Lennox G21Q3-40 Installation and operation manual

ELECTRONICALLY CONTROLLED
MOTOR
ICM1
ICM2
Page 1 1998 Lennox Industries Inc.
Litho U.S.A.
Corp. 9815−L9
G21/GSR21
Service Literature Revised 10−2001
G21/GSR21 SERIES UNITS
G21 and GSR21 units are condensing furnaces utilizing
the pulse combustion process. Initially, combustion
takes place in an enclosed chamber. Then, as combus
tion products pass through the heat exchanger system
into a coil, the latent heat of combustion is extracted and
water is condensed from the exhaust gas.
The unit uses a redundant gas valve to assure safety
shutoff as required by A.G.A. The units are manufac
tured for natural gas application. L.P. kits for A.G.A. /
C.G.A. units are available for field changeover.
An electronic direct spark ignition control initiates com
bustion. The ignition control serves four functions: pre
purge, ignition, flame sensing and postpurge. Controls
reset in the event of nuisance lockout. The control also
verifies loss of combustion during a cycle, closing the
gas valve. A differential pressure switch shuts down the
unit immediately if there are obstructions in the exhaust
outlet or air intake.
A purge blower clears the combustion chamber before and af
ter each heating cycle to ensure proper air mixture for startup.
All units feature direct drive multispeed blower motors.
V" series units feature an electronically controlled blower
motor (ICM1 or ICM2) controlled by a VSP control (VSP1 or
VSP2−1). The VSP controls blower CFM using either a
PWM (pulse width modulation) signal or fixed 24 VAC or
VDC signal.
These signals are gener
ated by optional controls
such as the Harmony zone
control system, Efficiency
Plus Humidity Control
(CCB1) or a thermostat. A
PWM signal is generated by
the Harmony zone control
system. A fixed DC signal is
generated by the Efficiency
Plus Humidity Control
(CCB1) and a 24 VAC signal
is generated by a thermo
stat.
The Harmony zone control system produces a PWM signal
to regulate blower CFM. The CCB1 varies indoor blower
speed in response to indoor humidity demand. The CCB1
produces a DC signal.
When a twospeed condensing unit is used with the Harmony
zone control system or CCB1, compressor speed as well as
indoor blower CFM are controlled more accurately than with
a conventional two stage thermostat.
All specifications in this manual are subject to change.
PULSE COMBUSTION PROCESS
EXHAUST
BACK PRESSURE IGNITION
COMBUSTION
AIR
GAS
OPEN CLOSED
COMBUSTION
CHAMBER
TAILPIPE
SPARK
1 − Gas and air enter and mix in combustion
chamber
2 − To start the cycle a spark is used to ignite the
gas and air mixture (this is one "pulse").
3 − Positive pressure from combustion closes
flapper valves and forces exhaust gases down
the tailpipe.
4 − Exhaust gases leaving chamber create a
negative pressure. This opens the flapper valve
drawing in gas and air.
5 − At the same instant, part of the pulse is
reflected back from the tailpipe causing the
new gas and air mixture to ignite. No spark is
needed. (this is anotherpulse").
6 − Steps 4 and 5 repeat 60 to 70 times per second
forming consecutive pulses" of 1/4 to 1/2
Btuh each.
7 − Latent heat is removed from combustion
products and condensate (water) is formed in
the condensate coil.
7
4
5
3
1
432
SENSOR

Page 2
TABLE OF CONTENTS
General Page 1
Parts Arrangement Page 3
Specifications Page 4−6
Blower Data Page 6−10
Component Illustration Page 11
I Application Page 12
II Unit Components Page 12−36
VSP1 Blower Control Page 15−16
VSP2 Blower Control Page 17−19
Ignition Control Page 21−24
Blower Motor Q Units Page 29
ICM1 and ICM2 Page 30−36
III Installation Page 36−44
IV Maintenance Page 45−46
V Unit Checkout Page 46−53
VI Troubleshooting Page 54−56
VII Troubleshooting Charts Page 57−68
GC1 Ignition Control Page 57
GC3 Ignition Control Page 58
G891 Ignition Control Page 58
Electrical Checkout Page 59
Gas, Air, Spark Check Page 60
Electrical Testing Page 61
ICM2−VSP2 Page 62
ICM1−VSP1 Page 63−65
Additional Charts Page 66−68
VIII Wiring and Sequence of Operation
G21Q Page 69−70
GSR21Q Page 71−72
G21V Page 73−74
GSR21V Page 75−76
G21Q W/ GC3 Page 77−78
G21Q W/ GC1 Page 79−80
GSR21Q W/ GC3 Page 81−82
GSR21Q W/ GC1 Page 83−84
G21V W/ GC1 Page 85−86
G21V W/ GC3 Page 87−88
GSR21V W/ GC1 Page 89−90
GSR21V W/ GC3 Page 91−92
Jumper Summary Tables Page 93−99
CAUTION
Electrostatic discharge can affect electronic com
ponents. Take precautions during furnace installa
tion and service to protect the furnace’s electronic
controls. Precautions will help to avoid control ex
posure to electrostatic discharge by putting the
furnace, the control and the technician at the same
electrostatic potential. Neutralize electrostatic
charge by touching hand and all tools on an un
painted unit surface, such as the gas valve or blow
er deck, before performing any service procedure.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures

Page 3
PARTS ARRANGEMENT (G21 UPFLOW UNIT)
AIR INTAKE
CONNECTION
LIMIT
CONTROL
LOW VOLTAGE
TERMINAL
STRIP
GAS VALVE
EXPANSION
TANK
CONTROL
BOX
DOOR
INTERLOCK SWITCH
DIFFERENTIAL
PRESSURE
SWITCH
G21Q SERIES
BLOWER MOTOR
AIR VALVE AND
AIR HOUSING
BLOWER
ICM2 BLOWER
MOTOR
HEAT EXCHANGER
ASSEMBLY
IGNITION
CONTROL
GAS FLAPPER VALVE
& ORIFICE ASSEMBLY
ICM1 BLOWER
MOTOR
IGNITION CONTROL
PARTS ARRANGEMENT (GSR21 DOWNFLOW HORIZONTAL UNIT)
AVAILABLE ELECTRONICALLY
CONTROLLED BLOWER MOTOR
V" MODELS
AIR INTAKE
PVC CONNECTION
EXHAUST OUTLET
CONTROL BOX
HEAT EXCHANGER
ASSEMBLY
DOOR INTERLOCK
SAFETY SWITCH
EXPANSION TANK
GAS VALVE
AIR FLAPPER AND
HOUSING
BLOWER
COMPARTMENT
LOW VOLTAGE
TERMINAL STRIP
PURGE BLOWER

