Lennox EL280UHE Series Service Literature page 35

Applications Using A Single-Stage Thermostat
See figure 27 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 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.
3 - 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 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
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 (depending on setting)
high speed CAI
high fire gas valve
indoor blower high heat
HEATING OPERATION WITH TWO-STAGE THERMOSTAT
1 15
Pre-Purge
Ignitor Warm-up
HEATING OPERATION WITH SINGLE STAGE THERMOSTAT
1 15
Pre-Purge
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 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 and the HUM contacts are
de-energized. The indoor blower is de-energized at
the end of the off delay as well as the 120V ACC
terminal.
35 39 80
30 second
blower
Trial For
"on" delay
Ignition
30 seconds
RECOGNITION PERIOD
Figure 27
35 39 80
30 second
blower
Trial For
"on" delay
Ignition
Figure 28
Page 35
5 SEC
blower
"off"
Post
delay
Purge
5 SEC
blower
"off"
Post
delay
Purge