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Lennox F8AUH series Service Manual

Gas furnaces
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See also: User's Information 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 (NO
) Standards and California Seasonal Efficiency re-
x
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.

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F8AUH
Gas Furnaces
Up-Flow/Horizontal
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)
SERVICE MANUAL
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Page 1
Corp 9912−L5
Revised 08−2004

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Summary of Contents for Lennox F8AUH series

  • Page 1: Table Of Contents

    Add-On Cooling - 1 thru 5 Tons (3.5 to 17.6 kW) SERVICE MANUAL Corp 9912−L5 Revised 08−2004 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.

  • Page 2: Specifications

    SPECIFICATIONS F8AUH**−24−045 F8AUH**−36−060 F8AUH**−36−075 Model No. F8AUH**−24−060 F8AUH**−24−075 F8AUH**−24−045X F8AUH**−36−060X 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%...
  • Page 3 SPECIFICATIONS F8AUH**−48−075 F8AUH**−48−100 F8AUH**−60−100 F8AUH**−60−120 Model No. F8AUH**−48−120 F8AUH**−48−075X F8AUH**−48−100X F8AUH**−60−100X 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%...

  • Page 4: Blower Data

    BLOWER DATA F8AUH**−24−045, F8AUH**−24−060 AND F8AUH**−24−75 BLOWER PERFORMANCE Air Volume at Various Blower Speeds External Static External Static Pressure High Medium−High Medium−Low in. w.g. 1270 1245 1220 1195 1170 1140 1110 1060 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 Air Volume at Various Blower Speeds External Static...
  • Page 5 BLOWER DATA F8AUH**−60−100 AND F8AUH**−60−120 BLOWER PERFORMANCE Air Volume at Various Blower Speeds External Static External Static Pressure High Medium−High Medium Medium−Low in. w.g. 2450 1155 2160 1020 1970 1700 1500 2440 1150 2155 1015 1965 1695 1500 2430 1145 2150 1015 1960...

  • Page 6: Horizontal Venting

    HORIZONTAL VENTING REQUIREMENTS (Requires Optional Side Wall Power Venting Kit) Vent Pipe Diameter Vent Pipe Minimum Vent pipe Maximum Horizontal Venting Furnace Furnace Connection Equivalent Length Equivalent Length Transition Required Model No Model No. feet meters feet meters F8AUH**−24−045 3 to 4 76 to 102 F8AUH**−24−060 18.0...

  • Page 7: Parts Identification

    F8AUH PARTS ARRANGEMENT HEAT EXCHANGER ASSEMBLY COMBUSTION AIR BLOWER PRESSURE PRIMARY SWITCH LIMIT GAS VALVE BURNER ACCESS PANEL BURNER ASSEMBLY TRANSFORMER BLOWER ASSEMBLY CONTROL BOARD BLOWER ACCESS PANEL FIGURE 1 HEATING COMPONENTS (shown in horizontal position) PRIMARY LIMIT ROLLOUT SWITCH (2) COMBUSTION AIR PROVE SWITCH MANIFOLD...

  • Page 8: I Unit Components

    F8AUH BURNER ASSEMBLY (shown in upflow position) CARBIDE ROLLOUT IGNITOR SWITCHES BURNER VEST PANEL UPPER BURNER MOUNTING RAIL LOWER BURNER MOUNTING RAIL FLAME BURNER SENSOR HEAT EXCHANGER PRIMARY LIMIT Back ORIFICE Left GAS VALVE Right Front BURNER MANIFOLD Bottom FIGURE 3 I−UNIT COMPONENTS (Figures 1, 2, 3) 1−...
  • Page 9 3− Furnace Control (A3) a−Electronic Ignition (See Figure 5) On a call for heat the 50A55 control board monitors the All F8AUH model units are equipped with an integrated combustion air blower pressure switch. The control will not ignition blower control (50A55), which controls ignition, begin the heating cycle if the pressure switch is closed (by−...
  • Page 10 The 50A55 control board is equipped with two LED lights for troubleshooting. The diagnostic codes are listed below in table TABLE 3 DIAGNOSTIC CODES MAKE SURE TO ID LED’S CORRECTLY: REFER TO INSTALLATION INSTRUCTIONS FOR CONTROL BOARD LAYOUT. LED #1 LED #2 DESCRIPTION Power −...
  • Page 11 50A55 INTEGRATED CONTROL BOARD TABLE 4 (Silicon Carbide Ignition System) TERMINAL DESIGNATIONS COOL−H Blower − Cooling Speed (120VAC) HEAT−H Blower − Heating Speed (120VAC) EAC−H Electronic Air Cleaner (120VAC) HUM−H Humidifier (120VAC) XFMR−H Transformer (120VAC) LINE−H Input (120VAC) LINE−N Input (Neutral) HUM−N Humidifier (Neutral) EAC−N...
  • Page 12 5−Blower Motors and Capacitors 7−Flame Rollout Switches (S47) Flame rollout switch is a high temperature limit located on All F8AUH units use direct drive blower motors. All motors top of the burner box. Each furnace is equipped with two used are 120V permanent split capacitor motors to ensure identical switches.
  • Page 13 LIMIT CONTROL FOR (Pressure) Switch (S18) F8AUH SERIES UNITS F8AUH series units are equipped with a combustion air THIS TYPE AUTO-RESET LIMIT prove switch located on the vestibule panel. The switch is IS USED FOR THE PRIMARY LIMIT (S10) AND...

