Lennox F8AUH Series User manual

Page 1
Corp 9912−L5
Revised 08−2004
AFUE - Up to 80.5%
Heating Input - 45,000 to 120,000 Btuh (13.2 to 35.2 kW)
Add-On Cooling - 1 thru 5 Tons (3.5 to 17.6 kW)
F8AUH
Gas Furnaces
Up-Flow/Horizontal
SERVICE MANUAL
F8AUH series units are mid−efficiency gas furnaces used
for upflow or horizontal applications only, manufactured
with tubular heat exchangers formed of aluminized steel.
F8AUH units are available in heating capacities of 45,000 to
120,000 Btuh and cooling applications up to 5 tons. Refer to
Engineering Handbook for proper sizing.
Units are factory equipped for use with natural gas. Kits are
available for conversion to LPG operation. F8AUH model
units are equipped with a hot surface silicon carbide ignition
system. The F8AUHX unit meets the California Nitrogen Ox-
ides (NOx) Standards and California Seasonal Efficiency re-
quirements. All units use a redundant gas valve to assure
safety shut−off as required by A.G.A. or C.G.A. Units may be
installed in upflow or horizontal position.
All specifications in this manual are subject to change. Pro-
cedures outlined in this manual are presented as a recom-
mendation only and do not supersede or replace local or
state codes. In the absence of local or state codes, the
guidelines and procedures outlined in this manual (except
where noted) are recommended only and do not constitute
code.
TABLE OF CONTENTS
Introduction 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Specifications 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blower Data 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Horizontal Venting 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High Altitude 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parts Identification 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I Unit Components 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II Installation 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III Start Up 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV Heating System Service Checks 14. . . . . . . . . . . . . .
V Typical Operating Characteristics 16. . . . . . . . . . . . . .
VI Maintenance 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII Wiring and Sequence of Operation 19. . . . . . . . . . . .
VIII Control Board Troubleshooting Guide 23. . . . . . . .

Page 2
SPECIFICATIONS
Model No. F8AUH**−24−045
F8AUH**−24−045X F8AUH**−24−060 F8AUH**−36−060 F8AUH**−36−060X F8AUH**−24−075 F8AUH**−36−075 F8AUH**−36−075X
Input Btuh (kW) 45,000 (13.2) 60,000 (17.6) 60,000 (17.6) 75,000 (22.0) 75,000 (22.0)
Output Btuh (kW) 39,600 (10.8) 49,200 (14.4) 49,200 (14.4) 61,700 (18.1) 61,700 (18.1)
AFUE 80.1% 80.5% 80.5% 80.1% 80.0%
California Seasonal Efficiency 75.4% 76.4% 75.9% 76.8% 76.8%
Flue size connection dia. − in. (mm) round 3 (76) 3 (76) 3 (76) 4 (102) 4 (102)
Temperature rise range − _F (_C) 30 − 60 (17 − 33) 45 − 75 (25 − 42) 45 − 75 (25 − 42) 45 − 75 (25 − 42) 45 − 75 (25 − 42)
High static AGA/CGA cert. − in wg. (Pa) .50 (125) .50 (125) .50 (125) .50 (125) .50 (125)
Gas Piping Size IPS Nat. or LPG/propane 1/2 (13) 1/2 (13) 1/2 (13) 1/2 (13) 1/2 (13)
Blower wheel nominal in. 9 x 7 9 x 7 10 x 7 9 x 7 10 x 7
Blower
wheel
nominal
diameter x width mm 229 x 178 229 x 178 254 x 178 229 x 178 254 x 178
Blower motor output − hp (W) 1/4 (187) 1/4 (187) 1/3 (224) 1/4 (187) 1/3 (224)
Nominal coolin
g
Tons 1, 1.5 or 2 1, 1.5 or 2 2, 2.5 or 3 1, 1.5 or 2 2, 2.5 or 3
Nominal
cooling
that can be added kW 3.5, 5.3 or 7.0 3.5, 5.3 or 7.0 7.0, 8.8 or 10.6 3.5, 5.3 or 7.0 7.0, 8.8 or 10.6
Shipping weight − lbs. (kg) 1 package 130 (59) 130 (59) 135 (61) 135 (61) 135 (61)
Electrical characteristics 120 volts − 60 hertz − 1 phase (less than 12 amps) All models
OPTIONAL ACCESSORIES − Must Be Ordered Extra
Hanging Bracket Kit 46J66 46J66 46J66 46J66 46J66
High Altitude Pressure Switch Kits No Change No Change No Change 88J80 88J80
LPG/propane kit 38K84 38K84 38K84 38K84 38K84
Sidewall Power Venting Kit 79J15 79J15 79J15 79J15 79J15
Twinning Kit 15L38 15L38 15L38 15L38 15L38
Up−Flow/Horizontal Filter and Filter Rack Kits
No. & sizeoffilters− in. (mm)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
*Variable Field.
Annual Fuel Utilization Efficiency based on US DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating
for non−weatherized furnaces.
For proper operation at altitudes over 4500 ft. (1370 m).
Required for horizontal venting applications.
Polyurethane frame type filter is furnished with kit.

