Lennox EL297UH110XV60C - Electronic Ignition Sequence

Page 94
CAUTION
Failure to use properly sized wiring and circuit
breaker may result in property damage. Size wiring
and circuit breaker(s) per Product Specications
bulletin (EHB) and unit rating plate.
WARNING
Fire Hazard. Use of aluminum wire with this product
may result in a re, causing property damage,
severe injury or death. Use copper wire only with
this product.
WARNING
Electric Shock Hazard. Can cause injury
or death. Unit must be properly grounded
in accordance with national and local
codes.
Electronic Ignition
The two-stage, variable speed integrated control used
in EL297UHV 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 thermo-
stat 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 a Two-Stage Thermostat
See FIGURE 58 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 rst-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 re pressure switch does not close
the combustion air inducer will switch to high re.
After a 15 second pre-purge the high re pressure
switch will close and the unit will begin operation on
high re. After 10 to 20 seconds of high re operation
the unit will switch to low re.
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 re (rst 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 re
heating speed, the HUM contacts close energizing
the humidier and 120V ACC terminal is energized.
The furnace will continue this operation as long as
the thermostat has a rst-stage heating demand.
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.
NOTE - If the indoor thermostat is set on
CONTINUOUS FAN ON mode, the furnace will
light on high re (second-stage) for 60 seconds to
improve heat exchanger warm up. After 60 second
warm-up period, furnace will switch to low re (rst-
stage).
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 re
(second stage) pressure switch to make sure it is
closed. The high re (second stage) gas valve is
energized and the indoor blower motor is energized
for operation at the high re heating speed.
7 - When the demand for high re (second stage) heat
is satised, the combustion air inducer is switched
to the low-re heating speed and the high-re
(second stage) gas valve is de-energized. The
low-re (rst stage) gas valve continues operation.
The indoor blower motor is switched to the low-re
heating speed.
8 - When the thermostat demand for low-re (rst
stage) heat is satised, the gas valve is de-
energized and the eld-selected indoor blower o󰀨
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, the HUM contacts as well
as the 120V ACC terminals are de-energized. The
indoor blower is de-energized at the end of the o󰀨
delay.