  • Page 14: Installation

    II−PLACEMENT AND INSTALLATION knob on Honeywell valve clockwise to OFF. Do not force. Make sure unit is installed in accordance with installation 5 − Replace access panel. instructions and applicable codes. C-Safety or Emergency Shutdown III−START-UP Turn off unit power. Close manual and main gas valves. A−Preliminary and Seasonal Checks D-Extended Period Shutdown 1 −...
  • Page 15 When checking piping connections for gas leaks, use pre- Manifold Adjustment Procedure: 1 − Connect a test gauge to outlet pressure tap on gas ferred means. Kitchen detergents can cause harmful corro- valve. Start unit and allow 5 minutes for unit to reach sion on various metals used in gas piping.

  • Page 16: Typical Operating Characteristics

    To Measure Flame Signal − Ignition Control: WARNING A transducer is required Fire and explosion hazard. to measure flame signal if These instructions MUST be followed exactly. meter used will not read a Can cause a fire or explosion resulting in property low micro amp signal.

  • Page 17: Maintenance

    C−External Static Pressure operation. The filter is located in the return air duct or return air register. Filters must be cleaned or replaced when dirty 1 − Tap locations shown in figure 19. to assure proper unit operation. 2 − Punch a 1/4" diameter hole STATIC PRESSURE B−...
  • Page 18 2 − Check for correct voltage. 1 − Check and clean blower wheel. 3 − Check amp−draw on blower motor. 2 − Motors used on the F8AUH series units are perma- nently lubricated and need no further lubrication. Page 18...

  • Page 19: Wiring And Sequence Of Operation

    VII− Wiring and Sequence of Operation 1 − When there is a call for heat, W1 of the thermostat en- 5 − Gas valve opens for a 4−second trial for ignition (igni- ergizes W of the furnace control with 24VAC. tor will remain energized for the first second).

  • Page 20: Heating Sequence Of Operation

    HEATING SEQUENCE OF OPERATION ABNORMAL HEATING MODE NORMAL HEATING MODE GAS VALVE OFF. COMBUSTION AIR BLOWER OFF. POWER ON INDOOR BLOWER DELAY OFF. LED #1 ON LED #2 ON CONTROL SELF−CHECK OKAY? (RESET CONTROL BY TURNING MAIN POWER OFF.) IS POLARITY REVERSED? POLARITY REVERSED.
  • Page 21 HEATING SEQUENCE CONTINUED NORMAL HEATING MODE ABNORMAL HEATING MODE 15-SECOND COMBUSTION AIR BLOWER PREPURGE INITIATED BY CLOSED PRESSURE SWITCH. LEDS SIGNAL IS THERE A PROPER GROUND? IGNITOR WARM-UP −− 11 TO 21 SECONDS. ALTERNATING FAST FLASH 4-SECOND TRIAL FOR IGNITION. GAS VALVE OPENS.
  • Page 22 COOLING SEQUENCE OF OPERATION NORMAL COOLING MODE ABNORMAL COOLING MODE POWER ON IGNITION CONTROL MAIN POWER ON. GAS VALVE OFF. COMBUSTION AIR BLOWER OFF. INDOOR BLOWER OFF WITH NORMAL DELAY. CONTROL SELF DIAGNOSTIC CHECK. SIGNAL CIRCUIT BOARD FAILURE AT LED. IS CONTROL OPERATING NORMALLY? INTERRUPT MAIN POWER TO RESET CONTROL.

  • Page 23: Control Board Troubleshooting Guide

    VIII-Troubleshooting- Silicon Carbide Ignition System PROBLEM: 1 UNIT FAILS TO OPERATE IN THE COOLING, HEATING, OR CONTINUOUS FAN MODE Condition Possible Cause Corrective Action / Comments 1.1.1 ACTION 1 − Check 120V main voltage. − Both diagnostic lights fail to light up. Main voltage 120V not supplied to Determine cause of main power failure.
  • Page 24 PROBLEM 1: UNIT FAILS TO OPERATE IN THE COOLING, HEATING, OR CONTINUOUS FAN MODE Condition Possible Cause Corrective Action / Comments ACTION 1 − Check that the unit is properly 1.5.1 − On initial power−up the combustion grounded. ACTION 2 − Install a proper main ground to the air blower remains energized.
  • Page 25 PROBLEM 3: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER DOES NOT ENERGIZE (CONT.). Condition Possible Cause Corrective Action/Comments − Unit operates with a cooling and con- 3.3.1 ACTION 1 − Check for correct wiring and loose tinuous fan demand.
  • Page 26 PROBLEM 5: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER ENERGIZES, IGNITOR IS ENERGIZED. Condition Possible Cause Corrective Action/Comments ACTION 1 − Check line pressure at the gas 5.1.1 valve. Pressure should not exceed 13" WC for - Unit operates with a cooling and con both natural and propane.
  • Page 27 PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN PREMATURELY (CONT.) Condition Possible Cause Corrective Action/Comments ACTION 1 − Check that the manifold pressure matches value listed on nameplate. See installa- − Combustion air blower energizes tion instructions for proper procedure. with a heating demand.
  • Page 28 PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN PREMATURELY (CONT.) − Combustion air blower energizes with a heating demand. − Burners light. − Diagnostic lights flash watch guard 6.5.1 ACTION 1 − Check that the sensor is properly lo- flame failure.

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