Page 3
SPECIFICATIONS
Model No. F8AUH**−48−075
F8AUH**−48−075X
F8AUH**−48−100
F8AUH**−48−100X
F8AUH**−60−100
F8AUH**−60−100X F8AUH**−48−120 F8AUH**−60−120 F8AUH**−60−120X
Input Btuh (kW) 75,000 (22.0) 100,000 (29.3) 100,000 (29.3) 120,000 (35.2) 120,000 (35.2)
Output Btuh (kW) 61,700 (18.1) 82,000 (24.0) 82,000 (24.0) 98,400 (28.8) 98,400 (28.8)
AFUE 80.0% 80.1% 80.0% 80.0% 80.1%
California Seasonal Efficiency 76.3% 76.5% 77.0% Not available 75.5%
Flue size connection dia. − in. (mm) round 4 (102) 4 (102) 4 (102) 4 (102) 4 (102)
Temperature rise range − _F (_C) 45 − 75 (25 − 42) 45 − 75 (25 − 42) 35 − 65 (19 − 36) 45 − 75 (25 − 42) 45 − 75 (25 − 42)
High static AGA/CGA cert. − in wg. (Pa) .50 (125) .50 (125) .65 (162) .50 (125) .50 (125)
Gas Piping Size IPS Nat. or LPG/propane 1/2 (13) 1/2 (13) 1/2 (13) 1/2 (13) 1/2 (13)
Blower wheel nominal in. 11 x 8 12 x 8 12 x 9 10 x 10 12 x 9
Blower
wheel
nominal
diameter x width mm 279 x 203 305 x 203 305 x 229 254 x 254 305 x 229
Blower motor output − hp (W) 1/2 (373) 1/2 (373) 3/4 (560) 1/2 (373) 3/4 (560)
Nominal coolin
g
Tons 2 thru 4 2 thru 4 3.5 thru 6 2 thru 4 3.5 thru 6
Nominal
cooling
that can be added kW 7.0 thru 14.1 7.0 thru 14.1 12.3 thru 21.1 7.0 thru 14.1 12.3 thru 21.1
Shipping weight − lbs. (kg) 1 package 140 (64) 175 (79) 175 (79) 175 (79) 175 (79)
Electrical characteristics 120 volts − 60 hertz − 1 phase (less than 12 amps) All models
OPTIONAL ACCESSORIES − Must Be Ordered Extra
Hanging Bracket Kit 46J66 46J66 46J66 46J66 46J66
High Altitude Pressure Switch Kits 88J80 18L24 18L24 18L24 18L24
LPG/propane kit 38K84 81J14 81J14 81J14 81J14
Sidewall Power Venting Kit 79J15 79J15 79J15 79J15 79J15
Twinning Kit 15L38 15L38 15L38 15L38 15L38
Up−Flow/Horizontal Filter and Filter Rack Kits
No. & sizeoffilters− in. (mm)
32J02 (single)
66K64 (ten)
(1) 16 x 20 x 1
(406 x 508 x 25)
46J14 (single)
66K65 (ten)
(1) 20 x 20 x 1
(508 x 508 x 25)
46J14 (single)
66K65 (ten)
(1) 20 x 20 x 1
(508 x 508 x 25)
46J14 (single)
66K65 (ten)
(1) 20 x 20 x 1
(508 x 508 x 25)
46J14 (single)
66K65 (ten)
(1) 20 x 20 x 1
(508 x 508 x 25)
*Variable Field.
Annual Fuel Utilization Efficiency based on US DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating
for non−weatherized furnaces.
For proper operation at altitudes over 4500 ft. (1370 m).
Required for horizontal venting applications.
Polyurethane frame type filter is furnished with kit.

Page 4
BLOWER DATA
F8AUH**−24−045, F8AUH**−24−060 AND F8AUH**−24−75 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
External
Static
Pressure High Medium−High Medium−Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1270 600 980 460 770 365 570 270
.05 12 1245 590 975 460 770 365 565 265
.10 25 1220 575 975 460 770 365 565 265
.15 37 1195 565 965 455 765 360 560 265
.20 50 1170 550 960 455 760 360 560 265
.25 62 1140 540 950 450 760 360 555 260
.30 75 1110 525 940 445 760 360 550 260
.40 100 1060 500 910 430 750 355 545 255
.50 125 990 465 880 415 740 350 540 255
.60 150 900 425 810 380 690 325 530 250
.70 175 800 380 740 350 630 295 520 245
NOTE − All air data is measured external to unit with 1 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table
F8AUH**−36−060 AND F8AUH**−36−075 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
External
Static
Pressure High Medium−High Medium−Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1425 670 1240 585 1000 470 800 380
.05 12 1415 670 1230 580 995 470 800 380
.10 25 1400 660 1220 575 990 465 795 375
.15 37 1385 655 1200 565 985 465 795 375
.20 50 1370 645 1180 555 980 460 790 375
.25 62 1350 635 1160 545 970 460 780 370
.30 75 1330 630 1140 540 955 450 770 365
.40 100 1280 605 1095 515 925 435 750 355
.50 125 1210 570 1040 490 900 425 720 340
.60 150 1135 535 985 465 860 405 680 320
.70 175 1070 505 920 435 800 380 630 300
NOTE − All air data is measured external to unit with 1 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table
F8AUH**−48−075, F8AUH**−48−100 AND F8AUH**−48−120 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
External
Static
Pressure High Medium−High Medium Medium−Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1830 865 1600 755 1325 625 1070 505 880 415
.05 12 1815 855 1585 750 1320 625 1070 505 880 415
.10 25 1800 850 1570 740 1315 620 1070 505 880 415
.15 37 1875 885 1550 730 1310 620 1065 505 875 415
.20 50 1750 825 1530 720 1300 615 1060 500 875 415
.25 62 1725 815 1515 715 1290 610 1050 495 870 410
.30 75 1700 800 1500 710 1275 600 1040 490 870 410
.40 100 1650 780 1460 690 1245 590 1020 480 860 405
.50 125 1600 755 1420 670 1210 570 1000 470 840 395
.60 150 1550 730 1380 650 1170 550 980 460 820 385
.70 175 1480 700 1330 630 1130 535 960 455 790 375
NOTE − All air data is measured external to unit with 1 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table

Page 5
BLOWER DATA
F8AUH**−60−100 AND F8AUH**−60−120 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
External
Static
Pressure High Medium−High Medium Medium−Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
0 0 2450 1155 2160 1020 1970 930 1700 800 1500 710
.05 12 2440 1150 2155 1015 1965 925 1695 800 1500 710
.10 25 2430 1145 2150 1015 1960 925 1690 800 1495 705
.15 37 2415 1140 2135 1010 1950 920 1685 795 1495 705
.20 50 2400 1135 2120 1000 1940 915 1680 795 1490 705
.25 62 2380 1125 2105 995 1930 910 1675 790 1480 700
.30 75 2360 1115 2090 985 1915 905 1670 790 1470 695
.40 100 2310 1090 2050 965 1870 880 1650 780 1440 680
.50 125 2260 1065 2000 945 1810 855 1610 760 1410 665
.60 150 2180 1030 1950 920 1750 825 1560 735 1370 645
.70 175 2100 990 1890 890 1700 800 1520 715 1330 630
NOTE − All air data is measured external to unit with 1 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table
FILTER AIR RESISTANCE
cfm (L/s) in. w.g. (Pa)
0 (0) 0.00 (0)
200 (95) 0.01 (2)
400 (185) 0.03 (7)
600 (280) 0.04 (10)
800 (375) 0.06 (15)
1000 (470) 0.09 (22)
1200 (560) 0.12 (30)
1400 (655) 0.15 (37)
1600 (750) 0.19 (47)
1800 (845) 0.23 (57)
2000 (935) 0.27 (67)
2200 (1030) 0.33 (82)
2400 (1125) 0.38 (95)
2600 (1220) 0.44 (110)

