Lennox G25MV Series User manual

Page 1 1995 Lennox Industries Inc.
LITHO U.S.A.
Corp. 9505–L2
G25MV
Service Literature
G25MV SERIES UNITS
G25MV series units are mid–efficiency gas furnaces
manufactured with tubular steel heat exchangers
formed of aluminized steel. G25MV units are available
in heating capacities of 60,000 to 120,000 Btuh (17.6 to
35.2 kW) and cooling applications from 1.5 through 5
tons (3.5 through 17.6 kW). Refer to Engineering Hand-
book for proper sizing.
Units are factory equipped for use with natural gas. All
G25MV units use a hot surface ignition system. The
G25MV units meet the California Nitrogen Oxides (NOx)
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. or C.G.A.
All G25MV units are equipped with an electronic vari-
able speed (VSM) fan motor. The VSM motor maintains
a specified air volume throughout the entire external
static range.
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 is field re-
moved and reinstalled so it is upright when the cabinet
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 Lennox 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.
G25MV

Page 2
SPECIFICATIONS
Model No. G25MV3-60 G25MV3-75 G25MV5-100 G25MV5-120
Input Btuh (kW) high fire 60,000 17.6) 75,000 (22.0) 100,000 (29.3) 120,000 (35.2)
Output Btuh (kW) high fire 48,000 (14.1) 60,000 (17.6) 80,000 (23.4) 96,000 (28.1)
A.F.U.E. 80.0% 80.0% 80.0% 80.0%
Input Btuh (kW) low fire 40,800 (12.0) 51,000 (15.0) 68,000 (20.0) 81,600 (24.0)
Flue size connection diameter — in. (mm) round 3 (76) 4 (102) 4 (102) 4 (102)
Temperature rise range — F (C) 30 – 60 (17 – 33) low fire
40 – 70 (22 – 39) high fire
High static certified by A.G.A./C.G.A. — in wg. (Pa) .80 (200) .80 (200) .80 (200) .80 (200)
Gas Piping Size I.P.S. Natural gas only 1/2 (13) 1/2 (13) 1/2 (13) 1/2 (13)
Blower wheel nominal
di id h
in. 10 x 7 10 x 7 12 x 9 12 x 9
diameter x width mm 254 x 178 254 x 178 305 x 229 305 x 229
Blower motor output — hp (W) 1/2 (373) 1/2 (373) 1 (746) 1 (746)
Unit minimum circuit ampacity (amps) 12.2 18.2
Unit maximum fuse or circuit breaker size 15.0 20.0
Electrical characteristics 120 volts — 60 hertz — 1 phase
Nominal cooling
hbddd
Tons 2, 2-1/2 or 3 2, 2-1/2 or 3 3-1/2, 4 or 5 3-1/2, 4 or 5
g
that can be added kW 7.0, 8.8 or 10.6 7.0, 8.8 or 10.6 12.3, 14.1 or 17.6 12.3, 14.1 or 17.6
Up-flow/Horizontal Filter Kit (furnished) — filter size (1) 16 x 20 x 1 (406 x 508 x 25) (1) 20 x 20 x 1 (508 x 508 x 25)
Shipping weight — lbs. (kg) 1 package 135 (61) 135 (61) 175 (79) 175 (79)
Optional Accessories (Must Be Ordered Extra) 
Down-flow
Fil Ki
Catalog No. LB-69843A (32J01) — 3 lbs. (1 kg)
Filter Kit No. & Size of Filters — in. (mm) (2) 16 x 20 x 1 (406 x 508 x 25)
Down-flow Combustible Floor Base LB-79239A (67J91) — 10 lbs. (4 kg) LB-79239B (67J92) — 10 lbs. (4 kg)
Hanging Bracket Kit LB-69957 (46J66) — 15 lbs. (7 kg)
Annual Fuel Utilization Efficiency based on U.S. DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating for non-weatherized
furnaces.
Filters are not furnished with kit and must be ordered extra. Polyurethane frame type filter is furnished with kit.
2 in. x 3 in. (51 mm x 76 mm) flue adaptor furnished with -60 input furnaces for connection to furnace induced draft blower.
2 in. x 4 in. (51 mm x102 mm) flue adaptor furnished with -75, -100 & -120 input furnaces for connection to furnace induced draft blower.
BLOWER PERFORMANCE DATA
G25MV3-60 & G25MV3-75 BLOWER PERFORMANCE
0 through 0.80 in. w.g. (0 Through 200 Pa) External Static Pressure Range
VSP2-1 Blower Control — Factory Settings
G25MV3-60 = Low Speed — 1 / High Speed — 4 / Heat Speed — 1
G25MV3-75 = Low Speed — 3 / High Speed — 4 / Heat Speed — 2
“ADJUST”
VSP2-1 Jumper Speed Positions
“ADJUST”
Jumper
“LOW” Speed (Cool Or Continuous Fan) “HIGH” Speed (Cool) “HEAT” Speed
Jumper
Setting 1 2 3 4 1 2 3 4 1 2 3 4
g
cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
NORM 640 300 740 350 890 420 1030 485 1060 500 1100 520 1260 595 1340 630 850 400 1050 495 1280 605 1370 645
—600 285 680 320 810 380 920 435 930 440 1020 480 1130 535 1200 565 780 370 945 445 1160 545 1240 585
NOTE — The effect of static pressure and filter resistance is included in the air volumes listed.
G25MV5-100 & G25MV5-120 BLOWER PERFORMANCE
0 through 0.80 in. w.g. (0 Through 200 Pa) External Static Pressure Range
VSP2-1 Blower Control — Factory Settings
G25MV5-100 = Low Speed — 2 / High Speed — 4 / Heat Speed — 1
G25MV5-120 = Low Speed — 3 / High Speed — 4 / Heat Speed — 2
“ADJUST”
VSP2-1 Jumper Speed Positions
“ADJUST”
Jumper
“LOW” Speed (Cool Or Continuous Fan) “HIGH” Speed (Cool) “HEAT” Speed
Jumper
Setting 1 2 3 4 1 2 3 4 1 2 3 4
g
cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
NORM 690 325 1170 550 1440 680 1480 700 1620 765 1820 860 2000 945 2100 990 1420 970 1720 810 2030 960 2150 1015
—580 275 1050 495 1280 605 1360 640 1350 635 1550 730 1700 800 1800 850 1280 605 1550 730 1800 850 1900 895
NOTE — The effect of static pressure and filter resistance is included in the air volumes listed.
NOTE — Do not adjust jumper settings (CFM) below tap# 2 “NORMAL” for both “LOW” speed and “HEAT” speed for the G25MV5–120 units only.

Page 3
FIGURE 1
G25MV
PARTS IDENTIFICATION
VENT ADAPTER CABINET TOP
G25MV
CABINET
CABINET
BOTTOM
HEAT EXCHANGER
ASSEMBLY
BURNER
ASSEMBLY
BLOWER
ASSEMBLY
COMBUSTION AIR
BLOWER
FLUE BOX
SECONDARY
LIMITS
FRONT LOUVERED
PANEL
DOOR INTERLOCK SWITCH
BURNER
CONTROL
BOARD
TRANSFORMER
PRESSURE
SWITCHES
NOx
TURBULATOR
VSP2
BOARD
FLUE THERMOSTAT
VARIABLE SPEED
BLOWER MOTOR
PRIMARY
LIMIT
G25MV BLOWER DOOR COMPONENTS – BLOWER ACCESS
FIGURE 2
W
W
BRN
BRN
BLK
BURNER
CONTROL (A3)
VSP2
CONTROL (A24)
TRANSFORMER
(T1)
DOOR INTERLOCK
SWITCH (S51)
TRANSFORMER
CIRCUIT BREAKER (CB8)
G25MV SHOWN
IN UPFLOW
POSITION

