Lennox SGA420 Guide

10/08
*2P1008*
505,111M
*P505111M*
SGA420, 480, 540, 600
©2006
Litho U.S.A.
505,111M
10/2008
Supersedes 4/2006
WHAT TO DO IF YOU SMELL GAS:
FIRE OR EXPLOSION HAZARD
Failure to follow safety warnings ex-
actly could result in serious injury
death or property damage.
WARNING
Do not store or use gasoline or other
flammable vapors and liquids in the
vicinity of this or any other appliance.
Installation and service must be per-
formed by a qualified installer, ser-
vice agency or the gas supplier.
DDo not try to light any appliance.
DDo not touch any electrical switch; do not
use any phone in your building.
DImmediately call your gas supplier from a
neighbor’s phone. Follow the gas suppli-
er’s instructions.
DIf you cannot reach your gas supplier, call
the fire department.
DExtinguish any open flames.
ROOFTOP
UNITS
READ ALL INSTRUCTIONS IN THIS MANUAL
AND RETAIN FOR FUTURE REFERENCE
See unit nameplate for manufacturer
and address.
(35, 40, 45 and 50 TONS)
DLeave the building immediately.

Page 1
POWER EXHAUST
DISCHARGE
(OPTIONAL)
EVAPORATOR
COILS
CONDENSATE
DRAIN
HUMIDITROL
COILS
(OPTIONAL)
BURNERS
GAS VALVES (2)
CONDENSER
COILS (4)
CONTROL
BOX
COMPRESSORS (4)
CONDENSER
FANS (6)
BLOWERS
FILTERS ELEVEN − 16 x 25 x 2" (406 x 635 x 51mm) OR 16 X 25 X 4" (406 X 635 X 102mm)
ECONOMIZER
(OPTIONAL)
ENERGY RECOVERY
WHEEL (OPTIONAL)
FILTER
DRIERS
SGA420, 480, 540, 600 PARTS ARRANGEMENT

Page 2
Safety
1− Keep unit area clear and free of combustible
materials, gasoline and other flammable vapors and
liquids.
2− Do not obstruct air flow to unit. Unit must receive
an unobstructed flow of combustion and
ventilating air.
WARNING
Danger of explosion and fire. Failure to
follow safety warnings exactly could
result in dangerous operation, serious
injury, death or property damage.
WARNING
Improper installation, adjustment, alteration, ser-
vice or maintenance can cause property damage,
personal injury or loss of life. Installation and ser-
vice must be performed by a qualified installer, ser-
vice agency or the gas supplier
WARNING
Electric shock hazard. Can cause injury
or death. Before attempting to perform
any service or maintenance, turn the
electrical power to unit OFF at discon-
nect switch(es). Unit may have multiple
power supplies.
WARNING
Product contains fiberglass wool.
Disturbing the insulation in this product during
installation, maintenance, or repair will expose you
to fiberglass wool. Breathing this may cause lung
cancer. (Fiberglass wool is known to the State of
California to cause cancer.)
Fiberglass wool may also cause respiratory, skin,
and eye irritation.
To reduce exposure to this substance or for further
information, consult material safety data sheets
available from address shown below, or contact
your supervisor.
P.O. Box 79990
Dallas, TX 75379−9900 USA
CAUTION
Label all wires prior to disconnection when servic-
ing controls. Wiring errors can cause improper and
dangerous operation. Verify proper operation after
servicing.
WARNING
Danger of electrical shock, explosion
and fire. Improper servicing could
result in dangerous operation, serious
inujry, death or property damage.
WARNING
Do not use this furnace if any part has been under
water. A flood−damaged furnace is extremely dan-
gerous. Attempts to use the furnace can result in
fire or explosion. A qualified service agency
should be contacted to inspect the furnace and to
replace all gas controls, control system parts,
electrical parts that have been wet or the furnace
if deemed necessary.
WARNING
Danger of explosion. Can cause injury
or product or property damage. Should
the gas supply fail to shut off or if
overheating occurs, shut off the gas
valve to the furnace before shutting off
the electrical supply.
Unit Operation
FOR YOUR SAFETY READ BEFORE LIGHTING
BEFORE LIGHTING smell all around the furnace area for
gas. Be sure to smell next to the roof because some gas is
heavier than air and will settle on the roof.
Use only your hand to push in or turn the gas control knob.
Never use tools. If the knob will not push in or turn by
hand, do not try to repair it, call a qualified service
technician. Force or attempted repair may result in a fire
or explosion.
The unit is equipped with an automatic spark ignition
system. There is no pilot. In case of a safety shutdown,
move thermostat switch to OFF and return the thermostat
switch to HEAT. Electromechanical thermostat is shown
in figure 1. For logic units and units with electronic
temperature controls, shut off the main disconnect and
then turn it on to reset the ignition control.

