Lennox SL280DFNV Installation Instructions Manual page 40

Applications Using A Single-Stage Thermostat
See figure 28 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
minutes).
1 - On a call for heat, thermostat first-stage contacts
close sending a signal to the integrated control. The
integrated 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 energized at low speed.
2 - Once the control receives a signal that the low
pressure switch has closed, the combustion air
inducer begins 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 opera-
tion the unit will switch to low fire.
3 - After the pre-purge is complete, a 20-second initial
ignitor warm-up period begins. The combustion air
inducer continues to operate at low speed.
ON
OFF
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, communicating thermostat set for 30 adjustable 15 to 45 seconds.
ON
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, communicating thermostat set for 30 adjustable 15 to 45 seconds.
HEATING OPERATION WITH TWO-STAGE THERMOSTAT
1 15
Pre-Purge
Ignitor Warm-up
30 seconds
HEATING OPERATION WITH SINGLE STAGE THERMOSTAT
15
1
Ignitor Warm-up
Pre-Purge
4 - After the 20-second warm-up period has ended, the
gas valve is energized on low fire (first stage) and
ignition occurs. At the same time, the control module
sends a signal to begin an indoor blower 30-second
ON-delay. When the delay ends, the indoor blower
motor is energized on the low fire heating speed and
the HUM contacts are energized. The integrated
control also initiates a second-stage on delay
(factory-set at 7 minutes; adjustable to 12 minutes).
5 - If the heating demand continues beyond the
secondstage 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
complete, 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.
35 39 80
30* second
blower
Trial For
"on" delay
Ignition
RECOGNITION PERIOD
Figure 27
35 39
80
30* second
Trial For
blower
Ignition
"on" delay
Figure 28
Page 40
5 SEC
blower
"off"
Post
delay
Purge
5 SEC
blower
"off"
Post
delay
Purge