycle starts in maximum-heat, the
furnace control CPU increases the inducer speed after flame
sense.
Blower-On delay – If the burner flame is proven the blower-
6.
ON delay for intermediate-heat and maximum-heat are as
follows:
Intermediate-heat – 45 seconds after the gas valve GV is
opened the blower motor BLWM is turned ON at the
modulating heat airflow.
Maximum-heat – 25 seconds after the gas valve GV is opened
the BLWM is turned ON at maximum-heat airflow.
Simultaneously, the humidifier terminal HUM and electronic air
cleaner terminal EAC-1 are energized and remain energized
throughout the heating cycle.
Switching from Intermediate-Heat to a Low Range Input - If
7.
the furnace control switches from intermediate-heat to a low
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440 08 4001 00
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