Page 3
TYPICAL ELECTROMECHANICAL THERMOSTAT
FIGURE 1
Placing Unit Into Operation
WARNING
Danger of explosion and fire. Can cause
injury or product or property damage.
You must follow these instructions ex-
actly.
Gas Valve Operation for Honeywell
VR8205Q/VR8305Q (figure 2)
HONEYWELL VR8205Q/VR8305Q SERIES GAS VALVE
Gas valve knob is shown in OFF position.
FIGURE 2
LOW FIRE
ADJUSTMENT
HIGH FIRE
ADJUSTMENT
INLET
PRESSURE
TAP
MANIFOLD
PRESSURE
TAP
1− Set thermostat to lowest setting.
2− Turn off all electrical power to furnace.
3− This furnace is equipped with an ignition device which
automatically lights the burner. Do not try to light the
burner by hand.
4− Open or remove the heat section access panel.
5− Turn the knob on the gas valve clockwise to
OFF". Do not force.
6− Wait five (5) minutes to clear out any gas. If you then
smell gas, STOP! Immediately call your
gas supplier from a neighbor’s phone. Follow the gas
supplier’s instructions. If you do not smell gas, go to
the next step.
7− Turn the knob on the gas valve counterclockwise
to ON". Do not force.
8− Close or replace the heat section access panel.
9− Turn on all electrical power to furnace.
10− Set thermostat to desired setting.
11− The ignition sequence will start.
12− If the furnace does not light the first time (gas line not
fully purged), it will attempt up to two more ignitions
before locking out.
13− If lockout occurs, repeat steps 1 through 10.
14− If the furnace will not operate, follow the instructions
Turning Off Gas to Furnace" and call your service
technician or gas supplier.
Turning Off Gas to Furnace
1− If using an electromechanical thermostat, set to the
lowest setting.
2− Before performing any service, turn off all electrical
power to the furnace.
3− Open or remove the heat section access panel.
4− Turn the knob on the gas valve clockwise to
OFF". Do not force.
5− Close or replace the heat section access panel.
WARNING
Danger of explosion. Can cause injury or
death. Do not attempt to light manually.
Unit has a direct spark ignition system.

Page 4
Burner Flame
WARNING
Danger of explosion and fire. Can cause
injury or product or property damage.
Periodically inspect burner flame to en-
sure proper unit operation.
The primary air is permanently set for normal operation.
The flame will be basically blue with some clear yellow
streaking in the end of the flame. Inspect the burner flame
periodically during the heating season using the
inspection port provided on the burner access panel.
Flue Passage and Vent Inspection
Annually, before heating season, inspect the combustion
air louvers, vent cap, heat exchanger, burners and
combustion air inducer for corrosion, deterioration or
deposits of debris. Remove any obstructions or blockage.
See figure 3.
Service
To maintain efficiency and longevity, your equipment
must be serviced yearly by a qualified service technician.
Failure to provide proof of service can void warranty.
CAUTION
Label all wires prior to disconnection when ser-
vicing controls. Wiring errors can cause improper
and dangerous operation. Verify proper operation
after servicing.
Servicing Filter
Units are equipped with eleven 16 X 25 X 2" (406 X 635 X
51mm) or 16 X 25 X 4" (406 X 635 X 102mm) filters. Filters
should be checked monthly and replaced when
necessary with filters of like kind and size. Take note of air
flow direction marking on filter frame when reinstalling
filters. See figure 4.
NOTE − Filters must be ULC approved or equivalent for
use in Canada.
HEAT EXCHANGER ASSEMBLY
HEAT
EXCHANGER
GAS VALVE
COMBUSTION
AIR INDUCER
PROVE
SWITCH
COLLECTOR
BOX
BURNER
ROLLOUT
SWITCH
FIGURE 3
VENT
CONNECTOR
Secures vent
to combustion
air inducer