Page 6
HORIZONTAL VENTING REQUIREMENTS
(Requires Optional Side Wall Power Venting Kit)
Furnace
Model No
Vent Pipe Diameter
Furnace Connection
Vent Pipe Minimum
Equivalent Length
Vent pipe Maximum
Equivalent Length
Horizontal Venting
Transition Required
Model No. in. mm feet meters feet meters in. mm
F8AUH**−24−045
F8AUH**−24−060
F8AUH**−36−060 3 76 10 3.0 60 18.0 3 to 4 76 to 102
F8AUH**−24−075
F8AUH**−36−075
F8AUH**−48−075
F8AUH**−48−100
F8AUH**−60−100
F8AUH**−48−120
F8AUH**−60−120
4 102 10 3.0 60 18.0 − − − − − − − −
VENTING NOTES − Elbows − One 3 inch (76 mm) diameter 45_elbow is equivalent to 3 feet (1.0 m) of straight vent pipe.
One 4 inch (102 mm) diameter 45_elbow is equivalent to 4 feet (1.2 m) of straight vent pipe.
One 3 inch (76 mm) 90_elbow is equivalent to 5 feet (1.5 m) of straight vent pipe.
One 4 inch (102 mm) 90_elbow is equivalent to 7 feet (2.1 m) of straight vent pipe.
Two 45_elbows are equal to one 90_elbow.
Tees − One 3 inch (76 mm) diameter tee is equivalent to 19 feet (5.8 m).of straight vent pipe.
One 4 inch (102 mm) diameter tee is equivalent to 25 feet (7.6 m).of straight vent pipe.
Transition − 3 inch to 4 inch (76 mm to 102 mm) transition is equivalent to 2 feet (0.61 m) of straight vent pipe.
Transition furnished with power venter should be installed on top of flue adaptor at induced draft blower.
NOTE − All horizontal venting applications require optional Sidewall Power Venting Kit.
HIGH ALTITUDE DERATE
Refer to table below for manifold pressure settings for
Installations at different altitudes and different fuels.
NOTE−In Canada, certification for installations over
4500 ft. (1372m) above sea level is the jurisdictionof
the local authorities.
Manifold Absolute Pressure in. w.c. (kPa)
ALTITUDE ft. (m)
FUEL 0−4500
(0−1372)
4500−5500
(1372−1676)
5500−6500
(1676−1981)
6500−7500
(1982−2286)
NAT 3.5 (0.87) 3.4 (0.85) 3.3 (0.82) 3.2 (0.80)
LP 9.5 (2.36) 9.2 (2.29) 8.9 (2.21) 8.6 (2.14)
The combustion air blower pressure switches are factory
set and are not to be adjusted.
At elevations of 4500 ft. (1372m) or greater, change factory
installed pressure switch to switch listed in table below.
UNIT MODEL PRESSURE SWITCH
PART NUMBER
F8AUH**−24−045(X) NO CHANGE
F8AUH**−24−060
F8AUH**−36−060(X) NO CHANGE
F8AUH**−24−075
F8AUH**−36−075(X)
F8AUH**−48−75(X)
88J80
F8AUH**−48−100(X)
F8AUH**−60−100(X) 18L24
F8AUH**−48−120
F8AUH**−60−120(X) 18L24

Page 7
FIGURE 1
BLOWER
ACCESS PANEL
HEAT EXCHANGER
ASSEMBLY
BURNER
ASSEMBLY
COMBUSTION AIR
BLOWER
CONTROL
BOARD
TRANSFORMER
PRESSURE
SWITCH
GAS VALVE
PRIMARY
LIMIT
BURNER
ACCESS PANEL
BLOWER
ASSEMBLY
F8AUH PARTS ARRANGEMENT
FIGURE 2
HEAT EXCHANGER
COMBUSTION AIR BLOWER
MANIFOLD
ROLLOUT SWITCH (2)
COMBUSTION AIR
PROVE SWITCH
GAS VALVE
BURNERS
COLLECTOR
PRIMARY LIMIT
Front
Right
Top
Bottom
Left
Back
HEATING COMPONENTS (shown in horizontal position)

Page 8
FIGURE 3
F8AUH BURNER ASSEMBLY (shown in upflow position)
Top
FLAME
SENSOR
MANIFOLD
ORIFICE
GAS VALVE
HEAT EXCHANGER
VEST PANEL
BURNER
UPPER
BURNER
MOUNTING
RAIL
LOWER
BURNER
MOUNTING
RAIL
BURNER
BOX
BURNER
BOX
TOP
Front Right
Bottom
Left
Back
PRIMARY LIMIT
ROLLOUT
SWITCHES
CARBIDE
IGNITOR
I−UNIT COMPONENTS (Figures 1, 2, 3)
F8AUH unit components are shown in figure 1. The gas
valve, combustion air blower and burners can be ac-
cessed by removing the burner access panel. Electrical
components are mounted to the blower housing.
F8AUH units are factory equipped with bottom return air pan-
els in place. The panels are designed to be field removed as
required for bottom air return. Knockout panels are provided
for side return air.
CAUTION
Electrostatic discharge can affect electronic
components. Take precautions during furnace
installation and service to protect the furnace’s
electronic controls. Precautions will help to
avoid control exposure to electrostatic dis-
charge by putting the furnace, the control and
the technician at the same electrostatic poten-
tial. Neutralize electrostatic charge by touching
hand and all tools on an unpainted unit surface,
such as the gas valve or blower deck, before per-
forming any service procedure.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
1− Control Transformer (T1)
A transformer located on the blower housing provides
power to the low voltage section of the unit. Transformers
on all models are rated 30VA with a 120V primary and a
24V secondary.
2−Door Interlock Switch (S51)
A door interlock switch rated 16A at 125VAC is wired in se-
ries with line voltage. When the blower door is removed the
unit will shut down.
DANGER
Shock hazard.
Spark related components contain high voltage.
Disconnect power before servicing. Control is
not field repairable. If control is inoperable, sim-
ply replace entire control.
Can cause injury or death. Unsafe operation will
result if repair is attempted.

