V" Modulation/Limit Control; Modulation Control S78; Limit Control S10 - Lennox G21Q3-40 Unit Information

TABLE 5
FAN/LIMIT CONTROL Q" MODELS
UNIT
G21Q3 40
G21Q3/4 60
G21Q3 80
G21Q4/5 80
G21Q3 100
G21Q4/5 100
GSR21Q3/4 50
GSR21Q3 80
GSR21Q4/5 80
GSR21Q4/5 100
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.
LIMIT SETTING
200°
160°
210°
200°
230°
210°
170°
170°
160°
160°
Page 20
TABLE 6
MODULATION CONTROL S78 V" MODELS
UNIT
G21V3 60
G21V3 80
G21V5 80
G21V5 100
GSR21V3 80
GSR21V5 80
GSR21V−100
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.
MODULATION SETTING
OPEN CLOSE
140°F 110°F
145°F 115°F
135° 105°F
F
140°F 110°F
140°F 110°F
130° 100°
F F
130° 100°
F F