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setpoint, the thermostat closesW1 contacts the first
stage electric heat contactor (AH or AH and CH) is
energized. If the first stage of electric heat cannot
satisfy the heating requirement, the thermostat closes
W2.
When the W2 contacts close, the second stage electric
heat contactor (BH or BH and DH) is energized, if
applicable. The thermostat cycles both the first and
second stages of heat "On" and "Off" as required to
maintain the zone temperature setpoint.
Heating Operation (for Gas Units) -
Standard Economizer
When the system switch is set to the "Heat" position
and the zone temperature falls below the heating
setpoint, the Ignition module (IGN) initiates a heat
cycle.
Ignition Module Low, Medium and High
Heat
Two-stage (IGN) runs self-check (including verification
that the gas valve is de-energized). (IGN) checks the
high-limit switches (TC01 and TC01) for normally
closed contacts, the pressure switch (PS) for normally
open contacts, and the flame rollout (FR) switch for
continuity. (IGN) energizes inducer blower on high
speed to check pressure switch closure. If the pressure
switch is closed, the inducer blower starts a 20 second
pre-purge (15 seconds on high speed followed by 5
seconds on low speed).
If the pressure switch (PS) is still open, the inducer
blower will continue to be energized on high speed
until pressure switch closure.
After pre-purge completes, the (IGN) energizes the first
stage of the gas valve, initiates spark for 2 seconds
minimum, 7 seconds maximum (ignition trial) and
detects flame and de-energizes spark. From this point,
a fixed 45 second indoor blower delay on timing starts.
After the indoor blower delay on is completed, the
(IGN) energizes the indoor blower. The (IGN) enters a
normal operating loop where all inputs are
continuously monitored. If the first stage of gas heat
cannot satisfy the heating requirement, the thermostat
closes W2. The (IGN) energizes the second stage of the
gas valve and the second stage of inducer blower.
When the zone thermostat is satisfied, the (IGN) de-
energizes the gas valve. The (IGN) senses loss of flame.
The (IGN) initiates a 5 second inducer blower post
purge and 90 second indoor blower delay off at current
speed. The (IGN) de-energizes the inducer blower at
the end of the post purge. The (IGN) de-energizes the
indoor blower at the end of the selected indoor blower
delay off.
Table 9. Ignition module diagnostics
Steady light
Module is powered up, but no active call
for heat.
Blinking at
continuous steady
rate
Active call for heat.
One blink
Loss of communication.
Two blinks
System lockout (failure to ignite, no
spark, low/ no gas pressure, etc.)
Three blinks
Pressure switch (no vent air flow, bad
CBM, closed at initial call for heat). Auto
reset.
Four blinks
High limit (excessive heat in combustion
chamber, low airflow). Auto reset.
Five blinks
Flame sensed and gas valve not
energized or flame sensed and no call
for heat.
Six blinks
Flame rollout (CBM failure, incorrect gas
pressure, and incorrect primary air).
Requires manual reset of the switch.
Seven blinks
W1 and W2 swapped (electromechanical
3-10 tons units).
Drain Pan Condensate Overflow Switch
(Optional)
The Condensate Overflow Switch (COF) is utilized to
prevent water overflow from the drain pan. The float
switch is installed on the corner lip of the drain pan.
When the condensate level reaches the trip point, the
COF relay energizes and opens the 24 Vac control
circuit which disables the unit. Once the 24 Vac control
circuit is opened, a delay timer will prevent unit start-
up for three minutes.
Low Ambient Operation (Optional)
During low ambient operation, outside air temperature
below 50°F, the Low Ambient Controller will cycle the
outdoor fan motor "Off" and "On" based on discharge
pressure. The indoor fan motor (IDM) will continue to
operate and the outdoor fan will return to normal
operation once the ambient temperature is above 50°F.
10 Horsepower Motor Overload
10HP indoor fan motors are not internally protected.
For proper protection, ensure the adjustable dial on the
front of the overload (located in the control box) is set
to the nameplate FLA value of the indoor motor.
Two-Speed Indoor Fan Control
For units equipped with two-speed indoor fan control,
standard unit operation for cooling and heating will
continue and all unit functions will operate normally,
except for the control of the indoor fan speed. All units
configured with any type of heating will heat the supply
air and space as normal utilizing full speed indoor fan
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