Page 9
3− Furnace Control (A3)
All F8AUH model units are equipped with an integrated
ignition blower control (50A55), which controls ignition,
safety circuits, blower operation, fan off timing and allows
for thermostat connection and troubleshooting. The sys-
tem consists of ignition control board (figure 6 with control
terminal designations in table 4) and a silicon carbide ig-
nitor (figure 7). The board features two LED lights for
troubleshooting and two accessory terminals rated at (4)
four amps. See table 3 for troubleshooting diagnostic
codes. Table 1 and 2 show jack plug terminal designa-
tions. Units equipped with this control board board can be
used with either electronic or electro−mechanical thermo-
stats without modification. Each time power is applied to
the furnace, the control board board performs a selfcheck
including energizing the combustion air blower for a peri-
od of 1 second.
TABLE 1
CONTROL BOARD J156 (J2) TERMINAL
DESIGNATIONS
PIN # FUNCTION
1Combustion Air Blower Line
2Ignitor Line
3Combustion Air Blower Neutral
4Ignitor Neutral
TABLE 2
CONTROL BOARD J58 (J1) TERMINAL
DESIGNATIONS
PIN # FUNCTION
1High Limit Out
2Not Used
324V
4Not Used
5Rollout Switch In
624V
7High Limit In
8Ground
9Gas Valve In
10 Pressure Switch In
11 Rollout Switch Out
12 Gas Valve Out
a−Electronic Ignition (See Figure 5)
On a call for heat the 50A55 control board monitors the
combustion air blower pressure switch. The control will not
begin the heating cycle if the pressure switch is closed (by−
passed). Once the pressure switch is determined to be
open, the combustion air blower is energized. When the
differential in the pressure switch is great enough, the
pressure switch closes and a 15−second pre−purge be-
gins. If the pressure switch is not proven within 2−1/2 min-
utes, the control goes into Watchguard−Pressure Switch
mode for a 5−minute re−set period.
After the 15−second pre−purge period, the carbide ignitor
warms up for 11 to 21 seconds during which the gas valve
opens at 19 seconds for a 4−second trial for ignition. The
ignitor stays energized for the first second of the 4−second
trial. If ignition is not proved during the 4−second period,
the control will try four more times with an inter purge and
warm−up time between trials of 37 seconds. After a total of
five trials for ignition (including the initial trial), the control
goes into Watchguard−Flame Failure mode. After a
60−minute reset period, the control will begin the ignition
sequence again.
b−Fan Time Control
The fan on time of 45 seconds is not adjustable. Fan off
time (time that the blower operates after the heat demand
has been satisfied) can be adjusted by flipping the dip
switches located on the integrated control. The unit is
shipped with a factory fan off setting of 90 seconds. Fan off
time will affect comfort and is adjustable to satisfy individu-
al applications. See figure 4.
FIGURE 4
FAN-OFF TIME ADJUSTMENT
To adjust fan−off timing, flip dip switch to desired setting.
60sec
.
90sec
.
120sec
.
180sec
.

Page 10
The 50A55 control board is equipped with two LED lights for troubleshooting. The diagnostic codes are listed below in table
3.
TABLE 3
DIAGNOSTIC CODES
MAKE SURE TO ID LED’S CORRECTLY: REFER TO INSTALLATION INSTRUCTIONS FOR CONTROL BOARD LAYOUT.
LED #1 LED #2 DESCRIPTION
SIMULTANEOUS
SLOW FLASH
SIMULTANEOUS
SLOW FLASH
Power − Normal operation
Also signaled during cooling and continues fan.
SIMULTANEOUS FAST
FLASH
SIMULTANEOUS FAST
FLASH Normal operation − signaled when heating demand initiated at thermostat.
SLOW FLASH ON
Primary or Secondary limit open. Units with board 12L42: Limit must close within
5 trials for ignition or board goes into one hour limit Watchguard. Units with board
56L85: Limit must close within 3 minutes or board goes into one hour limit Watch-
guard.
OFF SLOW FLASH
Pressure switch open or has opened 5 times during a single call for heat; OR:
Blocked inlet/exhaust vent; OR: Condensate line blocked; OR: Pressure switch
closed prior to activation of combustion air blower.
ALTERNATING SLOW
FLASH
ALTERNATING SLOW
FLASH Watchguard − burners fail to ignite.
SLOW FLASH OFF Flame sensed without gas valve energized.
ON SLOW FLASH Rollout switch open. OR: 9 pin connector improperly attached.
ON
ON
OFF
ON
OFF
ON
Circuit board failure or control wired incorrectly.
FAST FLASH SLOW FLASH Main power polarity reversed. Switch line and neutral.
SLOW FLASH FAST FLASH Low flame signal. Measures below .7 microAmps. Replace flame sense rod.
ALTERNATING FAST
FLASH
ALTERNATING FAST
FLASH
Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR:
Open ignitor circuit.
NOTE − Slow flash equals 1 Hz (one flash per second). Fast flash equals 3 Hz (three flashes per second). Drop out flame sense current < 0.15 microAmps
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉ
DEMAND
CAB
GAS VALVE
15
ON
OFF
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉ
ÉÉ
40
ÉÉÉÉÉÉÉÉ
IGNITOR
36
1
Pre -Purge Ignitor Warmup Trial for
Ignition
Post
Purge
5 SEC82
*Blower on time will be 45 seconds after ignition. Blower off time will depend on OFF TIME" Setting.
INDOOR BLOWER
ÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉ
FIGURE 5
Blower On"
Delay
50A55 Control Board Ignition Sequence
37