Page 4
I–UNIT COMPONENTS (Figure 1)
G25MV unit components are shown in figure 1. The
blower controls, gas valve and burners can be accessed
by removing the front access panel. A separate blower
access door is located behind the front access panel.
Electrical control components are mounted to the
blower access door. Units are factory equipped with
bottom return air panels
A–Blower Door Components (Figure 2)
Electrical burner control and blower control components
are located on the outside surface of the blower access
door. Jackplugs allow the blower door to be easily re-
moved for blower service.
Located on the blower door are the unit transformer
(T1), burner control (A3), VSP2 blower control board
(A24), and door interlock switch (S51).
1– Control Transformer (T1)
A transformer located on the blower door provides power
to the low voltage section of the unit. Transformers on all
models are rated 50VA with a 120V primary and a 24V
secondary. The transformer is protected by a 3.0 amp
circuit breaker.
2–Door Interlock Switch (S51)
A door interlock switch rated 16A at 125VAC is located
on the blower access door. The switch is wired in series
with line voltage. When the blower door is removed the
unit will shut down.
3– Burner Control (A3)
All G25MV units utilize a burner control manufactured
by RAM Electronics Corporation. The “RAM” board is
a printed circuit board which controls the gas valve,
combustion air blower and ignition. It also monitors
the flame, limit and gas valve operation. The board
utilizesboth120and24VAC.Seefigure3.Theboardis
equipped with a diagnostic LED for use when trouble-
shooting the unit.
Whenthe furnaceis idle(blower offand noheating or
cooling demand), the diagnostic LED flashes at a
slow steady rate. On a call for heat, the diagnostic
LED begins flashing at a fast rate and the combustion
air blower is energized. The LED flashes different
codes to indicate problem conditions. The diagnostic
LED lights red (not flashing) to indicate control board fail-
ure. Table 1 shows how to interpret the other LED modes.
Pre-Purge
On a call for heat, the combustion air blower begins
operating. If the combustion air prove switch closes,
the combustion air blower continues to operate for
30seconds (pre-purge) before allowing ignition. Pre-
purge allows the heat exchanger to be cleared of
combustion products and to introduce fresh air for
combustion. If the combustion air prove switch does
notclose,the combustionair blower continues torun
indefinitely (until the prove switch closes).
Post-Purge
After a demand, the combustion air blower continues
to operate for 5 seconds (post-purge) before stopping.
Post-purge allows the heat exchanger to be cleared of
combustion products.
Ignition Control
The ignition control is a hot surface ignition control
module integral to the burner control. See table 2 and
figure 3. When there is a call for heat, the control delays
ignition until combustion air blower operation has been
proved and pre-purge period has elapsed. It then ener-
gizes a hot surface for 35 seconds. At the end of the 35
seconds the main gas valve opens on high fire. Trial for
ignition lasts for 7 seconds. At the same time, the control
begins monitoring the flame sensor. If the flame current
is too weak (less than 1 microamp) or if the burners do
not ignite (within the 7 second ignition trial), the control
will shut off the hot surface ignitor and the combustion
air blower and de-energize the gas valve. Flame current
should be between 1 and 5 microamps to keep the gas
valve open.
The control will attempt to ignite the burners up to two
more times. Each time the control restarts the ignition
sequence, it begins with a 30 second pre-purge. If flame
is not sensed after the third trial, the control locks out.
Lockout means that the control shuts off the gas valve,
spark and combustion air blower for 60 minutes. At the
end of 60 minutes the control completely resets and
will attempt ignition up to three times. The control
can be manually reset before the end of 60 minutes
by momentarily turning off power to the unit.
When flame is sensed, the indoor blower starts after a
45 second delay. Gas valve remains open and blower
continues to run until demand stops, flame sensor
senses loss of flame, a limit opens or the combustion
air prove switch opens. If any of these events occur
during a thermostat demand, the gas valve closes
and the diagnostic LED registers the error condition.

Page 5
G25MV BURNER CONTROL (A3)
FIGURE 3
LINE VOLTAGE
TERMINAL
CONNECTIONS
24VAC VOLT
TERMINAL
CONNECTIONS
THERMOSTAT
CONNECTIONS (TB1)
DIAGNOSTIC
LED
See Table 3 for
Terminal
Functions
PLUG P20
12
1
See Table 2 for
Terminal
Functions
3
Diagnostic LED
The burner control is equipped with a diagnostic LED
used for troubleshooting the unit and the control. LED
functions are shown in table 1.
TABLE 1
Burner Control A3 Diagnostic LED
LED State Meaning Remedy
Steady On Control Failure Replace Control
Slow Flash Normal Operation
and No Call For Heat - - - -
Fast Flash Normal Operation
with Call For Heat - - - -
Two
Flashes Control Lockout
Failed to Sense or Sustain
Flame. Check Gas Valve,
Burners, Hot Surface Ignitor
and Wire, Flame Sensor.
Replace Control If All OK.
Three
Flashes Pressure Switch Open
Failed to Prove Combustion
Blower Operation or
Blocked Vent. Repair or
Replace as Necessary.
Four
Flashes Open Limit
Check Primary Limit, Rollout
Switches and Secondary
Limits. Find source of Over
temperature. If all OK, Reset
or Replace Limits as
Necessary.
Five
Flashes Flame Sensed and Gas Valve Not Energized. Check Gas Valve. If OK, Check Flame Sensor.
Six
Flashes Failed Hot Surface Ignitor
Check Hot Surface Ignitor
Voltage. If OK,
Replace Ignitor
DANGER
Shock hazard.
Disconnect power before servicing. Control is not
field repairable. If control is inoperable, simply
replace entire control.
Can cause injury or death. Unsafe operation will
result if repair is attempted.
TABLE 2
BURNER CONTROL JACK/PLUG 20 TERMINAL DESIGNATIONS
Pin # Function
1High Fire Pressure Switch Output
2Low Valve Output
3Low Fire Pressure Switch Output
4Blank
5Limits Input
6High Limit Input
7Flame Sensor
8Limits Output
9Valve Return
10 Blank
11 Ignitor Return
12 Ignitor Output

