Lennox EL280UHX Series Unit Information page 36

Electronic Ignition
The two-stage integrated control used in EL280UHE(X)
units has an added feature of an internal Watchguard
control. The feature serves as an automatic reset device
for ignition 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 sequence.
NOTE - The ignition control thermostat selection DIP
switch is factory-set in the "TWO-STAGE" position.
Applications Using
See FIGURE 27 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
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 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.
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.
a Two-Stage Thermostat
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 terminal
is energized. The furnace will continue this operation
as long as the thermostat has a first-stage heating de-
mand.
NOTE - If the indoor thermostat is set on
CONTINUOUS FAN ON mode, the furnace will
light on high fire (second-stage) for 60 seconds to
improve heat exchanger 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
second- 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
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.
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
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 a s the 120V ACC
terminal.
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