Page 5
REMOVE FILTERS
FIGURE 4
Lubrication
All motors are lubricated at the factory. No further
lubrication is required.
Blower shaft bearings are prelubricated. For extended
bearing life, relubricate at least once every two years
with a lithium base grease, such as Alvania 3 (Shell Oil),
Chevron BRB2 (Standard Oil) or Regal AFB2 (Texas
Oil). Use a hand grease gun for relubrication. Add only
enough grease to purge through the bearings so that a
bead of grease appears at the seal lip contacts.
Manifold Pressures
Manifold pressures are shown in table 1. Refer to figure 2
to locate pressure ports.
TABLE 1
MANIFOLD PRESSURES in.wg. (kPa)
Natural Gas Propane (LP) Gas
Unit 1st Stage
+0.2(+.05)
2nd Stage
+0.3(+.08)
1st Stage
+0.2(+.05)
2nd Stage
+0.3(+.08)
420−600 1.6 (0.40) 3.7 (0.92) 5.5 (1.37) 10.5(2.61)
See Optional Modulating Gas Valve (MGV) section on
units equipped with MGVs.
Burners
1− Periodically examine burner flames for proper
appearance during the heating season.
2− Inspect the position of the flame sensor. Sensor
should be centered in the path of the flame.
3− Before each heating season examine the burners for
any deposits or blockage which may have occurred.
Clean burners as follows.
4− Turn off the electrical power and gas supply to the unit
and open heat section access panel.
5− Remove and retain two screws securing burners to
the burner support. See figure 5. Clean as necessary.
Replace burners and secure with retained screws.
WARNING
Danger of explosion. Can cause injury or
death. Do not overtighten main burner
mounting screws. Snug tighten only.
BURNER BOX ASSEMBLY
FIGURE 5
GAS
MANIFOLD
FLAME
SENSOR
GAS
VALVE
BURNERS
REMOVE
SCREWS
TO CLEAN
BURNERS
BURNER
SUPPORT
IGNITOR
6− Remove and retain the two screws securing the
ignitor to the burner support. Remove the burner and
check the spark gap with appropriately sized twist
drills or feeler gauges. See figure 6.
7− Replace the ignitor and secure in place with retained
screws.
FI
GU
RE
6
IGNITOR
SPARK GAP
SHOULD BE 1/8"
(3mm)
8− Restore electrical power and gas supply. Follow
lighting instructions attached to unit and use
inspection port in access panel to check flame.

Page 6
Combustion Air Inducer
A combustion air inducer proving switch checks
combustion air inducer operation before allowing power
to the gas controller. The gas controller will not operate if
the inducer is obstructed.
Under normal operating conditions, the combustion air
inducer wheel should be checked and cleaned prior to the
heating season. However, it should be examined
periodically during the heating season to establish an
ideal cleaning schedule.
Clean the combustion air inducer as follows:
1− Turn off the electrical power and the gas supply to the
unit.
2− Disconnect the pressure switch air tubing from the
combustion air inducer port.
3− Remove and retain the screws securing the
combustion air inducer to the flue box. Remove and
retain the screw securing the combustion air inducer
to the vent connector. See figure 3.
4− Clean the inducer wheel blades with a small brush
and wipe off any dust from the housing. Clean
accumulated dust from the front of the collector box
cover.
5− Return the combustion air inducer motor to the
original location and secure with retained screws. It
is recommended that the combustion air inducer
gasket be replaced during reassembly.
6− Use a small brush to clean the combustion air inlet
louvers on the heat access panel.
Flue Passageway and Flue Box
1− Remove the combustion air inducer assembly as
described in the Combustion Air Inducer subsection.
2− Remove the flue box cover. Clean the flue box with a
wire brush as required.
3− Clean the tubes with a wire brush.
4− Reassemble the unit. The flue box cover gasket and
combustion air inducer gasket should also be
replaced during reassembly.
Service Reminder
Call a qualified service technician if the unit is inoperative.
Before calling, always check the following to be sure
service is required:
1− Be sure electrical disconnect switches are ON.
2− Check room thermostat for proper setting.
3− Replace any blown fuses or reset circuit breakers.
4− Gas valve should be ON.
5− Air filters should not be plugged, limiting air flow.
6− Make sure all access panels are in place.
Repair Parts Listing
When ordering repair parts, include the complete model number and serial number listed on the ETL rating plate − e.g.
SGA480H4BH1Y.
Gas Heat Section Parts
Heat Exchanger
Combustion Air Assembly
Combustion Air Proving Switch
Burner Assembly
Burner Manifold Assembly
Main Burner Orifices
Flame Roll−out Switches
Auxiliary Limit Controls
Ignition Electrode Assembly
Ignition Lead
Ignition Sensor Assembly
Sensor Lead
Combination Gas Valve
Limit Controls
Cooling Parts
Compressors
Condenser Fan Motors
Condenser Fan Blades
Condenser Fan Run Capacitors
Freezestats
Condenser Fan Mounting Bracket
Fan Grille
Indoor Blower Motors
Blower Wheel
Expansion Valve
Distributor
Exhaust Fans (Opt.)
Electrical Control Parts
IMC Control Board
Compressor Contactors
Circuit Breakers (Opt.)
Transformer (Control)
Transformer (Contactor)
Blower Contactor
Limit, Blower Relay
Heat Relays
Condenser Fan Relays
Capacitor CAB
Relay CAB
Crankcase Heaters
Disconnect Switch (Opt.)

