Lennox EL280UHE Series - Page 37

Page 37
Applications Using A Single-Stage Thermostat
See FIGURE 28 for ignition control sequence
B - Heating Sequence -- Integrated Control Thermostat
Selection DIP Switch 1 ON in “Single-Stage” Position
NOTE - In these applications, two-stage heat will be initi-
ated by the integrated control if heating demand has not
been satised after the eld adjustable period (7 or 12
minutes).
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.
3 - 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 and the HUM contacts are energized.
The integrated control also initiates a second-stage
on delay (factory-set at 7 minutes; adjustable to 12
minutes).
5 - If the heating demand continues beyond the
secondstage on 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.
6 - When the thermostat heating demand is satised,
the combustion air inducer begins a 5-second low
speed post-purge. The eld-selected indoor blower
o󰀨 delay begins. The indoor blower operates at the
low-re heating speed.
7 - 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 as the 120V ACC
terminal.
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
HEATING OPERATION WITH TWO-STAGE THERMOSTAT
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
FIGURE 27
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
HEATING OPERATION WITH SINGLE STAGE THERMOSTAT
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
7 or 12 minutes after heating demand begins, furnace switches to high fire (depending on setting)
FIGURE 28