Allied A97UH2E - Electronic Ignition

508433-01Issue 2345Page 60 of 70
Electronic Ignition
The two-stage, variable speed integrated control used
in A97UH2E 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 a Two-Stage Thermostat
See Figure 59 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.
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).
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 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 and the HUM contacts are de-
energized. The indoor blower is de-energized at the
end of the o󰀨 delay as well a s the 120V ACC terminal.
Figure 59. Heating Operation with Two-Stage Thermostat
1 stg heat demand
2 stg heat demand
15
ON
OFF
low speed CAI
35
1
Pre-Purge
Ignitor Warm-up
30 second
blower
“on” delay
Post
Purge
5 SEC80
ignitor
low fire gas valve
indoor blower low heat
high speed CAI
high fire gas valve
indoor blower high heat
39
Trial For
Ignition
blower
“off”
delay
RECOGNITION PERIOD
30 seconds