Lennox SL280UHNV Installation Instructions Manual page 42

Applications Using
See Figure 28 for ignition control sequence
B - Heating Sequence -- Integrated Control Thermostat
Selection DIP Switch 1 ON in "Single-Stage" Position
NOTE - In these applications, two-stage heat will be initiat-
ed 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 switch 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 pressure
switch has closed, the combustion air inducer begins
a 15-second pre-purge in low speed
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.
A Single-Stage Thermostat
HEATING OPERATION WITH TWO-STAGE THERMOSTAT
1 15
Pre-Purge
Ignitor Warm-up
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 second-
stage on delay, the integrated control energizes the
combustion air inducer at high speed. The gas valve
is energized on high fire 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
30 seconds
RECOGNITION PERIOD
FIgURE 27
35 39
30* second
Trial For
blower
Ignition
"on" delay
FIgURE 28
Page 42
5 SEC
Post
Purge
80
blower
"off"
delay
5 SEC
blower
"off"
Post
delay
Purge