Page 11
50A55 INTEGRATED CONTROL BOARD
(Silicon Carbide Ignition System)
FIGURE 6
4−Flame Sensor
A flame sensor is located on the left side of the burner sup-
port. See figure 8. The sensor is mounted on a bracket in
the burner support and the tip protrudes into the flame en-
velope of the left−most burner. The sensor is fastened to
burner supports and can be removed for service without
removing any part of the burners. During operation, flame
is sensed by current passed through the flame and sens-
ing electrode. The control board allows the gas valve to re-
main open as long as flame signal is sensed.
NOTE − The F8AUH furnace contains electronic com-
ponents that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.
PARK
TERMINAL DESIGNATIONS
COOL−H
HEAT−H
EAC−H
HUM−H
XFMR−H
LINE−H
LINE−N
HUM−N
EAC−N
XFMR−N
CIR−N
Blower − Cooling Speed (120VAC)
Blower − Heating Speed (120VAC)
Electronic Air Cleaner (120VAC)
Humidifier (120VAC)
Transformer (120VAC)
Input (120VAC)
Input (Neutral)
Humidifier (Neutral)
Electronic Air Cleaner (Neutral)
Transformer (Neutral)
Not Used
(FLAME SENSE)
TABLE 4
Alternate Blower Speeds (Dead)
Flame Sensor
FIGURE 7
Silicon Carbide Ignitor
1/4 in. 7 mm
1/32 in. 1 mm
9/16 in. 14.7 mm
1/32 in. 1 mm
FIGURE 8
NORMAL FLAME SIGNAL u0.7 MICROAMPS
LOW FLAME SIGNAL v0.7 MICROAMPS
MINIMUM FLAME SIGNAL v0.15 MICROAMPS
BURNER
FLAME
SENSOR
SIDE VIEW END VIEW
1/4 in. 7 mm
1/32 in. 0.79
mm
3/16 in.
4.7 mm
11/16 in. 18 mm
1/32 in. 0.79
mm
NORMAL FLAME SIGNAL 0.2 MICRAMPS
LOW FLAME SIGNAL 0.2 MICROAMPS
MIN. FLAME SIGNAL 0.15 MICROAMPS

Page 12
5−Blower Motors and Capacitors
All F8AUH units use direct drive blower motors. All motors
used are 120V permanent split capacitor motors to ensure
maximum efficiency. See table 5 for ratings.
TABLE 5
F8AUH BLOWER RATINGS 120V 1PH
BLOWER MOTOR HP
F8AUH24−045/060/075
F8AUH36-060/075
F8AUH48-075/100/120
CAP
1/2
1/3
1/4 5MFD 370V
5MFD 370V
7.5MFD 370V
3/4 40MFD 370V
F8UH60−100/120
FIGURE 9
Front
Bottom
Right
Left
Top
Back
BLOWER
MOTOR
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
MOTOR
CAPACITOR
SECONDARY
LIMITS (S21)
6−Combustion Air Blower (B6)
All F8AUH units use a combustion air blower to move air
through the burners and heat exchanger during heating
operation. The blower uses a 120VAC motor. The motor
operates during all heating operation and is controlled by
furnace control A3. The blower also operates for 15 se-
conds before burner ignition (pre-purge) and for 5 seconds
after the gas valve closes (post-purge)
A pressure switch connected to the combustion air blower
housing is used to prove combustion air blower operation.
The switch monitors air pressure in the blower housing. Dur-
ing normal operation, the pressure in the housing is nega-
tive. If pressure becomes less negative (signifying an ob-
struction) the pressure switch opens. When the pressure
switch opens, the furnace control (A3) immediately closes
the gas valve to prevent burner operation.
7−Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box. Each furnace is equipped with two
identical switches. One switch is located over the leftmost
burner and the other switch is located over the rightmost
burner. The limit is a N.C. SPST manual-reset limit con-
nected in series with the ignition control A3. When S47
senses rollout, the ignition control immediately stops igni-
tion and closes the gas valve. If unit is running and flame
rollout is detected, the gas valve will close and ignition con-
trol will be disabled. Rollout can be caused by a blocked
flue or lack of combustion air. The switch is factory set and
cannot be adjusted. The setpoint will be printed on the side of
the limit. The switch can be manually reset. To manually re-
set a tripped switch, push the reset button located on the
control.
FIGURE 10
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
8−Primary Limit Control (S10) Figure 12
The primary limit (S10) on F8AUH units is located in the
middle of the heating vestibule panel. When excess heat is
sensed in the heat exchanger, the limit will open. If the limit is
tripped, the furnace control energizes the supply air blower
and closes the gas valve. The limit automatically resets when
unit temperature returns to normal. The switch is factory set
and cannot be adjusted. The switch may have a different
setpoint for each unit model number. However, the setpoint
will be printed on the side of the limit.
9−Secondary Limit Controls (S21) Figure 12
The secondary limit (S21) on F8AUH units is located in the
blower compartment in the back side of the blower housing.
When excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the furnace control ener-
gizes the supply air blower and closes the gas valve. The limit
automatically resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted. F8AUH 2
ton units will be equipped with type limit in figure 11 and
F8AUH 3 ton and greater will have type limit in figure 12 .
FIGURE 11
F8AUH2 UNITS
LIMIT CONTROL FOR

Page 13
FIGURE 12
F8AUH SERIES UNITS
LIMIT CONTROL FOR
THIS TYPE AUTO-RESET LIMIT
IS USED FOR THE PRIMARY LIMIT (S10) AND
SECONDARY LIMIT (S21)
10−Gas Valve
The F8AUH uses a gas valve manufactured by White
Rodgers (figure 13) or Honeywell (figure 14). The valve is
internally redundant to assure safety shut−off. If the gas
valve must be replaced, the same type valve must be
used.
24VAC terminals and gas control knob are located on top of
the valve. All terminals on the gas valve are connected to
wires from the electronic ignition control. 24V applied to the
terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A regu-
lator adjustment screw is located on the valve.
LPG change over kits are available. Kits include burner ori-
fices and a gas valve regulator conversion kit. See optional
accessories in the SPECIFICATIONS section of this manual
for kit number.
WHITE RODGERS 36E SERIES GAS VALVE
GAS VALVE SHOWN IN OFF POSITION
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
MANIFOLD
PRESSURE
OUTLET
FIGURE 13
FIGURE 14
ON
OFF
HONEYWELL VR8205 SERIES GAS VALVE
GAS VALVE SHOWN IN OFF POSITION
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
MANIFOLD
PRESSURE
OUTLET
11−Combustion Air Blower Prove
(Pressure) Switch (S18)
F8AUH series units are equipped with a combustion air
prove switch located on the vestibule panel. The switch is
connected to the combustion air blower housing by means
of a flexible silicone hose. It monitors air pressure in the
combustion air blower housing.
The switch is a single-pole single-throw pressure switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the combus-
tion air blower is not operating or if the flue becomes ob-
structed.
FIGURE 15
PROVE SWITCH
COMBUSTION AIR BLOWER
PROVE SWITCH
Normally Open
Closes on Negative Pressure
PROVE
SWITCH
Sensing Tube
Attaches to Top
Side Of Blower
Top
Bottom
Left
Right
Back
Front
On start-up, the switch senses that the combustion air
blower is operating. It closes a circuit to the furnace control
when pressure inside the combustion air blower de-
creases to a certain set point. Set points vary depending
on unit size. The pressure sensed by the switch is relative
to atmospheric pressure. If the flue becomes obstructed
during operation, the switch senses a loss of negative
pressure (pressure becomes more equal with atmospher-
ic pressure) and opens the circuit to the furnace control
and gas valve. A bleed port on the switch allows dry relativ-
ity air in the vestibule to purge switch tubing, to prevent
condensate build up. The switch also has an internal inline
orifice, designed to prevent nuisance shut downs due to
erratic vent pressure fluctuations.
The switch is factory set and is not field adjustable. It is a
safety shut-down control in the furnace and must not be
by−passed for any reason. If switch is closed or by−passed,
the control will not initiate ignition at start up.