Page 4
SPECIFICATIONS (units equipped with conventional multispeed blower motor)
Model No. G21Q340 G21Q360 G21Q460
Input Btuh 40,000 60,000 60,000
Output Btuh 38,000 55,000 55,000
*A.F.U.E. 96.2% 94.1% 94.1%
California Seasonal Efficiency 90.7% 89.9% 88.8%
Temperature rise range (°F) 35  65 40  70 35  65
High static certified by A.G.A. (in wg.) .50 .50 .50
Gas Piping Size Natural 1/2 1/2 1/2
I.P.S. (in.) **LPG 1/2 1/2 1/2
Vent/Intake air pipe size connection (in.) 2 2 2
Condensate drain connection (in.) SDR11 1/2 1/2 1/2
Blower wheel nominal diameter x width (in.) 10 x 8 10 x 8 11 x 9
Blower motor hp 1/3 1/3 1/2
Number and size of filters (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 16 x 25 x 1
Tons of cooling that can be added 11/2  3 11/2  3 21/2  4
Electrical characteristics 120 volts  60 hertz  1 phase (less than 12 amps) All models
**LPG kit (optional) LB65810A LB65810B LB65810B
External Filter Mounting Part No. LB81871CA LB81871CA LB81871CA
Kit (optional) •Filter size (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 16 x 25 x 1
•Filter is not furnished with kit. Filter kit utilizes existing filter supplied with G21 unit.
*Annual Fuel Utilization Efficiency based on D.O.E. test procedures and according to F.T.C. labeling regulations.
Isolated combustion system rating for nonweatherized furnaces.
**LPG kit must be ordered extra for field changeover.
SPECIFICATIONS (units equipped with conventional multispeed blower motor)
Model No. G21Q380 G21Q480 G21Q580 G21Q3100 G21Q4/5100
Input Btuh 80,000 80,000 80,000 100,000 100,000
Output Btuh 73,000 73,000 74,000 93,000 95,000
*A.F.U.E. 93.9% 93.9% 93.2% 94.9% 94.5%
California Seasonal Efficiency 90.1% 88.9% 88.3% 90.8% 89.6%
Temperature rise range (°F) 45  75 40  70 35  65 55  85 40  70
High static certified by A.G.A. (in wg.) .50 .50 .50 .50 .50
Gas Piping Size Natural 1/2 1/2 1/2 1/2 1/2
I.P.S. (in.) **LPG 1/2 1/2 1/2 1/2 1/2
Vent/Intake air pipe size connection (in.) 2 2 2 2 2
Condensate drain connection (in.) SDR11 1/2 1/2 1/2 1/2 1/2
Blower wheel nominal diameter x width (in.) 10 x 8 11 x 9 12 x 12 10 x 8 12 x 12
Blower motor hp 1/3 1/2 3/4 1/2 3/4
Number and size of filters (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1
Tons of cooling that can be added 2  3 21/2  4 4 or 5 2  3 31/2  5
Electrical characteristics 120 volts  60 hertz  1 phase (less than 12 amps) All models
**LPG kit (optional) LB65810B LB65810B LB65810B LB65810C LB65810C
External Filter Mounting Part No. LB81871CA LB81871CA LB81871CB LB81871CB LB81871CB
Kit (optional) •Filter size (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1
•Filter is not furnished with kit. Filter kit utilizes existing filter supplied with G21 unit.
*Annual Fuel Utilization Efficiency based on D.O.E. test procedures and according to F.T.C. labeling regulations.
Isolated combustion system rating for nonweatherized furnaces.
**LPG kit must be ordered extra for field changeover.

Page 5
SPECIFICATIONS (contd.)
SPECIFICATIONS (units equipped with conventional multispeed blower motor)
Model No. GSR21Q350 GSR21Q45
0
GSR21Q38
0GSR21Q4/580 GSR21Q4/5100
Input Btuh 50,000 50,000 80,000 80,000 100,000
Output Btuh 47,000 47,000 71,000 72,000 92,000
*A.F.U.E. 94.8% 95.3% 91.7% 94.1% 92.0%
California Seasonal Efficiency 89.7% 90.1% 87.6% 88.6% 87.3%
Temperature rise range (°F) 30  60 25  55 40  70 30  60 45  75
High static certified by A.G.A. (in wg.) .50 .50 .50 .50 .50
Gas Piping Size Natural 1/2 1/2 1/2 1/2 1/2
I.P.S. (in.) •LPG 1/2 1/2 1/2 1/2 1/2
Vent/Intake air pipe size connection (in.) 2 2 2 2 2
Condensate drain connection (in.) SDR11 1/2 1/2 1/2 1/2 1/2
Blower wheel nominal diameter x width (in.) 10 x 8 10 x 10 10 x 10 12 x 12 12 x 12
Blower motor hp 1/3 1/2 1/3 3/4 3/4
Number and size of filters (in.) (1) 20 x 25 x 1
Tons of cooling that can be added 11/2  3 31/2  4 2  3 31/2  5 31/2  5
Electrical characteristics 120 volts  60 hertz  1 phase (less than 12 amps) All models
LPG kit (optional) **Furnished •LB65810C
Optional Horizontal Support Frame Kit  Ship. Weight LB56495CA (All Models)  18 lbs.
Optional Downflow Additive Base  Shipping Weight LB80639BB (All Models)  6 lbs.
•LPG kit must be ordered extra for field changeover.
*Annual Fuel Utilization Efficiency based on D.O.E. test procedures and according to F.T.C. labeling regulations.
Isolated combustion system rating for nonweatherized furnaces.
**LPG orifice furnished as standard with unit for field changeover. Convertible gas valve requires simple adjustment without adding any parts. See installation instructions.
SPECIFICATIONS (units equipped with electronically controlled blower motor)
Model No. G21V360 G21V380 G21V580 G21V5100
Input Btuh 60,000 80,000 80,000 100,000
Output Btuh 55,000 73,000 74,000 95,000
*A.F.U.E. 94.3% 94.5% 93.4% 94.5%
California Seasonal Efficiency 92.5% 92.4% 90.9% 91.5%
Temperature rise range (°F) 40  70 45  75 35  65 40  70
High static certified by A.G.A. (in wg.) .80 .80 .80 .80
Gas Piping Size Natural 1/2 1/2 1/2 1/2
I.P.S. (in.) **LPG 1/2 1/2 1/2 1/2
Vent/Intake air pipe size connection (in.) 2 2 2 2
Condensate drain connection (in.) SDR11 1/2 1/2 1/2 1/2
Blower wheel nominal diameter x width (in.) 10 x 8 10 x 8 111/2 x 9 111/2 x 9
Blower motor hp 1/2 1/2 1 1
Number and size of filters (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1
Tons of cooling that can be added 11/2, 2, 21/2 or 3 2, 21/2 or 3 31/2, 4 or 5 31/2, 4 or 5
Electrical characteristics 120V − 60hertz − 1 phase − 12.0 Amps Max 120V − 60hertz − 1 phase − 14.5 Amps Max
External Filter Cabinet (furnished)
•Filter size (in.) (1) 16 x 25 x 1 (1) 16 x 25 x 1 (1) 20 x 25 x 1 (1) 20 x 25 x 1
**LPG kit (optional) LB65810B LB65810B LB65810B LB65810C
•Filter is not furnished with cabinet. Filter cabinet utilizes existing filter supplied with G21V unit.
*Annual Fuel Utilization Efficiency based on D.O.E. test procedures and according to F.T.C. labeling regulations.
Isolated combustion system rating for nonweatherized furnaces.
**LPG kit must be ordered extra for field changeover.

