507335-02
Page 32 of 39
Issue 2001
High Altitude
The manifold pressure, gas orifice and pressure switch
may require adjustment or replacement to ensure proper
operation at higher altitudes. See Table 15 for manifold
pressures and Table 16 for pressure switch and gas
conversion kits.
For safety, shut unit OFF and remove manometer as
soon as an accurate reading has been obtained. Take
care to replace pressure tap plug.
WARNING
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.
Sequence of Operation
See Figure 35 through Figure 39
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 ramps up to 50% of 1st stage
heat speed and operates at that speed for one minute
(including ramp up time), then at 75% of 1st stage heat
speed for an additional minute. After that, 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 (see
Single Stage Thermostat Operation), 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 2 minutes at 82% of the selected heating speed
(low fire or high fire) before ramping down.
Capacity
High Altitude Pressure Switch Kit
High Altitude Natural
Gas Orifice Kit
Natural Gas to LP / Propane Kit
0 - 4500 ft.
4501 - 7500 ft.
7501 - 10000 ft.
7501 - 10000 ft.
0 - 7500 ft.
7501 - 10000 ft.
070
No Change
No Change
73W35
51W01
11K48
11K47
090
No Change
69W56
73W35
51W01
11K48
11K47
110
No Change
69W56
73W35
51W01
11K48
11K47
Table 16. High Altitude Pressure Switch and Gas Conversion Kits