Page 6
TABLE 3
BURNER CONTROL A3 TERMINAL DESIGNATIONS
Terminal Type Function
24VAC OUT 1/4” Spade 24VAC Out To VSP2
24VAC HOT 1/4” Spade 24VAC In From Transformer
24VAC RTN 1/4” Spade 24VAC In From VSP2
GROUND 1/4” Spade To Cabinet Ground
GScrew Strip Blower Demand
DS Screw Strip Input Harmony Zone Control
Y1 Screw Strip First Stage Cooling Demand
Y2 Screw Strip Second Stage Cooling Demand
CScrew Strip 24VAC Common
W2 Screw Strip Second Stage Heating Demand
W1 Screw Strip First Stage Heating Demand
RScrew Strip 24VAC to Thermostat
G1/4” Spade Blower Demand to VSP2
DS 1/4” Spade Harmony Zone To VSP2
IDR HI 1/4” Spade 120VAC Out To High Speed
Combustion Air Blower (B6)
IDR LO 1/4” Spade 120VAC Out To Lo Speed
Combustion Air Blower (B6)
120VAC RTN 1/4” Spade 120VAC Return From
Combustion Air Blower (B6)
120VAC RTN 1/4” Spade 120VAC Return From (T1)
120VAC HOT 1/4” Spade 120VAC In From Door
Interlock Switch (S51)
120VAC HOT 1/4” Spade 120VAC Output To Control
Transformer (T1)
120VAC HOT 1/4” Spade 120VAC To Blower Motor (B3)
ACCA 1/4” Spade Accessory Cooling Terminal
ACCH 1/4” Spade Accessory Heating Terminal
TERM #12 1/4” Spade 24VAC Accessory Input
S102 IN 1/4” Spade 24VAC In From High Fire
Pressure Switch
HI VLV 1/4” Spade 24VAC Out To Gas Valve (GV1)
S101 IN 1/4” Spade 24VAC In From Flue Thermostat
S101 OUT 1/4” Spade 24VAC Out To Flue Thermostat
VSP #3 1/4” Spade Second Stage Heat Output–VSP2
VSP #5 1/4” Spade Limit Output To VSP2
4–VSP2 Blower Control Board (A24)
G25MV units are equipped with a variable speed motor
that is capable of maintaining a specified CFM through-
out the external static range. The unit uses the VSP2–1
variable speed control board, located on the blower
access door, which controls the blower speed and
provides diagnostic LEDs. The control has both a
non–adjustable, factory preset “ON” fan timing delay and
an adjustable “OFF” fan timing delay (see figure 6).
The VSP2–1 also senses limit trip condition and turns on
the blower. The G25MV limit switch is located in the
middle of the vestibule wall. When excess heat is sensed
in the heat exchanger, the limit switch will open and
interrupt the current to the gas valve, while at the same
time the VSP2–1 energizes the blower on heating speed.
The limit automatically resets when the unit temperature
returns to normal and the blower is de–energized.
Diagnostic LEDs located on the VSP2–1 control board are
provided to aid in identifying the unit’s mode of operation.
Certain scenarios will arise depending on the jumper
positions. Refer to figure 4 for identification.
IMPORTANT
24 VAC half wave rectified (DC pulse), when
measured with a meter, may appear as a lower
or higher voltage depending on the make of
the meter. Rather than attempting to measure
the output voltage of A24, see G25MV BLOWER
& VSP2 BLOWER CONTROL BOARD TROUBLE-
SHOOTING FLOW CHART in the TROUBLE-
SHOOTING section of this manual.
JP2
HIGH LOW ADJUST HEAT
CFM
HI/LOW
ON/OFF
HEAT
HTG.
BLOWER
12
DS2
DS3
DS1
DS4
1
2
3
4
1
2
3
4
1
2
3
4
TEST
–
+
NORM
210
150
90
270
JP1
VSP2–1 VARIABLE SPEED CONTROL BOARD SELECTIONS
FIGURE 4
DIAGNOSTIC
DS LEDS
FAN “OFF”
TIMING PINS
JP46
13 PIN PLUG
(BOARD TO MOTOR)
HEATING STAGE
JUMPER SELECTOR PINS
JP73
15 PIN PLUG
(BOARD TO VARIOUS
POINTS IN FURNACE)
HIGH SPEED
SELECTOR PINS
(COOLING ONLY)
LOW SPEED
SELECTOR PINS
(COOLING AND HEATING)
HEATING SPEED
SELECTOR PINS
OPERATIONAL
SELECTOR PINS
(Affects both heating
and cooling modes)
1
1

Page 7
FIGURE 5
VSP2 BLOWER CONTROL BOARD (A24)
VOLTAGES INTO VSP2
VOLTAGES FROM VSP2 TO ELECTRONICALLY
CONTROLLED BLOWER MOTOR
34 volts
–34 volts
0 volts
Voltage across J73 pins 13 to 1 and 6 to 1 is 24VAC as shown here.
Refer to unit wiring diagram.
Voltage across J46 pins 6 to 3 and 1 to 3 is half-rectified AC as shown here.
Refer to unit wiring diagram.
Voltage across J73 pins 4 to 1 is approximately 15-20VDC (straight voltage) if
CCB is used. If Harmony is used a voltage of 0–25VDC should be present.
If CCB or Harmony is not used, pin 4 to 1 voltage is 21VAC.
Approx.
34 volts
0 volts
Voltage across J46 pins 8 and 9 to 3, is approximately 15-20VDC if CCB is used. If CCB
or Harmony is not used, pins 8 and 9 to 3 voltage is approximately 21VAC. If Harmony
is used a voltage of 0–25VDC should be present.
24VAC @ 60Hz.
24VAC Half-Rectified (DC Pulse)
@ 60Hz.
J46
HIGH LOW ADJUST HEAT
CFM
HI/LOW
ON/OFF
HEAT
HTG.
BLOWER
12
DS2
DS3
DS1
DS4
1
2
3
4
1
2
3
4
1
2
3
4
TEST
–
+
NORM
210
150
90
270
J73
1
1
J73
PIN 1 - C - 24 VAC common.
PIN 2 - G - Input signal from thermostat’s fan signal.
PIN 3 - W2 - Input signal for second stage heat from the thermostat.
PIN 4 - DS - Input signal for the blower speed regulation.
PIN 5 - Limit - Input signal from the external limit.
PIN 6 - R - 24 VAC power to the thermostat.
PIN 7 - C - 24 VAC common.
Pin 8 - C - 24 VAC common.
PIN 9 - CI - Input signal from the fan limit control.
PIN 10 - CO - Output signal to the burner control.
PIN 11 - HT - Input signal from the fan limit control.
PIN 12 - ACC - 24 VAC accessory output.
PIN 13 - 24V - Input 24 VAC power for the VSP2-1.
PIN 14 - 24V - Input 24 VAC power for the VSP2-1.
PIN 15 - V - Input signal from the gas line.
J46
PIN 1 - Heat - Heat speed input signal to the ICM2 motor.
PIN 2 - C - 24 VAC common.
PIN 3 - C - 24 VAC common.
PIN 4 - High Tap - High Speed programming input.
PIN 5 - Low Tap - Low speed programming input.
PIN 6 - On / Off - On / off output signal to the ICM2 motor.
PIN 7 - Adjust Tap - ICM2 mode selection.
PIN 8 - Hi / Low - Speed regulate input signal to the ICM2 motor.
PIN 9 - Hi / Low - Speed regulate input signal to the ICM2 motor.
PIN 10 - Ref. V - ICM2 reference voltage.
PIN 11 - Heat Tap - Heating blower speed programming.
PIN 12 - C - 24 VAC common.
PIN 13 - cfm - Motor speed diagnostic signal.
Diagnostic LED Lights
 
ON/OFF–DS3 indicates there is a demand for the blower
motor to run. When the ON/OFF LED–DS3 is lit, a demand
is being sent to the motor. In heating mode only, there is a
45 second fan “ON” delay in energizing ON/OFF
LED–DS3. The light will not go off until adjustable fan
“OFF” delay has expired.
If ON/OFF LED–DS3 is on and both HIGH/LOW LED–DS1
& HEAT LED–DS2 are off, the motor will operate in
low speed.
 
If HEAT LED–DS2 is on, the blower is running in the heat
speed according to the “HEAT” jumper setting. The
HEAT LED–DS2 comes on instantaneous and switches
off when the call for heat is satisfied.
NOTE–When the blower is in “OFF” delay mode, the
motor runs at low speed, therefore the HEAT LED–DS2
is off. It switches off when the call for heat is satisfied.
 
HIGH/LOW LED–DS1 indicates whether the blower is
operating in high or low speed. When the light is off, the
blower is running in low speed according to the “LOW”
jumper setting. When HIGH/LOW LED–DS1 is on, the
blower is operating in high speed according to the
“HIGH” jumper setting.
 
CFM LED–DS4 indicates the CFM the unit is operating,
according to the jumper settings. The light flashes
once for approximately every 100 CFM. For example, if
the unit is operating at 1000 CFM, CFM LED–DS4 will
flash 10 times. If the CFM is 2050, CFM LED–DS4 will
flash 20 full times plus one fast or half flash.
At times the light may appear to flicker or glow. This
takes place when the control is communicating with the
motor between cycles. This is normal operation.
The appropriate speed according to application and
CFM need is selected by moving jumper pins.
NOTE–On Harmony II zoning applications in the heating
mode, the highest speed obtainable is the same as the
highest cooling speed selection. Also, the heating speed
(heat jumper position) is only used when the primary limit
has been tripped. In non–zoning applications, refer to the
section on the VSP2–1 control.
Jumper Settings
IMPORTANT
Before changing jumper setting, make sure the
motor has completely stopped. Any jumper
setting change will not take place while the
motor is running.

