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Electronic Ignition
The two-stage integrated control used in EL280DFE(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
Watchguad 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 a Two-Stage Thermostat
See FIGURE 24 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 integrat-
ed control runs a self-diagnostic program and checks
high temperature limit switches for normally closed con-
tacts 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 ignit-
or 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.
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 demand.
NOTE - If the indoor thermostat is set on CONTINUOUS
FAN ON mode, the furnace will light on high fire (sec-
ond-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 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
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 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 31
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