Lennox G27M series Unit Information
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Lennox G27M series Unit Information

Lennox G27M series Unit Information

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Service Literature

G27M series units are mid-efficiency gas furnaces
manufactured with tubular aluminized steel heat ex
changers. G27M units are available in heating capaci
ties of 60,000 to 120,000 Btuh (17.6 to 35.2 kW) and cool
ing applications from 1.5 through 5 tons (3.5 through
17.6 kW). Refer to Engineering Handbook for proper siz
ing.
Units are factory equipped for use with natural gas only.
LP kits are not available. All G27M units feature the Len
nox SureLightT hot surface ignition system. The
G27MX units meet the California Nitrogen Oxides (NO
Standards and California Seasonal Efficiency require
ments. All units use a two-stage gas valve along with a
two-stage induced draft blower assembly. The gas
valve is redundant to assure safety shut-off as required
by A.G.A. and C.G.A.
Units may be installed in upflow, downflow or horizon
tal position. The heat exchanger is designed for upright
or horizontal use only. When the unit is installed in the
downflow position, the heat exchanger must be field
removed and reinstalled so it is upright when the cabi
net is inverted. No field conversion is required when
the unit is installed in the horizontal position.
The heat exchanger, burners and manifold assembly
can easily be removed for inspection and service by sim
ply disconnecting gas, unplugging wiring harnesses
and removing four screws holding the heat exchanger in
place. The heat exchanger slides out of the cabinet.
Information contained in this manual is intended for use
by qualified service technicians only. All specifications
are subject to change. Procedures outlined in this manu
al are presented as a recommendation only and do not
supersede or replace local or state codes.
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G27M
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1998 Lennox Industries Inc.
Litho U.S.A.
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Summary of Contents for Lennox G27M series

  • Page 1: Table Of Contents

    Service Literature G27M series units are mid-efficiency gas furnaces manufactured with tubular aluminized steel heat ex changers. G27M units are available in heating capaci ties of 60,000 to 120,000 Btuh (17.6 to 35.2 kW) and cool ing applications from 1.5 through 5 tons (3.5 through 17.6 kW).

  • Page 2: Specifications

    SPECIFICATIONS Model No. G27M2(X)-60 G27M3(X)-75 G27M4(X)-100 G27M5(X)-120 Input Btuh (kW) low fire 40,800 (12.0) 51,000 (14.9) 68,000 (19.9) 81,600 (23.9) Output Btuh (kW) low fire 32,600 (9.6) 40,800 (12.0) 54,400 (16.0) 65,300 (19.1) Input Btuh (kW) high fire 60,000 (17.6) 75,000 (22.0) 100,000 (29.3) 120,000 (35.2)
  • Page 3 1370 2100 1890 1700 1520 1330 NOTE — All air data is measured external to unit with 1 inch (25 mm) air filter in place. NOTE – Filter not furnished with unit. 1997 Lennox Industries Inc. Page 3 Litho U.S.A.

  • Page 4: I-Unit Components

    3– SureLight Ignition System (A3) G27M unit components are shown in figure 1. The All G27M units are equipped with the Lennox Sure – blower controls, gas valve and burners can be accessed Light ignition system. The system consists of an igni- by removing the front access panel.
  • Page 5 NOTE - The G27M furnace contains electronic compo great enough, the pressure switch closes and a 15-se cond pre-purge begins. If the pressure switch is not nents that are polarity sensitive. Make sure that the fur proven within 2-1/2 minutes, the control goes into nace is wired correctly and is properly grounded.
  • Page 6 TABLE 3 SureLight CONTROL BOARD SureLight BOARD J58 (J1) TERMINAL DESIGNATIONS PIN # FUNCTION Primary Limit In Not Used Roll Out Switch Out Gas Valve 24V Pressure Switch In Primary Limit Out Not Used Roll Out Switch In Ground E )DQ 7LPH &RQWURO The fan on time of 45 seconds is not adjustable.
  • Page 7 All G27M units use direct drive blower motors. All mo- tors used are 120V permanent split capacitor motors to ensure maximum efficiency. See table 4 for ratings. SECONDARY LIMIT SWITCHES Table 4 FOR G27M SERIES UNITS G27M BLOWER RATINGS 120V (1)S21 BLOWER MOTOR G27M2–60 5MF 370V G27M3–75...
  • Page 8 HEATING COMPONENTS (shown in horizontal position) COMBUSTION AIR ROLLOUT SWITCH (2) PROVE SWITCHES (S47) (S18 and S102) PRIMARY LIMIT (S10) ORIFICES Left Bottom Back MANIFOLD Front BURNERS Right HEAT EXCHANGER COMBUSTION AIR GAS VALVE (GV1) BLOWER (B6) FIGURE 8 G27M BURNER ASSEMBLY (shown in upflow position) ROLLOUT SWITCHES (S47)
  • Page 9 C–Heating Components (Figures 8 and 9 ) 3–Primary Limit Control (S10) Heating components are located inside the front louvered The primary limit (S10) is located in the middle of the panel. Inside the heating compartment are the combus- heating vestibule panel (see figure 9). S10 is the same tion air blower (B6), flame rollout switches (S47), primary type of limit as (1) S21 or (2) S21 as shown in figure 7.
  • Page 10 5–SureLight Ignitor 6–Flame Sensor Figure 12 and 14 shows the position of the SureLight Figure 14 shows the position of the flame sensor in per- hot surface ignitor in perspective with the rest of the spective with the rest of the burner assembly. The burner assembly.