Page 14
II−PLACEMENT AND INSTALLATION
Make sure unit is installed in accordance with installation
instructions and applicable codes.
III−START-UP
A−Preliminary and Seasonal Checks
1 − Inspect electrical wiring, both field and factory installed
for loose connections. Tighten as required.
2 − Check voltage at disconnect switch. Voltage must be
within range listed on the nameplate. If not, consult the
power company and have voltage condition corrected
before starting unit.
B-Heating StartUp
WARNING
Shock and burn hazard.
F8AUH units are equipped with a direct spark igni-
tion system. Do not attempt to light manually.
1 − STOP! Read the safety information at the beginning
of this section.
2 − Set thermostat to lowest setting.
3 − Turn off all electrical power to appliance.
4 − This appliance is equipped with an ignition device
which automatically lights the burners. Do not try to
light the burners by hand.
5 − Remove top access panel.
6 − White Rodgers 36E Gas Valve −− Switch lever to
OFF. See figure 13.
Honeywell VR8205 Gas Valve −− Turn knob on gas
valve clockwise to OFF. Do not force. See fig-
ure 14.
7 − 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 next step.
8 − White Rodgers 36E Gas Valve −− Switch gas valve lever
to ON.
Honeywell VR8205 Gas Valve −− Turn knob on gas
valve counterclockwise to ON. Do not force.
9 − Replace access panel.
10− Turn on all electrical power to unit.
11− Set thermostat to desired setting.
12− If the appliance will not operate, follow the instructions
To Turn Off Gas To Unit" and call your service techni-
cian or gas supplier.
Turning Off Gas To Unit
1 − Set thermostat to lowest setting.
2 − Turn off all electrical power to unit if service is to be
performed.
3 − Remove access panel.
4 − Switch lever on White Rodgers gas valve to OFF; turn
knob on Honeywell valve clockwise to OFF. Do not
force.
5 − Replace access panel.
C-Safety or Emergency Shutdown
Turn off unit power. Close manual and main gas valves.
D-Extended Period Shutdown
Turn off thermostat or set to UNOCCUPIED" mode. Close
all gas valves (both internal and external to unit) to guaran-
tee no gas leak into combustion chamber. Turn off power
to unit. All access panels, covers and vent caps must be in
place and secured.
IV−HEATING SYSTEM SERVICE CHECKS
A−A.G.A./C.G.A. Certification
All units are A.G.A. and or C.G.A. design certified without
modifications. Refer to the F8AUH Operation and Installa-
tion Instruction Manual Information.
B-Gas Piping
Gas supply piping should not allow more than 0.5”W.C. drop in
pressure between gas meter and unit. Supply gas pipe must not be
smaller than unit gas connection.
Compounds used on gas piping threaded joints should be
resistant to action of liquefied petroleum gases.
C-Testing Gas Piping
IMPORTANT
In case emergency shutdown is required, turn off
the main shut-off valve and disconnect the main
power to unit. These controls should be properly
labeled by the installer.
When pressure testing gas lines, the gas valve must be
disconnected and isolated. Gas valves can be damaged if
subjected to more than 0.5psig (14" W.C.). See figure 16. If
the pressure is equal to or less than 0.5psig (14"W.C.), use
the manual shut−off valve before pressure testing to iso-
late furnace from gas supply.
FIGURE 16
MANUAL MAIN SHUT−OFF VALVE
WILL NOT HOLD TEST PRESSURE
IN EXCESS OF 0.5 PSIG (14"W.C.)
GAS VALVE CAP
GAS PIPING TEST PROCEDURE
FIELD PROVIDED
LINE PRESSURE TAP

