59MN7C: Installation, Start-up, Operating and Service and Maintenance Instructions
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
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See Fig. 38 for thermostat connections.
NOTE: Heating Thermostat Type (Htt) set to (2St-LHt) selects the
minimum-heat only operation. When set to (2St-iHt) selects
intermediate-heat only operation mode. If set to either (2St-LHt) or set to
(2St-iHt), the furnace control will operate at two-stages only as
referenced in Two Stage Thermostat section below. If set to (1St), 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 (Htt).
Single-Stage Thermostat and Modulating Heating
(Adaptive Mode) Htt = 1St
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.
1. Inducer Prepurge Period - The furnace control CPU turns on
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 pre-purge, the first 45 seconds of intermediate-heat, 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 restriction. This evaluation is then used to determine the required
RPM necessary to operate the inducer motor in maximum-heat
pre-purge, or maximum-heat. The high pressure switch HPS should be
closed before ignition but the furnace control ignores this input until
after ignition occurs.
2. Igniter Warm-Up - At the end of the prepurge period, the
Hot-Surface Igniter HSI is energized for a 17-second igniter
warm-up period
3. Trial-For-Ignition Sequence - When the igniter warm-up period is
completed the main gas valve relay contact GVR closes to energize
the gas valve solenoid GV (pin 5). Simultaneously, the 24V
humidifier terminal HUM is energized and remains energized while
the gas valve solenoid is energized. The gas valve solenoid GV (pin
5) 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.
4. Flame-Proving - When the 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 (pin 5) open. If the burner flame is not proved within two
seconds, the furnace control CPU will close the gas valve GV (pin
5), 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, or 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.
5. Inducer Speed Change - If the cycle starts in intermediate-heat,
the furnace control CPU reduces the inducer speed slightly after
flame sense. If cycle starts in maximum-heat, the furnace control
CPU increases the inducer speed after flame sense.
6. Blower-ON delay — If the burner flame is proven, the blower-ON
delay for intermediate-heat and maximum-heat are as follows:
Intermediate-heat - 45 seconds after the gas valve GV (pin 5) is
opened the blower motor BLWM is turned ON at the modulating
heat airflow.
Maximum-heat - 25 seconds after the gas valve GV (pin 5) is
opened the BLWM is turned ON at maximum-heat airflow.
Simultaneously, the electronic air cleaner terminal EAC-1 is
energized and remain energized throughout the heating cycle.
7. Switching from Intermediate-Heat to a Low Range Input - If
the furnace control CPU switches from intermediate-heat to a low
range input (low range input is an input rate less than or equal to
51% of full rate), the furnace control CPU will turn the blower ON
or switch to the modulating heat airflow, energize the PSR relay to
open the NC contact, and slowly decrease the inducer motor speed
to the desired inducer RPM.
Switching from a Intermediate-Heat to a Medium Range Input
- If the furnace control CPU switches from intermediate-heat to a
different medium range input (medium range input is an input rate
between 52% and 71% of full rate), the furnace control CPU will
turn the blower ON or switch to the modulating heat airflow, and
continue to maintain the inducer motor speed or change it further if
required.
Switching from a Intermediate-Heat to a High Range Input - If
the furnace control CPU switches from intermediate-heat to a high
range input (high range input is an input rate greater than or equal to
72% of full rate), the furnace control CPU will turn the blower ON
or switch to the modulating heat airflow, and increase the inducer
motor speed to the desired inducer motor RPM.
Switching from Low Range Input to Maximum-Heat - If the
furnace control CPU switches from a low range input to
maximum-heat, the furnace control CPU will de-energize the PSR
relay to close the NC contact and slowly increase the inducer motor
speed until the medium pressure switch MPS closes. When the
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