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High Altitude
The manifold pressure, gas orifice and pressure switch
may require adjustment or replacement to ensure proper
operation at higher altitudes.
For safety, shut unit OFF and remove manometer as
soon as an accurate reading has been obtained. Take
care to replace pressure tap plug.
Primary and Secondary Limits
The primary limit is located on the heating compartment
vestibule panel. The secondary limits (if equipped) are
located in the blower compartment, attached to the back
side of the blower. These auto reset limits are factory set
and require no adjustment.
Flame Rollout Switches (Two)
These manually reset switches are located on (or inside of)
the burner box. If tripped, check for adequate combustion
air before resetting.
Pressure Switches
The pressure switch assembly (includes two switches)
is located in the heating compartment adjacent to
the combustion air inducer. These switches check for
proper combustion air inducer operation before allowing
ignition trial. The switches are factory-set and require no
adjustment.
Temperature Rise
Place the unit into operation with a second-stage heating
demand. After supply and return air temperatures have
stabilized, check the temperature rise. If necessary, adjust
the heating blower speed to maintain the temperature
rise within the range shown on the unit nameplate. See
Table 11 for allowable heating speeds. Increase the blower
speed to decrease the temperature rise. Decrease the
blower speed to increase the temperature rise. Failure to
properly adjust the temperature rise may cause erratic limit
operation.
Thermostat Heat Anticipation
Set the heat anticipator setting (if adjustable) according to
the amp draw listed on the wiring diagram that is attached
to the unit.
Page 32 of 39
WARNING
Sequence of Operation
See Figure 36 through Figure 40
Heating
On a call for heat from the room thermostat, the control
board performs a 1 second self check. Upon confirmation
that the pressure switch contacts are in an open position,
the control energizes the combustion blower on high
speed. The control then checks for adequate combustion
air by making sure the low-fire pressure switch contacts
are closed.
The igniter energizes and is allowed to warm up for 20
seconds before the gas valve energizes on 1st stage
and burners ignite. 45 seconds after the control confirms
ignition has occurred, the control drops the combustion
blower to low speed.
The circulating blower operates at full 1st stage heat
speed until either the heat call is satisfied or the thermostat
initiates a call for 2nd stage heat. On a call for 2nd stage
heat, the control energizes the circulating air blower on full
CFM 2nd stage heat.
If the automatic heat staging option is being used the
furnace does not switch to 2nd stage heat in response
to a call from the thermostat but instead operates at 1st
stage heat for the duration of the selected time before
automatically switching to 2nd stage heat.
When the call for heat is satisfied, the gas valve and
combustion air blower shut down. The control board shuts
off the gas valve and runs the combustion blower for an
additional 15 seconds. The circulating air blower continues
to run for selected blower off delay time (60/90/120/180
sec) before ramping down and shutting off.
In the event the unit loses ignition, the control will attempt
to recycle up to five times before it goes into a 1 hour
lockout. Lockout may be manually reset by removing
power from the control for more than 1 second or removing
the thermostat call for heat for more than 3 seconds.
If during a heating cycle the limit control senses an
abnormally high temperature and opens, the control board
de-energizes the gas valve and the combustion blower
while the circulating blower runs at 1st stage heat speed.
The circulating blower remains energized until the limits
are closed.
Fan On
When the thermostat is set for continuous fan operation
and there is no demand for heating or cooling, a call for fan
closes the R to G circuit and the circulating blower motor
runs at 38% of the selected cooling CFM until switched off.
When the call for fan is turned off, the control de-energizes
the circulating blower.
Issue 2302
507335-03
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