Page 6
SPECIFICATIONS (contd.)
SPECIFICATIONS (units equipped with electronically controlled blower motor)
Model No. GSR21V380 GSR21V580 GSR21V5100
Input Btuh 80,000 80,000 100,000
Output Btuh 71,000 72,000 92,000
*A.F.U.E. 94.5% 94.6% 92.0%
California Seasonal Efficiency 92.5% 92.1% 89.7%
Temperature rise range (°F) 40  70 30  60 45  75
High static certified by A.G.A. (in wg.) .80 .80 .80
Gas Piping Size Natural 1/2 1/2 1/2
I.P.S. (in.) •LPG 1/2 1/2 1/2
Vent/Intake air pipe size connection (in.) 2 2 2
Condensate drain connection (in.) SDR11 1/2 1/2 1/2
Blower wheel nominal diameter x width (in.) 10 x 8 111/2 x 9 111/2 x 9
Blower motor hp 1/2 1 1
Number and size of filters (in.) (1) 20 x 25 x 1
Tons of cooling that can be added 2  3 31/2  5 31/2  5
Electrical characteristics 120V − 60hertz − 1 phase
12.0 Amps Max 120V − 60hertz − 1 phase − 14.5 Amps Max
•LPG kit (optional) **Furnished LB65810C
Optional Horizontal Support Frame Kit  Ship. Weight LB56495CA (All Models)  18 lbs.
Optional Downflow Additive Base  Shipping Weight LB80639BB (All Models)  6 lbs.
•LPG kit must be ordered extra for field changeover.
*Annual Fuel Utilization Efficiency based on D.O.E. test procedures and according to F.T.C. labeling regulations.
Isolated combustion system rating for nonweatherized furnaces.
**LPG orifice furnished as standard with unit for field changeover. Convertible gas valve requires simple adjustment without adding any parts. See installation instructions.
BLOWER DATA
(units equipped with conventional multispeed blower motor)
G21Q340, G21Q360 AND G21Q380
BLOWER PERFORMANCE
External Static Air Volume @ Various Speeds
Pressure
(in. wg) High Medium Low
0 1585 1392 920
.05 1558 1364 917
.10 1533 1354 915
.15 1505 1335 912
.20 1477 1315 905
.25 1447 1294 893
.30 1418 1272 887
.40 1355 1225 858
.50 1282 1164 803
NOTE  All cfm data is measured external to unit with the air filter in place.
G21Q580 BLOWER PERFORMANCE
ExternalStatic Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh Medium MedLow Low
0 2460 2350 2155 1900 1695
.05 2430 2310 2130 1875 1675
.10 2395 2275 2100 1855 1655
.15 2355 2240 2065 1825 1625
.20 2315 2205 2035 1800 1600
.25 2275 2175 1995 1780 1570
.30 2235 2130 1960 1740 1540
.40 2155 2055 1880 1675 1480
.50 2070 1970 1790 1605 1410
.60 1980 1890 1710 1540 1345
NOTE  All cfm data is measured external to unit with the air filter in place.

Page 7
BLOWER DATA (contd.) (units equipped with conventional multispeed blower motor)
G21Q460 AND G21Q480
BLOWER PERFORMANCE
External Static Air Volume @ Various Speeds
Pressure
(in. wg) High Medium Low
0 1793 1295 1050
.05 1770 1290 1050
.10 1747 1285 1050
.15 1724 1280 1050
.20 1700 1275 1050
.25 1675 1267 1050
.30 1648 1258 1050
.40 1585 1233 1036
.50 1517 1193 1012
NOTE  All cfm data is measured external to unit with the air filter in place.
G21Q3100 BLOWER PERFORMANCE
External Static Air Volume @ Various Speeds
Pressure
(in. wg) High Medium Low
0 1850 1660 1500
.05 1805 1635 1470
.10 1760 1610 1440
.15 1720 1575 1420
.20 1680 1540 1400
.25 1635 1505 1375
.30 1590 1470 1350
.40 1500 1400 1290
.50 1400 1320 1220
.60 1290 1230 1140
NOTE  All cfm data is measured external to unit with the air filter in place.
G21Q4/5100 BLOWER PERFORMANCE
ExternalStatic Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh Medium MedLow Low
0 2450 2340 2140 1910 1690
.05 2420 2310 2110 1880 1670
.10 2390 2270 2080 1860 1640
.15 2350 2240 2050 1830 1620
.20 2320 2210 2020 1800 1590
.25 2280 2170 1990 1770 1570
.30 2250 2140 1960 1740 1540
.40 2180 2060 1890 1680 1480
.50 2100 1980 1810 1610 1420
.60 2005 1890 1740 1530 1350
NOTE  All cfm data is measured external to unit with the air filter in place.
GSR21Q350 BLOWER PERFORMANCE
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh MedLow Low
0 1640 1405 1070 875
.05 1620 1390 1065 870
.10 1595 1375 1060 865
.15 1570 1360 1055 860
.20 1545 1345 1045 855
.25 1520 1325 1035 850
.30 1490 1305 1025 840
.40 1430 1260 995 810
.50 1365 1200 960 775
.60 1285 1135 910 735
.70 1195 1055 840    
.80 1085 955 755    
NOTE  All cfm data is measured external to unit with the air filter in place.
GSR21Q380 BLOWER PERFORMANCE
External Static Air Volume @ Various Speeds
Pressure
(in. wg) High Medium Low
0 1735 1455 1095
.05 1720 1445 1090
.10 1700 1435 1085
.15 1675 1420 1080
.20 1640 1405 1070
.25 1615 1385 1060
.30 1585 1355 1045
.40 1520 1290 995
.50 1440 1210 930
.60 1330 1120 870
.70 1180 1015    
.80 1035 900    
NOTE  All cfm data is measured external to unit with the air filter in place.

Page 8
BLOWER DATA (contd.) (units equipped with conventional multispeed blower motor)
GSR21Q450 BLOWER PERFORMANCE
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh MedLow Low
0 1935 1725 1530 1225
.05 1900 1695 1515 1220
.10 1865 1665 1490 1210
.15 1825 1630 1465 1195
.20 1790 1595 1435 1175
.25 1745 1560 1400 1140
.30 1700 1520 1365 1105
.40 1585 1420 1285 1030
.50 1470 1320 1200 975
.60 1360 1215 1115 920
.70 1235 1115 1030    
.80 1105 1000 930    
NOTE  All cfm data is measured external to unit with the air filter in place.
GSR21Q4/580 BLOWER PERFORMANCE
ExternalStatic Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh Medium MedLow Low
0 2355 2205 1965 1740 1520
.05 2325 2175 1940 1715 1495
.10 2290 2150 1920 1695 1475
.15 2255 2115 1890 1670 1450
.20 2220 2085 1860 1645 1425
.25 2185 2050 1830 1620 1400
.30 2150 2020 1800 1595 1375
.40 2080 1950 1745 1540 1320
.50 2000 1880 1680 1475 1260
.60 1915 1805 1615 1410 1195
.70 1825 1720 1540 1330    
.80 1730 1635 1460 1240    
NOTE  All cfm data is measured external to unit with the air filter in place.
GSR21Q4/5100 BLOWER PERFORMANCE
ExternalStatic Air Volume (cfm) @ Various Speeds
Pressure
(in. wg) High MedHigh Medium MedLow Low
0 2275 2140 1940 1725 1520
.05 2245 2110 1915 1700 1490
.10 2215 2075 1885 1675 1465
.15 2185 2040 1860 1645 1435
.20 2150 2005 1830 1620 1410
.25 2115 1970 1805 1590 1380
.30 2075 1935 1775 1560 1350
.40 1990 1870 1710 1500 1290
.50 1925 1800 1645 1435 1235
.60 1835 1730 1570 1370 1175
.70 1760 1650 1490 1300    
.80 1675 1570 1400 1225    
NOTE  All cfm data is measured external to unit with the air filter in place.
Units Equipped With an Electronically Controlled Blower Motor ICM2
G21V3−60/80 BLOWER MOTOR PERFORMANCE
(For Static Pressure 0.0" to 0.8" w.g.)
ADJUST LOW SPEED HIGH (COOL) SPEED HEAT SPEED
ADJUST
JUMPER
SETTING
VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION
SETTING 1 2 3 4 1 2 3 4 1 2 3 4
+ 540 700 830 1000 1150 1260 1400 1410 1150 1250 1350 1420
NORM 490 630 740 880 1040 1140 1240 1265 1030 1140 1220 1300
− 440 560 670 800 940 1030 1140 1160 920 1020 1100 1190
NOTE: ADJUST position on JPB1 (NORM", +", or −") determines the row of CFM available to use.
NOTE  All air data is measured external to the unit with the air filter in place.
Blower maintains a constant CFM throughout a range of varying static pressures.