Page 8
To change jumper positions, gently pull the jumper off the
pins and place it on the desired set of pins. The following
section outlines the different jumper selections available
and conditions associated with each one. Refer to figure 4
for identification.
After the CFM for each application has been determined,
the jumper settings must be adjusted to reflect those
given in the tables on page 2. Using the tables, determine
which row of CFM volumes most closely matches the
desired CFM. Once a specific row has been chosen
(NORMALor–),CFMvolumes from other rowscannot
be used. Below are the descriptions of each of the
jumper selections.
Refer to table 4 for factory settings. Refer to the tables on
page 2 for the approximate air volume for each setting.
TABLE 4
MODEL
NUMBER
G25MV3-60
G25MV3-75
VSP2–1 FACTORY SETTINGS
HIGH LOW HEAT
141
243
ADJUST
NORM
NORM
G25MV5-100 241
243
NORM
NORM
G25MV5-120
 
The ADJUST pins allow the motor to run at normal speed
or approximately 10% lower than normal speed. The
tables on page 2 give two rows (NORMAL and –) with
their respective CFM volumes. The + adjustment setting
is not operable. Notice that the normal adjustment
setting for heat speed position #3 is 2030 CFM (960
L/s). After the adjustment setting has been determined,
chose the remaining speed jumper settings from
those offered in the table.
The TEST pin is available to bypass the VSP2–1 control
and run the motor at approximately 70% to test that the
motor is operational. This is beneficial primarily in
troubleshooting. G must be energized for motor to run.
  
For G25MV units, place the   jumper
across the second and third pins (position #2).
When W1 is energized, the  jumper selections are
activated. The HEAT jumper selections are activated
when W2 is energized.
NOTE–In Harmony II zoning applications, HEATING
BLOWER jumper must be in position #2.
 
The HEAT jumper is used to set the blower speed to
obtain the required CFM as outlined in HEAT SPEED
section of the tables on page 2.
The HEAT jumper selections are activated with a call for
second-stage heating (W2).
IMPORTANT
Before changing jumper setting, make sure the
motor has completely stopped. Any jumper set-
ting change will not take place while the motor
is running.
 
The HIGH jumper is used to determine the CFM during
cooling speed. These jumper selections are activated
when G and DS terminals are energized.
 
The LOW jumper is used to determine CFM during low
speed cooling. These jumper selections are activated
when G is energized. The LOW jumper may also be used
for low speed heating. See the “HEAT” section for details.
 
Fan “OFF” timings (time that the blower operates after
the heat demand has been satisfied) are determined by
the arrangement of a jumper on the VSP2–1 board. See
figure 6. To adjust fan “OFF” timings, gently disconnect
the jumper and reposition it across pins corresponding
with the new timing. Fan “OFF” time is factory set at
270 seconds. The control has a non–adjustable, factory
preset “on” fan timing (45 seconds).
      
    
FIGURE 6
FAN-OFF TIME ADJUSTMENT
270 210
150 90
To adjust fan–off timings:
Remove jumper from VSP2–1 and
select one of the other pin com-
binations to achieve the desired
time.
TIMING
JUMPER
TIMING PINS (seconds)
Leave jumper off to achieve
330 second fan–off timing.
Fan-off timing is factory
set at 270 seconds
NOTE—If fan “OFF” time is too low, residual heat in
heat exchanger may cause primary limit S10 to trip
resulting in frequent cycling of blower. If this occurs,
adjust blower to longer time setting.
Table 5 outlines the operation of the variable speed
motor in relation to specific modes of operation. Some
information has been repeated from the previous section
to provide an example. Refer to each diagnostic LED or
jumper settings section for more information.

Page 9
TABLE 5
VSP2–1 OPERATION
HEATING MODE COOLING MODE
UNITS WITH
SINGLE–STAGE HEATING
UNITS WITH
TWO–STAGE HEATING
UNITS WITH SINGLE–
SPEED COMPRESSOR
UNITS WITH TWO–SPEED
COMPRESSOR
NON–ZONED
APPLICATIONS
When a single-stage thermostat is
used and full heating input of the
unit is required, jump terminals W1
and W2 on the burner control
board.
The HEAT LED-DS2 is lit when the
thermostat calls for heat. The ON/
OFF LED-DS3 is lit after 110 seconds
(65 seconds pre-purge and 45 se-
conds fan “ON” time) from the time
a call for heat is made. This indicates
the blower is operating in heating
speed.
NON–ZONED
APPLICATIONS
Using a two-stage thermostat with
first-stage (W1) calling, the ON/OFF
LED-DS3 is lit to indicate the blower
is operating in low speed.
When the ON/OFF LED-DS3 and
HEAT LED-DS2 are lit, the blower is
operating in heating speed and se-
cond-stage (W2) heating is calling.
HEAT LED-DS2 is lit with a call for
heat from the thermostat. ON/OFF
LED-DS3 is lit after 110 seconds
from the time a call for heat is made.
NOTE—In two-stage operation and
when there is a call for first-stage
only, the unit will always start on
high heat (second-stage) and will
automatically switch to first-stage
operation after initial unit warm-up.
NON–ZONED
APPLICATIONS
The terminals “DS” and “Y” must be
jumpered together. With a call for
cooling, terminals G, Y and DS on
the unit control board are energized
from the thermostat. HI/LOW LED–
DS1 and ON/OFF LED–DS3 are lit to
indicate the blower is operating on
high speed.
NOTE—“Y” and “DS” must be
jumpered for single-stage cooling,
non-zoned applications.
NON–ZONED
APPLICATIONS
The ON/OFF LED–DS3 is lit to indicate
the blower is operating in first stage
cooling. This LED is energized on
when a 24VAC thermostat demand is
supplied to the control (terminal “G”
on the control box terminal strip).
In second stage, the ON/OFF LED–
DS3 and HI/LOW LED–DS1 are lit to
indicate the blower is operating on
high speed (24VAC is supplied to the
unit terminal strip “Y2” from “Y2”
on the thermostat).
NOTE—“Y2” must be jumpered to
“DS” in two–speed, non–zoned
applications.
HARMONY ZONED
APPLICATIONS
The blower speed is controlled by
the PWM (pulse width modulation)
signal sent from the control center
of the zoning system to the terminal
strip’s “DS” terminal. HI/LOW LED–
DS1 and ON/OFF LED–DS3 are lit to
indicate the blower is operating.
NOTE–In Harmony II zoning ap-
plications, HTG. BLOWER jumper
must be in position #2.
HARMONY ZONED
APPLICATIONS
The blower speed is controlled by
the PWM (pulse width modulation)
signal sent from the control center
of the zoning system to the terminal
strip’s “DS” terminal. HI/LOW LED–
DS1 and ON/OFF LED–DS3 are lit to
indicate the blower is operating.
NOTE–In Harmony II zoning ap-
plications, HTG. BLOWER jumper
must be in position #2.
HARMONY ZONED
APPLICATIONS
The blower speed is controlled by
the PWM (pulse width modulation)
signal sent from the control center
of the zoning system to the terminal
strip’s “DS” terminal. HI/LOW LED–
DS1 and ON/OFF LED–DS3 are lit to
indicate the blower is operating.
HARMONY ZONED
APPLICATIONS
The blower speed is controlled by
the PWM (pulse width modulation)
signal sent from the control center
of the zoning system to the terminal
strip’s “DS” terminal. HI/LOW LED–
DS1 and ON/OFF LED–DS3 are lit to
indicate the blower is operating.
NOTE: For zone applications with Harmony, remove wire from VSP #3 terminal on burner control board, insulate end and secure
to prevent from shorting.
B–Blower Compartment (Figure 8)
Located inside the blower compartment are the variable
speed blower motor (B3), coil choke (L13), and the
secondary limit switches [(1)S21 and (2)S21]. To access
the blower compartment follow the steps outlined below:
1– Remove the two screws holding the blower panel to
the unit.
2– Remove the two screws (located in the middle of the
blower panel) holding the grommet for the VSP2
JP46 wires.
3– Disconnect the jack (J46) from the VSP2 and feed it
through the blower panel.
4– Disconnect thermostat wires from the terminal strip
(TB1) on the burner control board.
5– Disconnect the 120VAC (black) wire from the burner
control board going to the coil choke and feed
through the blower panel grommet.
6– Disconnect line voltage wire (white) and feed through
the blower panel grommet.
7– Lift panel up and disconnect jack plugs JP1 and JP2
(brown wires) found in the blower compartment.
8– Remove wires from the wire clips located on the
blower panel allowing more wire for movement.
9– Place panel to the side, while servicing blower.
FIGURE 7
G25MV BLOWER DOOR COMPONENTS –
BLOWER ACCESS
W
W
BRN
BRN
BLK
G25MV
SHOWN
IN UPFLOW
POSITION
5
67
8
1
2
3
4
1
1– Blower Motor (B3)
G25MV units use a three-phase, electronically controlled
D.C. brushless motor (controller converts single phase
a.c. to three phase D.C.), with a permanent-magnet-type
rotor (figure 9). Because this motor has a permanent