Page 15
When checking piping connections for gas leaks, use pre-
ferred means. Kitchen detergents can cause harmful corro-
sion on various metals used in gas piping. Use of a specialty
Gas Leak Detector is strongly recommended.
Do not use matches, candles, flame or any other source of
ignition to check for gas leaks.
D−Testing Gas Supply Pressure
When testing supply gas pressure, connect test gauge to
inlet pressure tap (field provided). See figure 16. Check
gas line pressure with unit firing at maximum rate. Low
pressure may result in erratic operation or underfire. High
pressure can result in permanent damage to gas valve or
overfire. For F8AUH−45,60 and 75 BTUH natural gas
units, operating pressure at the unit must be a minimum
4.5"W.C. For the F8AUH−100 and 120 BTUH units, the op-
erating pressure must be a minimum of 5.0"W.C. For L.P.
gas units, operating pressure at unit gas connection must
be a minimum of 11.0" W.C.
On multiple unit installations, each unit should be checked
separately, with and without other units operating. Supply
pressure must fall within range listed in previous para-
graph.
E−Check Manifold Pressure
After line pressure has been checked and adjusted, check
manifold pressure. Move pressure gauge to outlet pres-
sure tap located on unit gas valve (GV1). Checks of man-
ifold pressure are made as verification of proper regulator
adjustment. Manifold pressure for the F8AUH can be mea-
sured at any time the gas valve is open and is supplying gas
to the unit. Normal manifold pressure for natural gas units is
3.5 in. W.C. For LP/propane gas the correct manifold pres-
sure is 9.5 in. W.C.
IMPORTANT
For safety, connect a shut-off valve between the
manometer and the gas tap to permit shut off of
gas pressure to the manometer.
Operating Pressure (outlet) in. W.C.
TABLE 6
GAS VALVE REGULATION
3.5 +0 −0.3Natural
L.P
.
9.5 + 0.5
Unit (Fuel)
The gas valve is factory set and should not require adjustment.
All gas valves are factory regulated. See table 6. See specifica
tions section of this manual for High Altitude manifold pressure
settings.
Manifold Adjustment Procedure:
1 − Connect a test gauge to outlet pressure tap on gas
valve. Start unit and allow 5 minutes for unit to reach
steady state.
2 − While waiting for the unit to stabilize, notice the flame.
Flame should be stable and should not lift from burner.
Natural gas should burn blue. L.P. gas should burn
mostly blue with some orange streaks.
3 − After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to values given in
table 6.
NOTE−Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to
replace pressure tap plug.
F− Proper Gas Flow (Approximate)
Furnace should operate at least 5 minutes before checking gas
flow. Determine time in seconds for two revolutions of gas
through the meter. (Two revolutions assures a more accurate
time.) Divide by two and compare to time in table 7 below. Ad
just manifold pressure on gas valve to match time needed.
NOTE− To obtain accurate reading, shut off all other
gas appliances connected to meter.
TABLE 7
GAS METER CLOCKING CHART
Seconds for One Revolution
FAUH
Ui
Natural LP
Unit 1 cu ft
Dial
2 cu ft
Dial
1 cu ft
Dial
2 cu ft
DIAL
−45 80 160 200 400
−60 60 120 150 300
−75 48 96 120 240
−100 36 72 90 180
−120 30 60 75 150
Natural−1000 btu/cu ft LP−2500 btu/cu ft
IMPORTANT
For safety, shut unit off and remove manometer as
soon as an accurate reading has been obtained.
Take care to replace pressure tap plug.
G−Flame Signal
A microamp DC meter is needed to check the flame signal
on the ignition control.
Flame (microamp) signal is an electrical current which passes
from the furnace control through the sensor during unit opera-
tion. Current passes from the sensor through the flame to
ground to complete a safety circuit.

FIGURE 17
Page 16
To Measure Flame Signal − Ignition Control:
A transducer is required
to measure flame signal if
meter used will not read a
low micro amp signal.
Seefigure17. The trans-
ducer converts micro-
amps to volts on a 1:1
conversion. Flame signal
for the control board should read 0.7 microamps with a lock-
out signal of .15 microamps. A digital readout meter must be
used. The transducer plugs into most meters. See fig-
ure 18 for proper use of transducer.
1 − Set the volt meter to the DC voltage scale. Insert
transducer into the VDC and common inputs. Ob-
serve correct polarities. Failure to do so results in
negative (−) values.
2 − Turn off supply voltage to control.
3 − Disconnect ignition control flame sensor wire from the
flame sensor.
4 − Connect (−) lead of the transducer to flame sensor.
5 − Connect (+) lead of transducer to the ignition control sen-
sor wire.
6 − Turn supply voltage on and close thermostat contacts to
cycle system.
7 − When main burners are in operation for two minutes, take
reading. Remember 1 DC volt = 1 DC microamp.
SET DIAL TO MEASURE VDC
(+)
(−)
(−) TO
FLAME SENSOR
NOTE−MUST USE DIGITAL METER
RED COLLAR
INDICATES
POSITIVE
LEAD
(+) TO
IGNITION
CONTROL
SENSOR
WIRE
FIGURE 18
WARNING
Fire and explosion hazard.
These instructions MUST be followed exactly.
Can cause a fire or explosion resulting in property
damage, personal injury or loss of life.
V−TYPICAL OPERATING CHARACTERISTICS
A−Blower Operation and Adjustment
NOTE− The following is a generalized procedure and
does not apply to all thermostat controls.
1 − Blower operation is dependent on thermostat control
system.
2 − Generally, blower operation is set at thermostat sub-
base fan switch. With fan switch in ON position, blower
operates continuously on heating speed. With fan
switch in AUTO position, blower cycles with demand or
runs continuously while heating or cooling circuit cycles.
3 − Depending on the type of indoor thermostat, blower
and entire unit will be off when the system switch is in
OFF position.
B−Temperature Rise
Temperature rise for F8AUH units depends on unit input,
blower speed, blower horsepower and static pressure as
marked on the unit rating plate. The blower speed must be
set for unit operation within the range of AIR TEMP. RISE
°F" listed on the unit rating plate.
To Measure Temperature Rise:
1 − Place plenum thermometers in the supply and return
air plenums. Locate supply air thermometer in the first
horizontal run of the plenum where it will not pick up ra-
diant heat from the heat exchanger.
2 − Set thermostat to highest setting.
3 − After plenum thermometers have reached their high-
est and steadiest readings, subtract the two readings.
The difference should be in the range listed on the unit
rating plate. If the temperature is too low, decrease
blower speed. If temperature is too high, first check
the firing rate. Provided the firing rate is acceptable, in-
crease blower speed to reduce temperature. To
change blower speed taps see the Blower Speed Taps
section in this manual.