Page 7
FIGURE 7
MODULATING GAS VALVE (MGV)
GV1
LEFT HEAT
SECTION GAS
VALVE
GV3
RIGHT HEAT
SECTION GAS
VALVE
GV5
RIGHT HEAT SECTION
MODULATING
GAS VALVE
GV4
LEFT HEAT SECTION
MODULATING
GAS VALVE
GAS
INLET
TO LEFT HEAT
SECTION
TO RIGHT
HEAT SECTION
MANIFOLD
PRESSURE
PORT
MANIFOLD
PRESSURE PORT
Optional Modulating Gas Valve (MGV)
Units equipped with optional modulating gas valves
(MGV) contain two modulating gas valves in addition to
two standard gas valves. See figure 7.
Operation
The IMC will control modulating gas valves to maintain
110°F (default) discharge air during the heating cycle.
The left heat section will operate when 25−50% of
nameplate heat is needed. Both heat sections will
operate when 50−100% of the nameplate heat is needed.
The normally open MGV will allow full heating capacity
should the MGV fail.
Start−Up
1− Operate the unit in heating mode according to the
Heating Start−Up section in this manual.
2− After the unit has operated for 5 minutes, turn the
OPT1 and OPT2 switches ON. See figure 8. The unit
will operate at maximum heating input.
3− Measure the manifold pressure at the gas valves.
Manifold pressures should be:
GV1 & GV3:
Natural − 4.0"w.c. LP − 10.8"w.c.
GV4 & GV5:
Natural − 3.7"w.c. LP − 10.5"w.c.
4− Turn the OPT2 switch OFF. The unit will operate at
minimum heating input.
5− Measure the manifold pressure at the gas valves.
Manifold pressures should be:
GV1 & GV3:
Natural − 4.0"w.c. LP − 10.8"w.c.
GV4 & GV5:
Natural − 0.9"w.c. LP − 2.6"w.c.
FIGURE 8
TURN UNIT OPT1 SWITCH
ON FOR MINIMUM MGV
HEATING OUTPUT
INITIATE MAXIMUM AND MINIMUM MGV
MODE
UNIT TEST
RECALL
ECTO
TEMP
OPT2
SHIFT
IMC BOARD
UNIT
HP
GAS
OPT1
1PH
TURN UNIT OPT1 AND
MODE OPT2 SWITCHES
ON FOR MAXIMUM MGV
HEATING OUTPUT
6− Turn OPT1 switch OFF.
NOTE − BOTH OPT1 AND OPT2 SWITCHES MUST BE
OFF FOR NORMAL UNIT OPERATION.
IMC Output
The IMC 0−10VDC output to the MGVs increases to
modulate valves further closed during a reduced heating
demand. The IMC 0−10VDC output to the MGVs
decreases to modulate valves further open during a
higher heating demand.
Heating
Demand
Reduces
IMC Output
To MGVs
Increases
MGVs
Modulate
Further
Closed
Heating
Output
Lowers
Heating
Demand
Increases
IMC Output
To MGVs
Reduces
MGVs
Modulate
Further
Open
Heating
Output
Increases