Page 10
magnet rotor it does not need brushes like conventional
D.C. motors. Internal components are shown in figure 10.
The stator windings are split into three poles which are
electrically connected to the controller. This arrangement
allows motor windings to turn on and off in sequence
by the controller.
FIGURE 8
SUPPLY AIR BLOWER
AND SECONDARY LIMITS
BLOWER
MOTOR (B6)
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
COIL
CHOKE (L13)
SECONDARY
LIMITS [(1)S21]
BLOWER
WHEEL
SECONDARY
LIMITS [(2)S21]
FIGURE 9
G25MV BLOWER MOTOR B3
MOTOR
CONTROLLER
SHAFT
J49
J48
G25MV BLOWER MOTOR COMPONENTS
FIGURE 10
STATOR
(WINDINGS)
OUTPUT
SHAFT
BEARING
ROTOR
A solid-state controller is permanently attached to
the motor. The controller is primarily an A.C. to D.C.
converter. Converted D.C. power is used to drive the
motor. The controller contains a microprocessor
which monitors varying conditions inside the motor
(such as motor workload).
The controller uses sensing devices to sense what posi-
tion the rotor is in at any given time. By sensing the posi-
tion of the rotor and then switching the motor windings
on and off in sequence, the rotor shaft turns the blower.
G25MV series blower motorratings are listed in table
6. All G25MV blower motors use single phase power.
An external run capacitor is not used. The motor uses
permanently lubricated ball-type bearings.
TABLE 6
ELECTRONICALLY CONTROLLED BLOWER MOTOR
CCW ROTATION
Unit Volts Phase HP
G25MV3 120 1
1
1/2
1G25MV5 120
Internal Operation
Each time the controller switches a stator winding (figure
10) on and off, it is called a “pulse.” The length of time
each pulse stays on is called the “pulse width.” By varying
the pulse width (figure 11), the controller varies motor
speed (called “pulse-width modulation”). This allows for
precise control of motor speed and allows the motor to
compensate for varying load conditions as sensed by the
controller. In this case, the controller monitors the static
workload on the motor and varies motor rpm in order to
maintain constant airflow (cfm).
The motor is equipped with 11 incremental taps which are
driven by the integral controller. The controller is capable
of controlling three of the 11 taps.
The motor controller is driven by the VSP2. The VSP2
receives its demand (PWM signal or fixed 24 VAC or
VDC signal) from optional controls such as the Harmony
zone control system, Efficiency Plus Humidity Control
(CCB1) or a conventional thermostat.
Motor rpm is continually adjusted internally to maintain
constant static pressure against the blower wheel. The
controller monitors the static work load on the motor and
motor amp-draw to determine the amount of rpm adjust-
ment. Blower rpm may be adjusted any amount in order
to maintain a constant cfm as shown in Blower Ratings
Tables. The amount of adjustment is determined by the
incremental taps which are used and the amount of motor
loading sensed internally. The cfm remains relatively
stable over a broad range of static pressure. Since the
blower constantly adjusts rpm to maintain a specified
cfm, motor rpm is not rated. Hence, the terms “blower
speed” and “speed tap” in this manual, on the unit wiring
diagram and on blower B3 refer to blower cfm regardless
of motor rpm.
When Harmony is used, speed taps are overridden and a
PWM signal generated by the Harmony controller contin-
uously varies motor speed based upon zone demands.
Initial Power Up
When line voltage is applied to B3, there will be a large
inrush of power lasting less than 1/4 second. This inrush
charges a bank of DC filter capacitors inside the controller.
If the disconnect switch is bounced when the disconnect
is closed, the disconnect contacts may become welded.
Try not to bounce the disconnect switch when applying
power to the unit.

Page 11
The DC filter capacitors inside the controller are con-
nected electrically to the speed tap wires. The capacitors
take approximately 5 minutes to discharge when the
disconnect is opened. For this reason it is necessary to
wait at least 5 minutes after turning off power to the
unit before attempting to change speed taps.
DANGER
Disconnect power from unit and
wait at least five minutes to allow
capacitors to discharge before at-
temptingtoadjustmotorspeedtap
settings. Failure to wait may cause
personal injury or death.
Motor Start-Up
When B3 begins start-up, the motor gently vibrates back
and forth for a moment. This is normal. During this time
the electronic controller is determining the exact position
of the rotor. Once the motor begins turning, the controller
slowly eases the motor up to speed (this is called “soft-
start”). The motor may take as long as 10-15 seconds to
reach full speed. If the motor does not reach 200rpm
within 13 seconds, the motor shuts down. Then the
motor will immediately attempt a restart. The shutdown
feature provides protection in case of a frozen bearing or
blocked blower wheel. The motor may attempt to start
eight times. If the motor does not start after the eighth try,
the controller locks out. Reset controller by momentarily
turning off power to unit.
One revolution
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
FIGURE 11
HIGH SPEED COOL (output from controller to motor windings)
MOTOR SPEED CONTROL WITH D.C. PULSE-WIDTH MODULATION
WINDING #1
WINDING #2
WINDING #3
LOW SPEED HEAT/COOL (output from controller to motor windings)
HIGH SPEED HEAT (output from controller to motor windings)
O volts
325VDC One revolution
O volts
325VDC
O volts
325VDC
WINDING #1
WINDING #2
WINDING #3
O volts
325VDC
O volts
325VDC
O volts
325VDC
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
WINDING #1
WINDING #2
WINDING #3
O volts
325VDC
O volts
325VDC
O volts
325VDC
Motor speed is determined by the size of the electrical pulse sent to
the motor windings. The longer the pulse, the faster the motor.
One
Pulse
ÇÇ
ÇÇ
ON PULSE OFF PULSE
ÇÇ
ÇÇ
WINDINGS TURNED OFF
The frequency of the pulses to the windings is 20KHz.
DO NOT ATTEMPT TO MEASURE THESE VOLTAGES.
WINDINGS TURNED ON
OUTPUT FROM CONTROLLER TO MOTOR WINDINGS
One revolution

