Lennox SL280DFV Installation Instructions Manual page 43

Applications Using A Single−Stage Thermostat
See figure 29 for ignition control sequence
B − Heating Sequence −− Control Thermostat Selection
DIP Switch 1 ON in Single−Stage" Position
NOTE − In these applications, two−stage heat will be initi-
ated by the integrated control if heating demand has not
been satisfied after the field adjustable period (7 or 12 min-
utes).
1 − On a call for heat, thermostat first−stage contacts close
sending a signal to the integrated control. The inte-
grated control runs a self-diagnostic program and
checks high temperature limit switches for normally
closed contacts and pressure switches for normally
open contacts. The combustion air inducer is ener-
gized at low speed.
2 − Once the control receives a signal that the low pres-
sure switch has closed, the combustion air inducer be-
gins a 15−second pre−purge in low speed.
NOTE − If the low fire pressure switch does not close
the combustion air inducer will switch to high fire. After
a 15 second pre−purge the high fire pressure switch
will close and the unit will begin operation on high fire.
After 10 to 20 seconds of high fire operation the unit
will switch to low fire.
3 − After the pre−purge is complete, a 20−second initial ig-
nitor warm−up period begins. The combustion air in-
ducer continues to operate at low speed.
1
É É
ON
Pre−Purge
OFF
É É
1 stg heat demand
low speed CAI
ignitor
low fire gas valve
indoor blower low heat
2 stg heat demand
high speed CAI
high fire gas valve
indoor blower high heat
* Conventional thermostat 30 seconds, icomfort Touch
1
ON
Pre−Purge
É É
OFF
heat demand
low speed CAI
ignitor
low fire gas valve
indoor blower low heat
7 or 12 minutes after heating demand begins, furnace switches to high fire (depends on setting)
high speed CAI
high fire gas valve
indoor blower high heat
* Conventional thermostat 30 seconds, icomfort Touch
HEATING OPERATION WITH TWO−STAGE THERMOSTAT
15 35
Trial For
Ignitor Warm−up
Ignition
30 seconds
HEATING OPERATION WITH SINGLE STAGE THERMOSTAT
15
Ignitor Warm−up
4 − After the 20−second warm−up period has ended, the gas
valve is energized on low fire (first stage) and ignition oc-
curs. At the same time, the control module sends a sig-
nal to begin an indoor blower 30−second ON−delay.
When the delay ends, the indoor blower motor is ener-
gized on the low fire heating speed and the HUM con-
tacts are energized. The integrated control also initi-
ates a second−stage on delay (factory−set at 7
minutes; adjustable to 12 minutes).
5 − If the heating demand continues beyond the second−
stage on delay, the integrated control energizes the
combustion air inducer at high speed. The control also
checks the high fire (second stage) pressure switch to
make sure it is closed. The high fire (second stage)
gas valve is energized and the indoor blower motor is
energized for operation at the high fire heating speed.
6 − When the thermostat heating demand is satisfied, the
combustion air inducer begins a 5−second low speed
post−purge. The field−selected indoor blower off delay
begins. The indoor blower operates at the low−fire
heating speed.
7 − When the combustion air post−purge period is com-
plete, the inducer, the HUM contacts as well as the
120V ACC terminals are de−energized. The indoor
blower is de−energized at the end of the off delay.
39 80
30* second
blower
on" delay
RECOGNITION PERIOD
®
thermostat set for 30 adjustable 15 to 45 seconds.
FIGURE 28
35 39
80
30* second
Trial For
blower
Ignition
on" delay
®
thermostat set for 30 adjustable 15 to 45 seconds.
FIGURE 29
Page 43
5 SEC
blower
off"
Post
delay
Purge
5 SEC
blower
off"
Post
delay
Purge