  • Page 11: Ii-Placement And Installation

    (Pressure) Switches (S128 and S102) corrected before starting unit. B--Heating Start-Up G27M series units are equipped with two combustion air 1 - - Set thermostat to OFF position. Close manual knob prove switches. S128 (low fire) and S102 (high fire) are lo- on gas valve.

  • Page 12: Iv-Heating System Service Checks

    Use of a C–Testing Gas Piping specialty gas leak detector is strongly recommended. It is available through Lennox under part number IMPORTANT 31B2001. See Corp. 8411–L10, for further details. Do not use matches, candles, flame or any other source In case emergency shutdown is required, turn off of ignition to check for gas leaks.
  • Page 13 F-- Proper Gas Flow (Approximate) On multiple unit installations, each unit should be checked separately, with and without other units oper- Furnace should operate at least 5 minutes before checking gas flow. Determine time in seconds for two ating. Supply pressure must be a minimum of 5.0” revolutions of gas through the meter.

  • Page 14: V-Typical Operating Characteristics

    If the temperature is nal transducer (part number 78H5401) available from too low, decrease blower speed. If temperature is Lennox to measure the flame signal. too high, first check the firing rate. Provided the firing Flame (microamp) signal is an electrical current which...

  • Page 15: Vi-Maintenance

    E--Blower Speed Taps Leaded Motors VI--MAINTENANCE --2 Models WARNING Blower speed tap changes are made on the SureLight control board. See figure 2. Unused taps must be se- Disconnect power before servicing unit. cured on dummy terminals ”PARK M1” and or ”PARK M2”...
  • Page 16 8 - - Reinstall burner box and gas supply piping. Recon- Heat Exchanger NOTE- -Use papers or protective covering in front of nect ignitor and flame sensor leads. furnace while cleaning furnace. 9 - - Carefully check all piping connections (factory and Due to dimples designed in the heatexchanger, clean- field) for gas leaks.
  • Page 17 VII--WIRING DIAGRAM AND SEQUENCE OF OPERATION TYPICAL G27M WIRING DIAGRAM (--2 Model 60 Hz. Units Only) 3DJH ...
  • Page 18 Sequence of Operation G27M 2 - - SureLight control energizes ignitor. A 20- -second warm- -up period begins. 1 - - When there is a call for heat, W1 of the thermostat Gas valve is energized on first stage. Gas valve energizes W of the furnace control with 24VAC.

  • Page 19: Heating Sequence Of Operation

    HEATING SEQUENCE OF OPERATION NORMAL HEATING MODE ABNORMAL HEATING MODE GAS VALVE OFF. COMBUSTION AIR BLOWER ON. POWER ON INDOOR BLOWER ON. LED# 1 ALTERNATING FAST FLASH LED# 2 ALTERNATING FAST FLASH CONTROL SELF–CHECK OKAY? CHECK FOR BROKEN IGNITOR OR OPEN IGNITOR CIRCUIT COMBUSTION AIR BLOWER ON FOR 1 SECOND.
  • Page 20 HEATING SEQUENCE CONTINUED NORMAL HEATING MODE ABNORMAL HEATING MODE PREPURGE WATCHGUARD 60 MINUTE DELAY IGNITION SEQUENCE BEGINS IGNITOR WARM-UP TWO-STAGE MODE W2 TIMED MODE SINGLE-STAGE MODE CALL FOR 1st STAGE HEAT SINGLE STAGE T'STAT HIGH HEAT ONLY WATCHGUARD 60 MINUTE DELAY TWO-STAGE MODE W2 TIMED MODE SINGLE-STAGE MODE...
  • Page 21 HEATING SEQUENCE CONTINUED NORMAL HEATING MODE ABNORMAL HEATING MODE PREPURGE (See top of previous page.) PRESSURE SWITCH CLOSED? GAS VALVE DE--ENERGIZED. INDOOR BLOWER DE--ENERGIZED. COMBUSTION AIR BLOWER ON FOR 2.5 MINUTES. TWO- -STAGE MODE TWO- -STAGE MODE CALL FOR 1st STAGE HEAT SINGLE STAGE T’STAT PRESSURE SWITCH CALL FOR 2nd STAGE...
  • Page 22 COOLING SEQUENCE CONTINUED GAS VALVE OFF . COMBUSTION AIR BLOWER OFF . INDOOR BLOWER OFF WITH NORMAL DELAY . ROLLOUT SWITCH MONITORED CONTINUOUSLY . SIGNAL CIRCUIT BOARD FAILURE AT LED. IS ROLLOUT SWITCH CLOSED? SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES. GAS VALVE OFF .

  • Page 23: Vii-Wiring Diagram And Sequence Of Operation 17 Viii-Troubleshooting

    VIII–SureLight Control Troubleshooting Guide UPON INITIAL POWER UP, REMOVE ALL THERMOSTAT DEMANDS TO THE UNIT 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. –...
  • 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 ground. – On initial power–up the combus- ACTION 2 – Install a proper main ground to the Improper ground to the unit.
  • 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 3.3.1 – Unit operates with a cooling and ACTION 1 – Check for correct wiring and loose continuous fan demand. Miswiring of furnace or improper connections.
  • Page 26 PROBLEM 5: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER ENERGIZES, IGNITOR IS ENERGIZED. (CONT.) 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 –...
  • Page 27 Lennox personnel. combustion air blower. ACTION 1 – Check for sooting deposits or oth- er restrictions in the heat exchanger assem- 6.3.4...
  • 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 flame failure. 6.5.1 ACTION 1 - - Check that the sensor is properly -- NOTE”...

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