Page 12
External Operation (Speed Tap Priority)
Figure 12 shows the two quick-connect jacks (J48 and
J49) which connect the motor to the G25MV. Jack J48 is
the power plug and jack J49 connects the unit controls
to the motor.
Line voltage must be applied to J48 pin 5 in order for the
motor to operate. When using 120VAC pins 1 and 2
must be jumpered. When control voltage is applied to
J49 pin 3 and 15 (single stage heating and cooling), the
motor is energized on the low speed heat/cool tap.
When voltage is applied to J49 pin 2 in addition to pin 3
and 15 (second stage heating), the blower is energized
on the high speed heating tap. When voltage is applied
to J49 pin 10 in addition to pin 3 and 15 (second stage
cooling), the blower is energized on the high speed
heating tap. The motor assigns priority to J49 pin 2 so that
if a call for cooling and a call for heating are concurrent,
heating call overrides and the blower operates on high
speed heating tap.
FIGURE 12
POWER
CONNECTOR J48
CONTROL
CONNECTOR J49
BLOWER B3 HARNESS CONNECTORS
J49
PIN 1 - C1
PIN 2 - W / W1
PIN 3 - C2
PIN 4 - Delay
PIN 5 - Cool
PIN 6 - Y1
PIN 7 - Adjust
PIN 8 - 0ut
PIN 9 - 0
PIN 10 - BK / PWM
PIN 11 - Heat
PIN 12 - R
PIN 13 - EM / W2
PIN 14 - Y / Y2
PIN 15 - G
PIN 16 - Out +
J48
PIN 1 - Jumper PIN 1 to PIN2 for 120VAC line input only.
PIN 2 - Jumper PIN 1 to PIN2 for 120VAC line input only.
PIN 3 - Ground
PIN 4 - AC Line
PIN 5 - AC Line
1
1
DANGER
Do not attempt to repair electronically controlled
blower motor or VSP2-1. There are no field service-
able parts. If either component appears to be faulty
after following checkout procedure, replace entire
component then recheck for proper operation.
2–Choke Coil (L13)
A choke coil is used on all G25MV units. The choke is
located on the blower housing and is used to block radio
frequency interference.
Precautions
If the G25MV or its electronically controlled blower motor
is improperly or inadequately grounded, it may cause
television interference (commonly known as RFI or radio
frequency interference).
This interference is caused by internal switching fre-
quencies of the motor controller (see figure 13). TV
interference may show up as small specks or lines
which randomly appear on the TV screen accompanied
by pops or clicks in the sound. Before attempting any ser-
vice, make sure the indoor unit is causing the interference.
To check, disconnect power to indoor unit then check TV
for continued signs of interference.
TV interference may be stopped by making sure the
motor is solidly grounded to the cabinet (metal to
metal) and by making sure the cabinet is solidly
grounded. If TV interference persists, make sure the
television (and all affected RF appliances) are moved
away from the G25MV. Also make sure affected ap-
pliances are connected to a separate electrical circuit.
ÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
ÇÇÇÇÇÇÇÇÇÇÇÇÇÇ
FIGURE 13
ECM SWITCHING FREQUENCY
WINDING #1
WINDING #2
WINDING #3
OUTPUT FROM CONTROLLER TO MOTOR WINDINGS
O volts
325VDC
One cycle
O volts
325VDC
O volts
325VDC
The electrical pulses to the windings cycle on and off at a
rate of 20,000 cycles per second (20Khz, this is called the
switching frequency). The effective D.C. voltage can be var-
ied by changing the pulse width. The switching frequency is
necessary in order to vary the speed of the motor. If the mo-
tor is not securely grounded to the cabinet and if the cabinet
is not securely grounded, the switching frequency may
cause television interference.
One Pulse
O volts
325VDC
ONE
ELECTRICAL PULSE
Switching Frequency
20Khz
Ç
PULSE ON
PULSE OFF
Ç
Ç
WINDING OFF

Page 13
3–Secondary Limit Switches
[(1)S21 and (2)S21]
The secondary limit switches [(1)S21 and (2)S21] on
G25MV units are located in the blower compartment on
the back side of the blower housing (see figure 8). When
excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the burner control
energizes the supply air blower and closes the gas valve.
The limit automatically resets when unit temperature
returns to normal. Two limits are supplied in each furnace
and each limit is a different style. Figure 14 shows both
styles. Both switches are N.C. SPST auto-reset switches
which actuate on a temperature rise. The switches are
factory set and cannot be adjusted. The (1)S21 limit
shown on the top opens at 140F + 5F (60.0C + 2.8C)
on a temperature rise and closes at 110F + 10F (43.3C
+ 5.6C). The (2)S21 limit shown on the bottom opens at
165F + 5F (73.9C + 2.8C) on a temperature rise and
closes at 135F + 8F (57.2C + 4.4C).
INSULATING COVER (s)
FIGURE 14
SECONDARY LIMIT SWITCHES
FOR G25MV SERIES UNITS
SPADE CONNECTORS
INSULATING COVER
LIMIT
LIMIT
(1)S21
(2)S21
FIGURE 15
HEATING COMPONENTS (shown in horizontal position)
HEAT EXCHANGER
COMBUSTION AIR
BLOWER (B6)
MANIFOLD
ROLLOUT SWITCH (2)
(S47)
COMBUSTION AIR
PROVE SWITCHES
(S72 and S102)
GAS VALVE (GV1)
BURNERS
FLUE LIMIT
THERMOSTAT (S101)
PRIMARY
LIMIT (S10)
ORIFICES
Front
Right
Top
Bottom
Left
Back

Page 14
Top
G25MV BURNER ASSEMBLY (shown in upflow position)
FIGURE 16
HOT SURFACE
IGNITOR
FLAME
SENSOR
MANIFOLD
ORIFICE
GAS VALVE
(GV1)
HEAT EXCHANGER
BURNER
UPPER
BURNER
MOUNTING
RAIL
LOWER
BURNER
MOUNTING
RAIL
BURNER BOX
BURNER BOX
TOP
ROLLOUT
SWITCHES (S47)
Front Right
Bottom
Left Back
PRIMARY LIMIT (S10)
VEST PANEL
C–Heating Components (Figures 15 and 16)
Heating components are located inside the front louvered
panel. Inside the heating compartment are the combus-
tion air blower (B6), flue limit thermostat (S101), flame
rollout switches (S47), primary limit control (S10), ignitor,
flame sensor, gas valve (GV1), and the combustion air
blower prove switches (S72 and S102).
1–Combustion Air Blower (B6)
All G25MV units use a two stage 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 con-
trolled by burner control A3. The blower also operates for
30 seconds before burner ignition (pre-purge) and for 5
seconds after the gas valve closes (post-purge).
2–Flue Limit Thermostat (S101)
Flue limit thermostat is a high temperature limit located
on top of combustion air blower collector box (see figure
15). The limit is a N.C. SPST auto-reset limit which opens
at 200F + 5F (93.3C + 2.8C) on a temperature rise
and closes at 160F + 8F (71.1C + 4.4C). The switch
is factory set and cannot be adjusted. The limit is electri-
cally connected to the burner control A3. The purpose of
the limit is to set the unit to low fire when temperature
reaches 200F (93.3C) in the heat exchanger on call for
first stage heat.
3–Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit switch
located on top of the burner box (see figure 16). 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 switch is
a N.C. SPST manual-reset switch which actuates at 267F
+12F (130.5C + 6.7C) on a temperature rise. The switch
is factory set and cannot be adjusted. To manually reset
a tripped switch, push the reset button located on the
control. The switches are connected in series with the
burner control A3. When S47 senses rollout, the burner
control immediately stops ignition and closes the gas
valve. If unit is running and flame rollout is detected, the
gas valve will close and ignition control will be disabled.
Rollout can be caused by a clogged heat exchanger or a
blocked flue.