Page 9
BLOWER DATA (contd.) (units equipped with an electronically controlled blower motor ICM2)
G21V5−80/100 BLOWER MOTOR PERFORMANCE
(For Static Pressure 0.0" to 0.8" w.g.)
ADJUST LOW SPEED HIGH (COOL) SPEED HEAT SPEED
ADJUST
JUMPER
SETTING
VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION
SETTING 1 2 3 4 1 2 3 4 1 2 3 4
+ 800 1050 1410 1620 1710 2030 2270* 2270* 1900 2140 2270* 2270*
NORM 720 950 1280 1500 1570 1850 2100 2220 1700 1940 2080 2200
− 620 850 1120 1310 1420 1650 1860 1990 1520 1730 1860 1940
NOTE: ADJUST position on JPB1 (NORM", +", or −") determines the row of CFM available to use.
*2300 CFM @ 0.2" w.g.; 2250 CFM @ 0.5" w.g.; 2200 CFM @ 0.8" w.g.
NOTE  All air data is measured external to the unit with the air filter in place.
Blower maintains a constant CFM throughout a range of varying static pressures.
GSR21V3−80 BLOWER MOTOR PERFORMANCE
(For Static Pressure 0.0" to 0.8" w.g.)
ADJUST LOW SPEED HIGH (COOL) SPEED HEAT SPEED
ADJUST
JUMPER
SETTING
VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION
SETTING 1 2 3 4 1 2 3 4 1 2 3 4
+ 520 670 800 960 1110 1220 1340 1420 1110 1210 1310 1420
NORM 480 600 740 880 1070 1160 1270 1300 1000 1100 1200 1280
− 420 550 650 770 950 1040 1150 1170 900 1000 1100 1160
NOTE: ADJUST position on JPB1 (NORM", +", or −") determines the row of CFM available to use.
NOTE  All air data is measured external to the unit with the air filter in place.
Blower maintains a constant CFM throughout a range of varying static pressures.
GSR21V5−80/100 BLOWER MOTOR PERFORMANCE
(For Static Pressure 0.0" to 0.8" w.g.)
ADJUST LOW SPEED HIGH (COOL) SPEED HEAT SPEED
ADJUST
JUMPER
SETTING
VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION VSP2−1 JUMPER POSITION
SETTING 1 2 3 4 1 2 3 4 1 2 3 4
+ 860 1100 1460 1740 1800 2090 2100*2100*1930 2100*2100*2100*
NORM 770 1020 1390 1580 1720 1990 2100*2100*1800 2000 2100*2100*
− 680 900 1180 1400 1450 1690 1940 2040 1580 1780 1920 2010
NOTE: ADJUST position on JPB1 (NORM", +", or −") determines the row of CFM available to use.
*2200 CFM @ 0.2" w.g.; 2100 CFM @ 0.5" w.g.; 2000 CFM @ 0.8" w.g.
NOTE  All air data is measured external to the unit with the air filter in place.
Blower maintains a constant CFM throughout a range of varying static pressures.

Page 10
Units Equipped With an Electronically Controlled Blower Motor ICM1
G21V360, G21V380 BLOWER PERFORMANCE
FACTORY BLOWER SPEED SETTINGS
G21V360 G21V380
Low Speed Heat/Cool  tap 2 Low Speed Heat/Cool  tap 2
High Speed Cooling  tap 11 High Speed Cooling  tap 11
High Speed Heat  tap 6 High Speed Heat  tap 7
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg.) Tap 1 Tap 2 Tap 3 Tap 4 Tap 5 Tap 6 Tap 7 Tap 8 Tap 9 Tap 10 Tap 11
0
thru
.80
    490 635 760 880 1030 1140 1220 1345 1420 1420
NOTE  All air data is measured external to the unit with the air filter in place.
*Blower maintains a constant CFM throughout a range of varying static pressures.
G21V580, G21V5100 BLOWER PERFORMANCE
FACTORY BLOWER SPEED SETTINGS
G21V580 G21V5100
Low Speed Heat/Cool  tap 2 Low Speed Heat/Cool  tap 2
High Speed Cooling  tap 11 High Speed Cooling  tap 11
High Speed Heat  tap 6 High Speed Heat  tap 7
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg.) Tap 1 Tap 2 Tap 3 Tap 4 Tap 5 Tap 6 Tap 7 Tap 8 Tap 9 Tap 10 Tap 11
0
thru
.80
    770 1015 1305 1510 1685 1820 2010 2050 2100 2100
NOTE  All air data is measured external to the unit with the air filter in place.
*Blower maintains a constant CFM throughout a range of varying static pressures.
GSR21V380 BLOWER PERFORMANCE
FACTORY BLOWER SPEED SETTINGS
GSR21V380
Low Speed Heat/Cool  tap 3
High Speed Cooling  tap 11
High Speed Heat  tap 7
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg.) Tap 1 Tap 2 Tap 3 Tap 4 Tap 5 Tap 6 Tap 7 Tap 8 Tap 9 Tap 10 Tap 11
0
thru
.80
        480 655 790 960 1120 1220 1365 1460 1460
NOTE  All air data is measured external to the unit with the air filter in place.
*Blower maintains a constant CFM throughout a range of varying static pressures.
GSR21V580, GSR21V5100 BLOWER PERFORMANCE
FACTORY BLOWER SPEED SETTINGS
GSR21V580 GSR21V5100
Low Speed Heat/Cool  tap 2 Low Speed Heat/Cool  tap 2
High Speed Cooling  tap 11 High Speed Cooling  tap 11
High Speed Heat  tap 6 High Speed Heat  tap 7
External Static Air Volume (cfm) @ Various Speeds
Pressure
(in. wg.) Tap 1 Tap 2 Tap 3 Tap 4 Tap 5 Tap 6 Tap 7 Tap 8 Tap 9 Tap 10 Tap 11
0
thru
.80
    890 990 1230 1425 1605 1735 1900 2015 2090 2090
NOTE  All air data is measured external to the unit with the air filter in place.
*Blower maintains a constant CFM throughout a range of varying static pressures.

