508187-01
Issue 2116
Page 41 of 51
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 energized on the low fire 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 second-
stage on 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.
6.
When the thermostat heating demand is satisfied, the
combustion air inducer begins a 5-second low speed
post-purge. The field-selected indoor blower off delay
begins. The indoor blower operates at the low-fire
heating speed.
7.
When the combustion air post-purge period is
complete, the inducer and the HUM contacts are
deenergized. The indoor blower is de-energized at the
end of the off delay as well as the 120 ACC terminals
Figure 22.
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 SEC
80
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
Figure 23.
Heating Operation with Single-Stage Thermostat
heat demand
15
ON
OFF
low speed CAI
35
1
Pre-Purge
Ignitor Warm-up
30 second
blower
“on” delay
Post
Purge
5 SEC
80
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
7 or 12 minutes after heating demand begins, furnace switches to high fire (depending on setting)