Page 15
FIGURE 17
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
4–Primary Limit Control (S10)
The primary limit (S10) on G25MV units is located in the
middle of the heating vestibule panel (see figure 16). S10
is the same type of limit as shown on the top of figure 14.
When excess heat is sensed in the heat exchanger, the
limit will open. If the limit is tripped, the burner control
energizes the supply air blower and closes the gas
valve. The limit automatically resets when unit temper-
ature returns to normal. The switch is factory set and
cannot be adjusted. The switch has different setpoints
for each unit model number. See table 7. The setpoint
will also be printed on the side of the limit.
TABLE 7
PRIMARY LIMIT CONTROL (S10)
UNIT ACTUATE TEMPERATURE
G25MV3–60
OPEN 200F + 5F (93.3C + 2.8C)
G25MV3
–
60
CLOSE 170F + 8F (76.7C + 4.4C)
G25MV3–75
OPEN 175F + 5F (79.4C + 2.8C)
G25MV3
–
75
CLOSE 145F + 8F (62.8C + 4.4C)
G25MV5–100
OPEN 190F + 10F (87.8C + 5.6C)
G25MV5
–
100
CLOSE 160F + 8F (71.1C + 4.4C)
G25MV5–120
OPEN 150F + 5F (65.6C + 2.8C)
G25MV5
–
120
CLOSE 120F + 8F (48.9C + 4.4C)
5–Gas Valve (GV1)
The G25MV uses a gas valve manufactured by White
Rodgers. The valve is a two-stage internally redundant
to assure safety shut–off. If the gas valve must be re-
placed, 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 burner control. 24V applied to
the terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the side of the
valve. Refer to figure 18 for location of valve features.
NOTE–Do not attempt to regulate low fire manifold
pressure.
FIGURE 18
WHITE RODGERS 36E SERIES GAS VALVE
GAS VALVE SHOWN IN OFF POSITION
HIGH FIRE
MANIFOLD
PRESSURE
ADJUSTMENT
ON SIDE
MANIFOLD
PRESSURE
OUTLET ON SIDE
6–Hot Surface Ignitor
Figure 20 shows the arrangement of the hot surface
ignitor in perspective with the rest of the burner assembly.
The burner control uses hot surface to ignite the two right-
most burners and the burners cross-light to the left. A
flame retention ring in the end of each burner is used to
maintain correct flame length and shape and to keep the
flame from lifting off the burner head. The hot surface
ignitor is rated at 1800F (982C) in 17 seconds with a
120VAC 4 amp power source.
7–Flame Sensor
Figure 20 shows the arrangement of flame sensor in
perspective with the rest of the burner assembly. The
flame sensor uses flame rectification to sense com-
bustion. Figure 19 shows the gap between tip of the
electrode and the burner surface.
FIGURE 19
FLAME SENSOR TO BURNER GAP
BURNER
FLAME SENSOR
3/4”

Page 16
FIGURE 20
TYPICAL BURNER/ELECTRODE ORIENTATION
view looking at flame end of burners
BURNER
FLAME RETENTION RING
UPPER BURNER
MOUNTING RAIL
LOWER BURNER
MOUNTING RAIL
MANIFOLD ORIFICE
Right Left
Top
Bottom
FLAME SENSOR
HOT SURFACE IGNITOR
FIGURE 21
PROVE SWITCH
COMBUSTION AIR BLOWER
PROVE SWITCH
Normally Open
Closes on Negative Pressure
PROVE
SWITCHES
Sensing Tube
“T’s” the two switches
together and attaches
to Side Of Blower
Top
Bottom
Left
Right
Back
Front
8–Combustion Air Blower Prove
(Pressure) Switches (S72 and S102)
G25MV series units are equipped with two combustion air
prove switches located on the vestibule panel. The
switches “T” together and are connected to the combus-
tion air blower housing by means of a flexible silicone
hose. The switches monitor air pressure in the combus-
tion air blower housing.
Both switches are N.O. SPST auto-reset pressure
switches electrically connected to the burner control
A3. The switches are marked low fire (S72) and high
fire(S102). The purpose of the switches are to prevent
burner operation if the combustion air blower is not
operating properly.
On start-up, the S72 and S102 switches sense the
combustion air blower is operating. S102 will drop
out of the circuit when high heat is satisfied (S101
opens). The switches close a circuit to the burner control
when pressure inside the combustion air blower de-
creases below a given set point (see table 8). The pressure
sensed by the switches are relative to atmospheric pres-
sure. If the flue becomes obstructed during operation, and
either switch senses a loss of negative pressure (pressure
becomes more equal with atmospheric pressure) the
switch opens the circuit to the burner control and gas
valve. Either switch, if opened, will open the circuit. The
switch trip pressures are different depending on unit size.
The trip pressures are printed on the side of the switches.
All the switches are factory set and are not field adjust-
able. The switches are a safety shut-down control in the
furnace and must not be bypassed for any reason.
TABLE 8
MODEL
NUMBER
G25MV3-60
Combustion Air Blower Prove Switches
0.46”W.C. + 0.05”W.C.
(114.37Pa + 12.43Pa)
NOTE: All measurements are negative.
S102
HIGH
S72
LOW
0.30”W.C. + 0.05”W.C.
(74.59Pa + 12.43Pa)
G25MV3-75 0.48”W.C. + 0.05”W.C.
(119.35Pa + 12.43Pa) 0.33”W.C. + 0.05”W.C. (82.05Pa + 12.43Pa)
G25MV5-100 0.40”W.C. + 0.05”W.C.
(99.46Pa + 12.43Pa) 0.20”W.C. + 0.03”W.C. (49.73Pa + 7.46Pa)
G25MV5-120 0.41”W.C. + 0.05”W.C.
(101.94Pa + 12.43Pa) 0.20”W.C. + 0.03”W.C. (49.73Pa + 7.46Pa)
II–PLACEMENT AND INSTALLATION
Make sure unit is installed in accordance with the
G25MV installation instructions.
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
1– Set thermostat to OFF position. Close manual knob
on gas valve.
2– Wait 5 minutes.
3– Open manual knob on gas valve, replace the front
access panel and turn on unit electrical supply.

Page 17
WARNING
Shock and burn hazard.
G25MV units are equipped with a hot surface
ignition system. Do not attempt to light manually.
4– Set fan switch to AUTO or ON and move system
selection switch to HEAT. Adjust thermostat to a
setting above room temperature.
5– If unit does not light the first time, it will attempt two
more ignitions before locking out.
6– If lockout occurs, repeat steps 1, 2, 3 and 4.
7– On start-up check operation of both high and low
fire. Refer to section IV item E for manifold pressure
check procedure.
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 guarantee 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 G25MV units are A.G.A. / C.G.A. design certified
without modifications. Refer to the G25MV Operation
and Installation Instruction Manual for 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 LP 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
22
. If the pressure is equal to or less than 0.5psig
(14”W.C.), use the manual shut–off valve before
pressure testing to isolate furnace from gas supply.
FIGURE 22
MANUAL MAIN SHUT–OFF VALVE
GAS 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
When checking piping connections for gas leaks, use
preferred means. Kitchen detergents can cause harmful
corrosion on various metals used in gas piping. Use of a
specialty Gas Leak Detector is strongly recommended. It
is available through Lennox under part number
31B2001. See Corp. 8411–L10, for further details.
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 22. 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 natural gas units, operating pressure at unit
gas connection must be a minimum of 4.5” W.C.
On multiple unit installations, each unit should be
checked separately, with and without other units oper-
ating. Supply pressure must be a minimum of 4.5”
W.C. for each unit.
E–Check Manifold Pressure
After line pressure has been checked and adjusted,
check manifold pressure. Move pressure gauge to out-
let pressure tap located on unit gas valve (GV1). Checks
of manifold pressure are made as verification of proper
regulator adjustment. Manifold pressure for the G25MV
can be measured 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” W.C. (870.2 Pa) for high
fire operation and 1.7” W.C. (422.7 Pa) for low fire.
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.
Thegasvalve is factorysetand should notrequire ad-
justment. All gas valves are factory regulated.