Page 11
UNIT COMPONENTS UPFLOW Q" UNITS
AIR INTAKE PVC CONNECTION
IGNITION CONTROL
EXPANSION TANK
GAS VALVE
CONTROL BOX
LIMIT CONTROL
SPARK PLUG & SENSOR ACCESS
PANEL
BLOWER
GAS FLAPPER VALVE
& ORIFICE ASSEMBLY
EXHAUST OUTLET
TERMINAL STRIP
TB1
DOOR INTERLOCK SAFETY SWITCH
AIR FLAPPER VALVE
INSULATION
HEATING COMPARTMENT
PURGE BLOWER
DIFFERENTIAL PRESSURE
SWITCH
SPARK PLUG
& SENSOR ACCESS
PANEL
UNIT COMPONENTS HORIZONTAL/REVERSE FLOW V" UNITS
(ICM2 SHOWN WITH VSP2−1)
PURGE BLOWER
AIR INTAKE PVC CONNECTION
IGNITION
CONTROL DIFFERENTIAL PRESSURE
SWITCH
EXPANSION
TANK
GAS VALVE
EXHAUST
OUTLET
AIR FLAPPER
VALVE
MODULATION/LIMIT
CONTROL
BLOWER
GAS FLAPPER VALVE
& ORIFICE ASSEMBLY
TERMINAL STRIP TB1
CONTROL
BOX
DOOR INTERLOCK
SAFETY SWITCH
INSULATION
HEATING
COMPARTMENT
VSP21
CONTROL BOARD

Page 12
I − APPLICATION
G21/GSR21 unit input range covers 40,000 through 100,000
Btuh. See specifications table.
G21/GSR21 models use the same cabinet size as the ex
istingG14/GSR14furnaceline.Allunitsinthe
G21/GSR21 series use direct drive blowers and accept
cooling coils in nominal tonnages up to 5 tons for the 80,
100. Consult the Engineering Handbook for proper sizing.
Slab filters are used for either bottom or side return air in
G21 models and top return air in GSR21 models.
II − UNIT COMPONENTS
A − Control Box (Figures 1 and 2)
The G21 control box is located below the air intake chamber.
40, 60 and 80 control boxes are designed to open over the
exhaust PVC line when the unit is set up for righthand dis
charge of exhaust. 100 control boxes are designed to open
over the exhaust PVC line when the unit is set up for lefthand
discharge of exhaust.
The GSR21 control box is located in the lower righthand
corner of the heating compartment in horizontal installa
tions and in the upper righthand corner of the heating
compartment in reverse flow applications.
GSR21Q CONTROL BOX
TRANSFORMER
T1
TERMINAL
STRIP
TB1
TERMINAL
BLOCK
TB2
DOOR INTERLOCK
SAFETY SWITCH
(S51)
INDOOR
BLOWER
RELAY (K3)
FAN TIMING CONTROL
(A28)
GSR21V CONTROL BOX
TRANSFORMER
T1
TERMINAL STRIP
TB1
TERMINAL BLOCK
TB2
FIGURE 1
GROUNDING LUG
GROUNDING LUG
DOOR INTERLOCK
SAFETY SWITCH
(S51)
ACCESSORY
RELAY K109
TERMINAL BLOCK TB2
TERMINAL
STRIP TB1
ACCESSORY
RELAY K109
TRANSFORMER
T1
G21V CONTROL BOX
FIGURE 2
G21Q CONTROL BOX
GROUNDING
LUG
GROUNDING
LUG
TRANSFORMER T1 INDOOR BLOWER
RELAY K3
TERMINAL
BLOCK
TB2
FAN TIMING CONTROL
(A28)
TERMINAL STRIP TB1
1 − Control Transformer T1
A transformer (T1) located inside the control box provides
power to the low voltage section of the unit. Transformers are
rated at 30VA for Q" models and 50VA for V" models with a
120V primary and 24V secondary.
2 − Transformer Fuse F1
Transformer T1 is protected by a fuse F1. See table 1 for fuse
ratings. The fuse is located on the TB1 terminal strip.
TRANSFORMER FUSE
F1
TABLE 1
APPLICATION FUSE RATING
V" MODELS
Q" MODELS
3 AMP MDX SLOW BLOW
2 AMP AGC FAST BLOW

Page 13
3 − Low Voltage Terminal Strip TB1
A low voltage terminal strip (TB1) with thermostat markings is
located outside the control box. See figures 3, 4 and 5.
FIGURE 3
FUSE 2 AMP AGC FAST
BLOW
Q" SERIES TERMINAL STRIP TB1
GBLOWER DEMAND INPUT
WHEATING DEMAND INPUT
CCOMMON
*Y is used as a terminal block to
connect the outdoor unit to the
thermostat. It makes no internal
connection to unit controls.
R24VAC OUTPUT
YCOOLING WIRING
WYRGC
V" SERIES IMC1 TERMINAL STRIP TB1
W1
W2
R
GY1Y2
DS
C
HB
FIGURE 4
FUSE
3 AMP MDX SLOW BLOW
R24VAC OUTPUT
W11st STAGE HEATING DEMAND INPUT
W22nd STAGE HEATING DEMAND INPUT
GBLOWER DEMAND INPUT
Y11st STAGE COOLING CONNECTION
Y22nd STAGE COOLING CONNECTION
DSPWM, AC OR DC INPUT
CCOMMON 24 VAC
HBSENSING OUTPUT FOR HEATING
BLOWER
*Y is used as a terminal block to connect the
outdoor unit to the thermostat. It makes no
internal connection to unit controls.
V" SERIES IMC2 TERMINAL STRIP TB1
W1
W2
GR C
DS
Y2
Y1
FIGURE 5
FUSE
3 AMP MDX SLOW BLOW
NM
The following terminal designations are unique to V" se
ries with the IMC1 motor:
R" This signal provides 24VAC to the thermostat and, in
zoning applications, to the zone control.
DS " This is the speed regulation input that switches the
blower from LOW to HIGH speed in cooling mode. DS is
the PWM (pulse width modulation) input in zoning ap
plications. When used with the CCB1 it is a 1217VDC
signal. When used without the CCB or Harmony it is a
24 VAC signal from the thermostat.
HB" This is an output signal, to tell the zone control when
the electronically controlled blower is energized from
either a heating demand or a tripped secondary limit.
The following terminal designations are unique to V" series
units with the IMC2 motor:
R" This signal provides 24VAC to the thermostat and, in Har
mony II zoning applications, to the control center.
DS" This is the speed regulation input that switches
the blower from LOW to HIGH speed, in cooling
mode. DS is the PWM (pulse width modulation) in
put in zoning applications that use Harmony II zon
ing system.
NM" This terminal is used for non−zoning (Non−Harmony),
non−modulating applications where heat exchanger mod
ulation is NOT desired. The heat demand from the ther
mostat should be wired to this terminal instead of W1.
CAUTION
For units with the IMC2 motor do not connect the
heat demand wire to both the W1 and the NM ter
minals. Damage to the unit will occur. Use the W1
terminal for zoning (Harmony) applications and
the NM terminal for non−zoning (Non−Harmony)
applications.
W1" This terminal is used for zoning (Harmony) modulating
applications where heat exchanger modulation IS de
sired. This terminal is also used for the first stage of a two−
stage heating application.
W2" This terminal is used for two−stage heating. Two−stage
heating is not available when using the non−modulating
NM terminal.
For field wiring to terminal strips see figures 3, 4 and 5. See
tables 2 and 3 for jumper connections.
TB1 TERMINAL STRIP JUMPERS V" MODELS
APPLICATION JUMPERS REQUIRED
Single Stage Heating
TABLE 2
IMC1 MOTOR
HB" Jumpered to W2"
TwoSpeed Compressor, No
CCB1, Without Harmony DS" Jumpered to Y2"
DS" Jumpered to G"
CCB1 With SingleSpeed or Two
Speed Compressor, Without Harmony No Jumpers Required*
SingleSpeed Compressor,
No CCB1, Without Harmony
*Never Jumper Y2" to DS" when a CCB1 control is used.
Damage to the CCB1 will occur.
TB1 TERMINAL STRIP JUMPERS V" MODELS
APPLICATION JUMPERS REQUIRED
TABLE 3
IMC2 MOTOR
TwoSpeed Compressor, No
CCB1, Without Harmony DS" Jumpered to Y2"
DS" Jumpered to G"
CCB1 With SingleSpeed or Two
Speed Compressor, Without Harmony No Jumpers Required*
SingleSpeed Compressor,
No CCB1, Without Harmony
*Never Jumper Y2" to DS" when a CCB1 control is used.
Damage to the CCB1 will occur.
Single Stage Heating See Figure 13
For Pin Setting
NOTE − For single stage heat application with the IMC2 mo
tor, VSP2−1 will have a selector pin for single or second
stage heating. See figure 13.
Refer to tables 18 through 22 for a complete listing of all jump
ers used.