FIGURE 19
STATIC PRESSURE
TEST
MANOMETER
F8AUH UNIT
Page 17
C−External Static Pressure
1 − Tap locations shown in figure 19.
2 − Punch a 1/4" diameter hole
in supply and return air ple-
nums. Insert manometer
hose flush with inside edge
of hole or insulation. Seal
around the hose with per-
magum. Connect the zero
end of the manometer to the
discharge (supply) side of the system. On ducted sys-
tems, connect the other end of manometer to the re-
turn duct as above. For systems with non−ducted re-
turns, leave the other end of the manometer open to
the atmosphere.
3 − With only the blower motor running and the evaporator
coil dry, observe the manometer reading. Adjust blow-
er motor speed to deliver the air desired according to
the job requirements.
4 − External static pressure drop must not be more than
0.5" W.C.
5 − Seal around the hole when the check is complete.
D−Blower Speed Taps
Blower speed tap changes are made on the control board.
See figure 6. Unused taps must be secured on two dummy
terminals labeled "PARK on the control board. The heat-
ing tap is connected to the "HEAT−H " terminal and the
cooling tap is connected to the "COOL−H" terminal. The
continuous blower tap is the same as the heating tap.
To change existing heat tap, turn off power then switch out
speed tap on "HEAT−H" with tap connected to "PARK" .
See table 8 for blower motor tap colors for each speed.
TABLE 8
VI−MAINTENANCE
At the beginning of each heating season, the system
should be checked as follows:
A−Filters
Return air filter is not supplied with unit. See Optional Ac-
cessaries section in this manual for filter and rack size. A
filter must be used in order to ensure long life and proper
operation. The filter is located in the return air duct or return
air register. Filters must be cleaned or replaced when dirty
to assure proper unit operation.
B− Heat Exchanger and Burners
Due to crimps designed in the heat exchanger, cleaning is
not recommended. Removal is for inspection only.
NOTE−Use papers or protective covering in front of fur-
nace while cleaning furnace.
To clean burners:
1 − Remove screws holding upper burner mounting rail.
Remove rail.
2 − Slide burners off each orifice pull burners from heat ex-
changer.
3 − Clean holes in burner head (retention ring) with a wire
brush. See figure 20.
FIGURE 20
4 − With a shop vacuum or rags, clean out soot and scale
deposits from burners.
5 − Remove screws securing flue box to vestibule panel.
Remove flue box from unit. Leave combustion air
blower attached to flue box.
6 − With a shop vacuum or rags, clean out soot and scale
deposits from heat exchanger tubes and flue box. If tur-
bulators are removed make sure they are re-installed
before reassembling units.
7 − Inspect heat exchanger for corrosion damage, holes or
cracks.
8 − Replace burners making sure to fully engage on orifice.
Resecure burner mounting rail and flue box. Inspect flue
box gasket. Replace gasket if necessary.
9 − Slide heat exchanger into cabinet and re-secure heat
exchanger screws.
10 − Reinstall filler piece making sure that there is a good
seal between the cabinet sides, blower and deck and
filler piece.
11 − Re-secure flue pipe, gas piping and access panels.
12 − Carefully check all piping connections (factory and
field) for gas leaks. Use a leak detecting solution or oth-
er preferred means.
13 − Turn on gas and electrical supply.

Page 18
FIGURE 21
HEAT EXCHANGER REMOVAL (unit shown in horizontal position)
REMOVE SCREWS (4)
SLIDE ENTIRE ASSEMBLY OUT OF CABINET
Front
Right
Top
Bottom
Left
Back
CAUTION
Potential for gas leaks, fire or explosion.
Some soaps used for leak detection are corrosive
to certain metals. Carefully clean piping thor-
oughly after leak detection has been completed.
Can cause damage to piping resulting in gas
leaks, fire or explosion.
C−Supply Air Blower
1 − Check and clean blower wheel.
2 − Motors used on the F8AUH series units are perma-
nently lubricated and need no further lubrication.
D−Flue and Chimney
Flue must conform to all AGA/GAMA venting require-
ments. Flue pipe deteriorates from the inside out and must
be disconnected in order to check thoroughly. Check flue
pipe, chimney and all connections for tightness and to
make sure there is no blockage or leaks.
E−Electrical
1 − Check all wiring for loose connections.
2 − Check for correct voltage.
3 − Check amp−draw on blower motor.

Page 19
VII− Wiring and Sequence of Operation
1 − When there is a call for heat, W1 of the thermostat en-
ergizes W of the furnace control with 24VAC.
2 − S10 primary limit switch and S47 rollout switch are
closed. Call for heat can continue.
3 − Control board energizes combustion air blower B6.
Combustion air blower runs until S18 combustion air
prove switch closes (switch must close within 2−1/2
minutes or control goes into 5 minute Watchguard
Pressure Switch delay). Once S18 closes, a 15−se-
cond pre−purge follows.
4 − Control board energizes ignitor. An 11 to 21 second
warm−up period begins.
5 − Gas valve opens for a 4−second trial for ignition (igni-
tor will remain energized for the first second).
6 − Flame is sensed, gas valve remains open for the heat
call.
7 − After 45−second delay, control board energizes in-
door blower B3.
8 − When heat demand is satisfied, W1 of the indoor ther-
mostat de−energizes W of the control board which de−
energizes the gas valve. Combustion air blower B6
continues a 5−second post−purge period, and indoor
blower B3 completes a selected OFF time delay.

Page 20
HEATING SEQUENCE OF OPERATION
NORMAL HEATING MODE ABNORMAL HEATING MODE
CONTROL SELF−CHECK OKAY?
BURNER OFF?
(CONTINUOUS FLAME CHECK)
NORMAL OPERATION:
LED #1 −− SLOW FLASH
LED #2 −− SLOW FLASH
YES
YES
YES
GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER DELAY OFF.
LED #1 ON
LED #2 ON
(RESET CONTROL BY TURNING MAIN POWER OFF.)
POLARITY REVERSED.
LED #1 −− FAST FLASH
LED #2 −− SLOW FLASH
POWER ON
IS POLARITY REVERSED?
ROLLOUT SWITCH CLOSED?
THERMOSTAT CALLS FOR HEAT:
LED #1 −− FAST FLASH
LED #2 −− FAST FLASH
IS COMBUSTION AIR
PRESSURE SWITCH OPEN?
GAS VALVE OFF. COMBUSTION AIR BLOWER ON.
INDOOR BLOWER ON.
LED #1 −− ON. LED #2 −− SLOW FLASH.
SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES.
GAS VALVE OFF. COMBUSTION AIR BLOWER ON.
INDOOR BLOWER ON HEATING SPEED.
LED #1 −− SLOW FLASH
LED #2 −− OFF
GAS VALVE OFF.
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF WITH DELAY.
LED #1 OFF
LED #2 SLOW FLASH
(Sequence holds until pressure switch
closes or thermostat resets control.)
NO
NO
YES
NO
NO
NO
IS COMBUSTION AIR BLOWER
ENERGIZED?
HAS COMBUSTION AIR PRESSURE
SWITCH CLOSED IN 2.5 MINUTES?
YES
YES
PRESSURE SWITCH IS IN WATCHGUARD MODE.
GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF WITH DELAY.
LED #1 −− OFF. LED #2 −− SLOW FLASH.
IS 5-MINUTE RESET PERIOD COMPLETE?
NO
PRIMARY AND SECONDARY LIMIT SWITCH.
CLOSED?
YES
YES
CONTINUED NEXT PAGE
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER ON
LED #1 SLOW FLASH RATE
LED #2 −− ON
NO
YES
NO
YES