Page 18
Manifold Pressure Check 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.
3– After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to the normal
manifold pressure of 3.5” W.C. (870.2 Pa) for high
fire operation or 1.7” W.C. (422.7 Pa) for low fire.
NOTE–Shut unit off, turn gas valve to “OFF” position
and remove manometer as soon as an accurate
reading has been obtained. Take care to replace
pressure tap plug.
F– Proper Gas Flow
To check for proper gas flow to combustion chamber,
determine Btuh input from unit rating plate or the
specifications tables on page 2. Divide input rating by
Btuh per cubic foot of available gas. Result is the number
of cubic feet per hour required. Determine flow of gas
through gas meter for two minutes. Multiply by 30 to get
hourly flow of gas to burner.
NOTE– To obtain accurate reading, shut off all other
gas appliances connected to meter.
G–High Altitude Derate
A.G.A. certified units must be derated 4 percent for each
1,000 feet (305m) above sea level. Thus, at an altitude of
4000 feet (1210 m), the unit would require a derate of 16
percent.
C.G.A. certified units must be derated 10 percent for
elevations 2000 feet (610 m) to 4500 feet (1370 m)
above sea level.
NOTE–In Canada, use the following high altitude
conversion kits: LB–79766A for G25MV3–60/75; and
LB–79766B for G25MV5–100/120.
NOTE–This is the only permissible field derate for
this appliance.
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.
H–Flame Signal
A 20 microamp DC meter is needed to check the flame
signal on the primary ignition control. Use a flame signal
transducer (part number 78H5401) available from Lennox
to measure the flame signal.
Flame (microamp) signal is an electrical current which
passes from the ignition control through the sensor
electrode during unit operation. Current passes from
the sensor through the flame to ground to complete a
safety circuit.
To Measure Flame Signal:
1– Remove front louvered panel and disconnect the
sensor wire from the flame sensor.
2– Place meter in series between flame sensor and
sensor wire. Connect the positive (+) lead of meter
to the sensor wire and the negative (–) lead of the
meter to the flame sensor.
3– Set thermostat for a heating demand and check
flame signal with unit operating. For G25MV series,
a reading of 4 to 6 microamps DC should occur. The
ignition control must see at least 1.5 microamps in
order to keep the gas valve energized.
Flame signal may rise above 5 microamps for the first few
seconds after ignition then level off within the range.
I–Electronically Controlled Blower Motor
To check–out the VSM blower and the VSP2–1 blower
control board, begin with the VSM blower. Refer to figure
23 and follow the check–out procedure as outlined. If the
blower fails any of these tests, do not attempt to repair
the VSM blower. There are no field serviceable parts in
this component. Replace the motor and repeat the check–
out procedure. To verify proper operation of the VSP2–1,
check the board as outlined in the troubleshooting flow-
charts on the last page.
VSM CHECK–OUT
1– Disconnect power to unit.
2– Disconnect 13 pin plug J46 from P46 located on
VSP2–1.
3– Connect voltage source as shown above.
4– Turn on power to unit. Blower should
operate at low speed.
VSM LOW SPEED CHECK–OUT
NOTE–Any A.C. voltage source less than 30 volts or any D.C. voltage
source less than 20 volts may be used to check out the motor. An ordi-
nary 9 volt battery is recommended. Unit transformer T1 secondary
may be used in lieu of a battery. A 9 volt battery will last for about one
day of normal operation. If transformer T1 is used, double check all
wiring connections before placing unit back in operation.
CHECK–OUT PROCEDURE USING J46
5– Disconnect power to unit.
6– Connect voltage source as shown above.
7– Turn on power to unit. Blower should operate at
high speed.
VSM HIGH SPEED
CHECK–OUT
8– Disconnect power to unit.
9– Connect voltage source as shown above.
10–Turn on power to unit. Blower should operate at
heating speed.
VSM HEATING SPEED CHECK–OUT
FIGURE 23
A kit is available from the Lennox parts center to use in testing
the variable speed motor. The kit 70J11 includes a test plug har-
ness to facilitate vsm check–out. Follow testing procedures
outlined in the instructions provided with the kit. The testing
procedures are different than those listed below.
If not using the kit to test the motor, follow the procedure below.
         
         
         
J46
J46
J46

FIGURE 24
STATIC PRESSURE
TEST
MANOMETER
G25MV UNIT
Page 19
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
subbase 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– In all cases, blower and entire unit will be off when
the system switch is in OFF position.
B–Temperature Rise
Temperature rise for G25MV 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 radiant heat from the heat exchanger.
2– Set thermostat to highest setting.
3– After plenum thermometers have reached their
highest 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, increase blower speed to reduce
temperature. To change blower speed taps see the
Blower Speed Taps section in this manual.
C–External Static Pressure
1– Measure tap locations as shown in figure 24.
2– Punch a 1/4” diameter hole
in supply and return air
plenums. Insert manome-
ter hose flush with inside
edge of hole or insulation.
Seal around the hose with
permagum. Connect the
zero end of the manometer
to the discharge (supply) side of the system. On
ducted systems, connect the other end of manometer
to the return duct as above. For systems with
non–ducted returns, 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 blower motor speed to deliver
theair desiredaccording tothe jobrequirements.
4– External static pressure drop must not be more
than 0.8” W.C. (198.9 Pa).
5– Seal around the hole when the check is complete.
VI–MAINTENANCE
At the beginning of each heating season, the system
should be checked as follows:
A–Filters
Return air filter is supplied with unit. 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–Cleaning Heat Exchanger and Burners
NOTE–Use papers or protective covering in front of
furnace while cleaning furnace.
To clean heat exchanger and burners:
1– Turn off both electrical and gas power supplies to
furnace. Refer to figure 1 during disassembly and
reassembly procedures.
2– Disconnect wires leading to burner and heat ex-
changer assembly.
3– Remove front louvered panel and and blower access
panels.
4– Remove flue pipe and draft hood.
5– Disconnect gas piping and move piping out of way
so heat exchanger can slide out front of cabinet.
6– Gas manifold, gas valve, burners and combustion
air blower do not need to be removed and can be
left in place.
7– Locate four screws which hold heat exchanger in
place (two on each side of cabinet). See figure 25.
Remove screws.
8– Slide heat exchanger out of cabinet.
To clean burners:
9– Remove screws holding upper burner mounting rail
(figure 16). Remove rail.
10– Slide burners off each orifice and pull burners from
heat exchanger.
11– Clean holes in burner head (retention ring) with a
wire brush. See figure 20.
12– With a shop vacuum or rags, clean out soot and
scale deposits from burners.
13– Remove screws securing flue box to vestibule
panel. Remove flue box from unit. Leave combustion
air blower attached to flue box.
14– With a shop vacuum or rags, clean out soot and scale
deposits from heat exchanger tubes and flue box.
15– Inspect heat exchanger for corrosion damage, holes
or cracks.

Page 20
16– Replace burners making sure to fully engage on
orifice. Resecure burner mounting rail and flue
box. Inspect flue box gasket. Replace gasket if
necessary.
17– Slide heat exchanger into cabinet and re-secure
heat exchanger screws.
18– Re-secure flue pipe, gas piping and access panels.
19– Carefully check all piping connections (factory and
field) for gas leaks. Use a leak detecting solution or
other preferred means.
20– Turn on gas and electrical supply.
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
Check and clean blower wheel. Motors are prelubricated
for extended life; no further lubrication is required.
B – 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.
C – Electrical
1 – Check all wiring for loose connections.
2 – Check for correct voltage.
3 – Check amp–draw on blower motor.
FIGURE 25
HEAT EXCHANGER REMOVAL (unit shown in horizontal position)
REMOVE SCREWS (4)
SLIDE ENTIRE ASSEMBLY OUT OF CABINET