FIGURE 7
WATCHGUARD
CONTROL BOARD
(A18)
Page 14
If a two speed condensing unit is used and Harmony or CCB1
is not used jumper DS to Y2. The blower will operate on the
low speed heat/cool tap during first stage cooling (low speed
compressor). During second stage cooling (high speed com
pressor), the blower will operate on the high speed cooling tap.
If a single speed condensing unit and no Harmony or CCB1 is
used, jumper DS to G. The blower will operate on the high
speed cooling tap during the cooling mode.
CAUTION
Never jumper Y2" to DS" if a CCB1 control is
used. Damage to the CCB1 control will occur.
IMPORTANT
Y2" must be jumpered to DS" in two−speed, non−
zoned application when CCB1 is not used.
4 − Terminal Block TB2 (Figure 6)
Line voltage is routed to the unit through a power supply termi
nal block (TB2) located inside the control box. The terminal
block is energized at all times.
In Q" units the accessory terminal (ACC) is energized any
time there is a blower demand. In V" units the accessory ter
minal is energized by the VSP control and is powered when
the blower is running.
The accessory terminal can be used for accessories such as
an electronic air cleaner.
TERMINAL BLOCK TB2
ACC NL1
FIGURE 6
5 − K3 Indoor Blower Relay Q" Models Only
A doublepole, doublethrow indoor blower relay is located in
side the control box to provide power to the blower. K3 relay
contacts also control the 120V accessory terminal located on
terminal strip TB2.
6 − Watchguard Control Board (A18)
GC1 Control Only
The watchguard control board is illustrated in figure 7.
Watchguard serves as an automatic electronic ignition
reset device. Watchguard is a N.C. SPST selfresetting
timer wired in series with W1 thermostat demand. It is
built into the ignition control on all units equipped with
GC3 or Johnson ignition control. On GC1 equipped
units it is externally mounted.
For GC1 equipped units Watchguard is enabled with a
heating demand. After one hour (unit locked out or run
ning) of continuous thermostat demand, watchguard
opens for two minutes then closes remaking thermostat
demand W1. This resets electronic ignition control A3.
After this break, a thermostat de
mand for heat will allow the unit to
fire. If ignition is not successful
unit lockout occurs. If successful
the unit operates. Lockouts are
usually attributed to low gas line
pressure. For units equipped with
a GC3 or Johnson ignition con
trol, the Watchguard is enabled only after the unit locks
out because of failed ignition attempts (five tries). The
watchguard will then break and remake thermostat de
mand after one hour.
7 − Fan Timing Control A28 Q"Units Only
(Figure 8)
A fan timing control
(A28) located in the con
trol box is used in Q"
models to regulate fan
on and fanoff timings.
Fanon timing is the
amount of time that the
unit operates in a heat
ing demand without the
blower running during initial start up. Fanoff timing is the
amount of time that the blower continues to run after heating
demand has been terminated. Fan timing control part number
51K4601 (figure 8) has a factory set fan−on time of 45 sec
onds and is adjustable from 30 to 60 seconds. Fan−off timing is
factory set at180 seconds and is adjustable from 120 to 240
seconds. Fan timing control with early part number 76H31 has
a fanon timing set at 45 seconds and is not adjustable. Fan
timing control with part number 97H03 and 79J65 has an ad
justable fanon timing from 30 to 60 seconds. Fanoff timing is
factory set at 180 seconds and is adjustable from 120 through
240 seconds. During fanoff timing blower operates on low
speed heat/cool tap.
FIGURE 8

Page 15
8 − K9 Isolation (Heat) Relay
Q" Models Only
Fan timing control (A28) with part number 79J65 con
tains a singlepole, singlethrow isolation relay (K9).
When there is a heat demand through W1, K9 is ener
gized closing the normally open contacts K91 in the
ignition circuit. The addition of an isolation relay in the
W1 circuit will eliminate electrical noise feeding from the
electronic thermostat back to the electronic ignition
control.
9 − K109 Accessory Relay V" Models Only
G21V−5 to −8 & GSR21V−10 to −15
A singlepole, singlethrow accessory relay is located inside
the control box to provide power to additional accessories
which may be used with the G21V / GSR21V.
10 − VSP1 Blower Control Board (A24)
V" Models Only G21V−1 to −3 &
GSR21V−1 to −9 (Figure 9)
FANOFF
TIMING PINS
VSP1 BLOWER CONTROL BOARD (A24)
L1
ACC
DS3 DS2 DS1
120V
ACCESSORY
TERMINAL
L1 LINE
VOLTAGE
J73
VSP1 CONTROL
PLUG
J46
OUTPUT
PLUG
210
90
150
270
FRONT VIEW
12
1
3
69
1
4
DIAGNOSTIC
LEDS
FIGURE 9
The VSP1 (A24), a printed circuit board located in the con
trol box, serves four primary functions:
1− Controls blower timings during heating to accommo
date the required initial heat−up and cool−down times of
the heat exchanger.
2− Senses limit trip condition and turns on the blower.
3− Controls the accessory relay.
4− Interfaces the 24VAC thermostat with the blower.
When operating in heating mode, VSP1 controls the blower
and monitors limit and gas valve operation. The VSP1 con
trols the fanon" and fanoff" timings. Fanon timings are
preset and non adjustable. Fanoff timings are adjustable.
Fanon timing is the amount of time the unit operates before
the blower is started. This period allows for heat exchanger
warmup. The fanon timing is preset at 45 seconds and is
not adjustable.
FanOff timings (time that blower operates after heating
demand has been satisfied) are determined by the ar
rangement of a jumper on the VSP1 board. To adjust fanoff
timings, gently disconnect jumper and reposition it across
pins corresponding with new timing (see figure 10). The
fanoff timing is factory set at 270 seconds.
IMPORTANT
If fanoff time is too low, residual heat in heat ex
changer may cause primary limit S10 to trip result
ing in frequent cycling of blower. If this occurs, ad
just blower to longer time setting.
DANGER
Shock Hazard. VSP1 fan control is
connected to line voltage. Discon
nect power to unit before changing
pin timings. Can cause personal
injury or control damage.
FANOFF TIME ADJUSTMENT
270 210
150 90
To adjust fan−off timings:
Remove jumper from VSP1 and se
lect one of the other pin combina
tions 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
FIGURE 10
The VSP1 includes a 120 VAC accessory terminal. The ter
minal is wired directly to terminal block TB2 and powers the
accessory connection on the terminal block. The terminal
is energized when the blower is running. It can be used for
any desired accessory equipment such as an electronic air
cleaner or humidifier.

