Lennox EL280UHE Series Unit Information page 35

Electronic Ignition
The two-stage integrated control used in EL280UHE(X)
units has an added feature of an internal Watchguard con­
trol. The feature serves as an automatic reset device for igni­
tion control lockout caused by ignition failure. After one hour
of continuous thermostat demand for heat, the Watchguard
will break and remake thermostat demand to the furnace
and automatically reset the control to begin the ignition se­
quence.
NOTE - The ignition control thermostat selection DIP switch is
factory-set in the "TWO-STAGE" position.
Applications Using a Two-Stage Thermostat
See figure 26 for ignition control sequence
A - Heating Sequence -- Integrated Control Thermostat
Selection DIP Switch 1 OFF in "Two-Stage" Position
(Factory Setting)
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.
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, the HUM contacts
close energizing the humidifier and 120V ACC termi­
nal is energized. The furnace will continue this opera­
tion as long as the thermostat has a first-stage heating
demand.
NOTE - If the indoor thermostat is set on CONTINU­
OUS FAN ON mode, the furnace will light on high fire
(second-stage) for 60 seconds to improve heat ex­
changer warm up. After 60 second warm-up period,
furnace will switch to low fire (first-stage).
5 - If second-stage heat is required, the thermostat sec­
ond-stage heat contacts close and send a signal to the
integrated control. The integrated control initiates a
30-second second-stage recognition delay.
6 - At the end of the recognition delay, the integrated con­
trol 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.
7 - When the demand for high fire (second stage) heat is
satisfied, the combustion air inducer is switched to the
low-fire heating speed and the high-fire (second
stage) gas valve is de-energized. The low-fire (first
stage) gas valve continues operation. The indoor
blower motor is switched to the low-fire heating speed.
8 - When the thermostat demand for low-fire (first stage)
heat is satisfied, the gas valve is de-energized and the
field-selected indoor blower off delay begins. The
combustion air inducer begins a 5-second post-purge
period.
9 - When the combustion air post-purge period is com­
plete, the inducer and the HUM contacts are de-ener­
gized. The indoor blower is de-energized at the end of
the off delay as well a s the 120V ACC terminal.
Page 35