Page 16
VSP1 provides an interface between the 24VAC indoor
thermostat signal and the direct current digital signal to the
blower motor. The control is responsible for energizing the
blower motor in response to thermostat demand and for
converting thermostat demand from 24VAC to 24VAC half
rectified (DC pulse) see figure 11. The motor controller (in
side the blower motor) is responsible for regulating blower
speed to maintain the desired CFM.
PIN 4 − 1217VDC INPUT
FROM HUMIDITY
CNTRL. TERM DS or
24VAC FROM TSTAT
TERMINAL Y2 IF CCB
OR HARMONY IS NOT
USED. PWM SIGNAL
FROM HARMONY.
FIGURE 11
VSP1 BLOWER CONTROL BOARD (A24)
PIN 1 − 24VAC COMMON
PIN 3 − 24VAC POWER TO
TSTAT
PIN 2 − 24VAC INPUT FROM
TSTAT TERM G
PIN 5 − OUTPUT SIGNAL
CONFIRMING HEATING
BLOWER SIGNAL
J73 Inputs
J46
Outputs
J73
Wires from J46 connect directly to indoor blower B3 jack J49. Volt
age on pins 2 and 3 are halfrectified AC (DC pulse). Measured volt
age will vary depending on the type of meter used.
J46
PIN 1 − Common
PIN 2 − 24VAC 1/2 rectified ON/OFF
PIN 3 − 24VAC 1/2 rectified HEAT
PIN 4 − 12−17VDC (Hi/Low) if CCB1 is used,
21VAC if CCB or Harmony is not used.
PWM signal if Harmony is used.
VOLTAGES INTO VSP1
VOLTAGES FROM VSP1 TO ELECTRONICALLY
CONTROLLED BLOWER MOTOR
34 volts
−34 volts
0 volts
Voltage across J73 pins 8 to 1 and 3 to 1 is 24VAC as shown here.
Refer to unit wiring diagram.
Voltage across J46 pins 2 to 1 and 3 to 1 is halfrectified AC as shown here.
Refer to unit wiring diagram.
Voltage across J73 pins 4 to 1 is approximately
1217VDC (straight voltage) if CCB is used. A PWM signal if Harmony is used.
If CCB or Harmony is not used, pin 4 to 1 voltage is 21VAC.
Approx.
34 volts
0 volts
Voltage across J46 pins 4 to 1, is approximately 1217VDC if CCB is used. If CCB or Har
mony is not used, pin 4 to 1 voltage is approximately 21VAC. A PWM signal if Harmony is
used.
24VAC @ 60Hz.
24VAC HalfRectified (DC Pulse)
@ 60Hz.
DS3
DS2
DS1
210
90
150
270
ACC
L1
1
2
3
4PIN 6 − 24VAC INPUT FROM
TSTAT TERM W2
PIN 7 − 24VAC INPUT SIGNAL
FROM EXTERNAL LIMIT
PIN 8 − 24VAC POWER FOR
VSP1
PIN 9 − 24VAC INPUT FROM
GAS VALVE
PIN 10 − 24VAC OUTPUT FROM
VSP1 TO IGNITION
CONTROL A3
PIN 11 − 24VAC INPUT FROM
FAN LIMIT CONTROL
PIN 12 − 24VAC INPUT FROM
FAN LIMIT CONTROL
13
6
9
12
IMPORTANT
24 VAC half wave rectified (DC pulse), when mea
sured with a meter, may appear as a lower or high
er voltage depending on the make of the meter.
Rather than attempting to measure the output
voltage of A24, see G21V/GSR21V V" BLOWER
AND VSP1 BLOWER CONTROL BOARD TROU
BLESHOOTING FLOW CHART in the TROUBLE
SHOOTING section of this manual.
Diagnostic LED Lights
Three diagnostic LED lights are provided on the control for
troubleshooting. The three lights DS1, DS2 and DS3 (fig
ure 12) are on/off," hi speed heat" and high speed cool."
In the heating and cooling mode, the on/off LED (DS1) is lit
indicating the blower is operating on low speed heat/cool
tap. It is lit whenever a 24VAC thermostat demand is sup
plied to the control (jackplug JP73 pin 2). When the hi
speed heat"(DS2) and the on/off (DS1) LED are both lit the
blower is operating on high speed heating tap (1217VDC
from CCB1 terminal DS or 24VAC from Y2 if CCB1 is not
used). During dehumidification mode, the CCB1 turns off
the DS output and the blower operates on low speed heat/
cool tap. When the high speed cool" (DS3) and the on/off"
(DS1) LED are both lit the blower is operating on high
speed cool tap.
If the unit is switched from a heating demand to a 2nd stage
cooling demand, all three lights (DS1, DS2 and DS3) may
be energized for a short time. During this period, blower
operates on high speed heating tap.
VSP1 BLOWER CONTROL BOARD (A24)
FIGURE 12
DS3 DS2 DS1
DIAGNOSTIC
LEDS
J73
VSP1 CONTROL
PLUG INPUTS
J46
OUTPUT
PLUG
FRONT VIEW (PARTIAL)
DS3 DS2 DS1
DS3 DS2 DS1
DS3 DS2 DS1
UNIT OPERATING ON LOW
SPEED
HEAT/COOL TAP
UNIT OPERATING ON HIGH SPEED HEAT TAP
UNIT OPERATING ON HIGH SPEED COOL TAP
NOT LIT LIT
Any other combination could indicate possible trouble with the
VSP1 refer to TROUBLESHOOTING section of this manual
3
1
69
12
1
DS3 DS2 DS1 UNIT SWITCHED FROM HEATING DEMAND TO
2nd STAGE COOLINGUNIT OPERATES ON
HIGH SPEED HEAT TAP MOMENTARILY

Page 17
B − VSP2 Blower Control Board (A24)
V" Models Only G21V−5 to −8 & GSR21V
−10 to −15 (Figure 13)
IMPORTANT
If fanoff time is too low, residual heat in heat ex
changer may cause primary limit S10 to trip result
ing in frequent cycling of blower. If this occurs, ad
just blower to longer time setting.
G21V / GSR21V units are equipped with a variable speed mo
tor that is capable of maintaining a specified CFM throughout
the external static range. The unit uses the VSP2−1 variable
speed control board, located in the blower compartment,
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 15).
The VSP2−1 also senses limit trip condition and turns on the
blower. The G21V / GSR21V limit switch is located in the vesti
bule 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 − ener
gized.
Diagnostic LEDs located on the VSP2−1 control board are pro
vided to aid in identifying the unit’s mode of operation. Certain
scenarios will arise depending on the jumper positions.
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
1
1
VSP2−1 BLOWER CONTROL BOARD (A24)
FIGURE 13
SELECTOR PINS
DIAGNOSTIC DS LEDS
HEATING STAGE
SELECTOR PIN
FAN OFF
TIMING PINS
JP2 13 PIN PLUG (BOARD TO MOTOR)
JP1 15 PIN (BOARD TO FURNACE)
FIGURE 14
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 ignition 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.

Page 18
IMPORTANT
24 VAC half wave rectified (DC pulse), when mea
sured with a meter, may appear as a lower or high
er voltage depending on the make of the meter.
Rather than attempting to measure the output
voltage of A24, see G21V/GSR21V V" BLOWER
AND VSP2 BLOWER CONTROL BOARD TROU
BLESHOOTING FLOW CHART in the TROUBLE
SHOOTING section of this manual.
Diagnostic LED Lights
1 − DS3 ON/OFF"
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.
a − DS2 HEAT"
If HEAT LED−DS2 is on, the blower is running in the heat
speed according to the HEAT" jumper setting. In heating
mode only, there is a 45 second delay in energizing HEAT
LED−DS2. Light will not go off until adjustable fan OFF"
delay has expired.
b − DS1 HI/LOW"
HIGH/LOW LED−DS1 indicates whether the blower is op
erating in high or low speed. When the light is off, the blow
er is running in low speed according to the LOW" jumper
setting. When HIGH/LOW LED−DS1 is on, the blower is op
erating in high speed according to the HIGH" jumper set
ting.
c − DS4 CFM"
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 be
tween 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 set
ting change will not take place while the motor is
running.
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 con
ditions associated with each one. Refer to figure 13 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 pages 7 and 8. Using the tables, determine
which row of CFM volumes most closely matches the de
sired CFM. Once a specific row has been chosen (+, NOR
MAL, or −), CFM volumes from other rows cannot be used.
Below are the descriptions of each of the jumper selec
tions.
Refer to table 4 for factory settings. Refer to the tables on
pages 7 and 8 for the approximate air volume for each set
ting.
TABLE 4
MODEL
NUMBER
G21V5−80
G21V3−60
G21V5−100
G21V3−80
VSP2−1 FACTORY SETTINGS
HIGH LOW HEA
T
141
241
ADJUST
NORM
NORM
GSR21V5−80 141
241
NORM
NORM
GSR21V5−100
a−ADJUST"
The ADJUST pins allow the motor to run at normal speed,
approximately 10% higher, or approximately 10% lower than
normal speed. The tables on pages 7 and 8 give three rows (
+, NORMAL, and −) with their respective CFM volumes. No
tice that the normal adjustment setting for heat speed position
#3 in the G21V5−80/100 blower data table is 2080 CFM. The
+ adjustment setting for that position is 2270 CFM and for the
− adjustment setting is 1860 CFM. After the adjustment set
ting has been determined, chose the remainder speed jump
er 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 mo
tor is operational. This is beneficial primarily in trouble
shooting. G must be energized for motor to run.

Page 19
b−HEATING BLOWER"
Place the HEATING BLOWER jumper across the first and
second pins for single−stage heating operation (position
#1). For two−stage operation, place the jumper across the
second and third pins (position #2).
The position of the jumper determines which set of speed
jumpers is activated. When the HEATING BLOWER jumper
is across the first and second pins, the HEAT jumper selec
tions are activated when W1 is energized.
If the jumper is across the second and third pins, the LOW
jumper selections are activated when W1 is energized.
HEAT jumper selections are activated when W2 is ener
gized.
NOTE−In Harmony II zoning applications, HEATING
BLOWER jumper must be in position #2.
c−HEAT"
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 pages 7 and 8.
If a lower heating speed (than one that is listed in HEAT
SPEED section) is required, the LOW jumper may be used
to set the heating speed. This is done by first placing the
LOW jumper in the desired CFM position and then placing
the HTG. BLOWER jumper across the second and third
pins (regardless of the actual stage). Doing so will activate
the low speed jumper setting when W1 is energized.
d−HIGH"
The HIGH jumper is used to determine the CFM during
cooling speed. These jumper selections are activated
when G and DS terminals are energized.
e−LOW"
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.
f−FAN OFF"
Fan OFF" timings (time that the blower operates after the
heat demand has been satisfied) are determined by the ar
rangement of a jumper on the VSP2−1 board. See fig
ure 15. 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.
Fan ON" time is factory set at 45 seconds and is not adjust
able.
WARNING − MAKE SURE TO DISCONNECT POWER
BEFORE CHANGING FAN OFF" TIMINGS.
FIGURE 15
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 combina
tions 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.
C − LimitControl−Modulation/Limit
Control (Figure 16)
S78 N.O.
HONEYWELL LIMIT CONTROL− MODULATION/
LIMIT CONTROL ALL MODELS S10/S78
FIGURE 16
S78
N.C.
S10
S10
S78 SWING
S78 N.C. IS
A 1/4"
QUICK
CONNECT
1 − Limit Control (S10) Q" Models
G21Q/GSR21Q models use a Honeywell limit control. See
unit components illustration (page 11) for exact location. If heat
exchanger temperatures rise above limit setting (see table 5),
the selfresetting limit interrupts power to ignition control A3,
and fan timing control A28 will maintain the blower to cool
down the unit. This is a safety shut down function. S10 auto
matically resets when temperatures inside the heat exchange
assembly drop below limit setting. Do not change factory limit
setting. S10 is set to limit maximum discharge air temperature.

Page 20
FAN/LIMIT CONTROL Q" MODELS
GSR21Q380
GSR21Q4/580
GSR21Q4/5100
G21Q3/460
G21Q380
G21Q4/580
G21Q4/5100
UNIT LIMIT SETTING
TABLE 5
160°
210°
200°
G21Q3100 230°
210°
GSR21Q3/450 170°
170°
160°
160°
G21Q340 200°
2 − V" Modulation/Limit Control (S78/S10)
G21V/GSR21V units use a Honeywell combination modula
tion/limit control. The limits are located in the same housing.
See unit components illustration (page 11).
a − Modulation Control S78
Modulation limit control (S78) is a selfresetting SPDT
limit wired in series with ignition control A3. See table 6
for modulation settings. For units equipped with the
VSP2 with the heating blower jumper in position 1 and
the thermostat wired to W1 in a singlestage applica
tion, the blower will operate on the low speed (heat/cool)
tap. For units equipped with the VSP1 and the thermo
stat wired to W1 in a single stage application, the blower
will operate on the low speed (heat/cool) speed tap.
Heat exchanger temperatures will rise to modulation
settings due to the low air flow across heat exchanger.
S78 will open interrupting power to gas valve GV1. VSP
(1 or 2) control board activates blower on low speed
(heat/cool) tap. Residual heat in heat exchanger is used
for heating. S78 closes when heat exchanger tempera
tures drop below modulation setting. If there is a ther
mostat demand, A3 will cycle through a normal ignition
timing sequence.
For units equipped with the VSP2 and the heating blow
er jumper in position 2, the blower will operate on heat
speed tap. Cycling will occur less frequently due to in
creased air flow across heat exchanger.
Use the NM" non modulating terminal instead of the
W1" for single stage nonzoning applications where
modulating is not desired. For units equipped with the
VSP1, and W2 and HB jumpered together, the blower
will operate on high speed tap. Regardless of VSP used
do not change the factory modulation settings. S78 is
set to prevent the unit from tripping limit S10.
110°F140°F
MODULATION CONTROL S78 V" MODELS
GSR21V580
UNIT MODULATION SETTING
TABLE 6
GSR21V−100
G21V360
G21V380
G21V580
110°F
G21V5100
GSR21V380
OPEN CLOSE
140°F
145°F115°F
135°
F
105°F
110°F140°F
100°
F
130°
F100°
F
130°
F
IMPORTANT
VSP2
The heating blower jumper must be in position num
ber 2 if a single stage thermostat is to be used and
heat speed is desired. This forces the blower to run
on the high speed heat tap and stops frequent cycl
ing which would occur. When the blower operates on
the low speed heat tap, reduced airflow causes heat
exchanger temperatures to rise above the modula
tion control (S78) setpoint. The limit will trip" termi
nating thermostat demand. The blower continues to
run and as temperatures decrease the limit resets.
Thermostat demand is remade.
IMPORTANT
VSP1
If a single stage thermostat is to be used, HB" and
W2" terminals must be jumpered together. This
forces the blower to run on the high speed heat
tap and stops frequent cycling which would occur.
When the blower operates on the low speed heat
tap, reduced airflow causes heat exchanger tem
peratures to rise above the modulation control
(S78) setpoint. The limit will trip" terminating
thermostat demand. The blower continues to run
and as temperatures decrease the limit resets.
Thermostat demand is remade.
b − Limit Control S10
If heat exchanger temperatures keep rising, limit
control S10 will open. S10 is a selfresetting N.C. lim
it wired in series with the thermostat. S10 opens on a
temperature rise interrupting power to thermostat,
terminating heating demand. See table 7 for limit set
tings. VSP2 control board activates blower on low
speed (heat/cool) tap.