24
18-CD24D1-2
Installer’s Guide
For complete shutdown: Turn the gas cock knob on the
main Gas Valve to the “OFF” position (See Figures 14 &
15).
Disconnect the electrical supply to the unit.
SEQUENCE OF OPERATION
NOTE: Whole House Air Cleaner is energized when
blower is operating and there is a call for heat or cool.
This can result in brief periods of blower operation
with the whole house air cleaner off. This can occur
during the heat fan off period or if the enhanced
mode of operation is chosen for cooling.
Comfort Control call for heat (2-Stage Comfort Control)
Call for 1st Stage only:
W1 Comfort Control contacts close signaling the control
module to run its self-check routine. After the control
module has verified that the 1st Stage Pressure Switch
contacts are open and the Limit Switch(es) contacts are
closed, the draft blower will be energized.
As the induced draft blower comes up to speed, the
Pressure Switch contacts will close and the ignitor
warm up period will begin. The ignitor will heat for
approx. 20 seconds, then the Gas Valve is energized in
1st Stage to permit gas flow to the burners.
The flame sensor confirms that ignition has been
achieved within the 4 second ignition trial period.
As the flame sensor confirms that ignition has been
achieved, the delay to fan ON period begins timing and
after approx. 45 seconds the indoor blower motor will be
energized at low speed and will continue to run during
the heating cycle. The whole house air cleaner will en-
ergize.
Call for 2nd Stage after 1st Stage:
W2 Comfort Control contacts close signaling a call for
2nd Stage Heat. After a 30 second delay, the induced
draft blower will be energized on high speed and the
2nd Stage Pressure Switch contacts will close. The Gas
Valve is energized in 2nd Stage and the indoor blower
motor in high speed.
2nd Stage satisfied, 1st Stage still called:
W2 Comfort Control contacts open signaling that 2nd
Stage Heating requirements are satisfied.
The induced draft blower is reduced to low speed allow-
ing the 2nd Stage Pressure Switch contacts to open and
the Gas Valve is reduced to 1st Stage. The indoor
blower motor is reduced to low speed.
1st stage satisfied:
W1 Comfort Control contacts open signaling that 1st
Stage heating requirements are satisfied. The Gas
Valve will close and the induced draft blower and the
whole house air cleaner will be de-energized. The in-
door blower motor will continue to run for the fan off
period (Field selectable at 60, 100, 140 or 180 seconds),
then will be de-energized by the control module.
Comfort Control call for heat (1-Stage Comfort
Control)
W1/ W2 (jumpered) Comfort Control contacts close sig-
naling a call for heat. 1st Stage sequence of operation
remains the same as above. 2nd Stage Heat will ener-
gize after the Stage delay timer (10 minutes) has ex-
pired.
Comfort Control satisfied:
W1/ W2 (jumpered) contacts close signaling the control
module to close the Gas Valve. The induced draft blower
is switched to low speed and de-energized after the post
purge timer has expired. The indoor blower motor will
continue to operate after the flames are extinguished
and then is switched to low heat speed for the FAN-
OFF period. The whole house air cleaner will be de-en-
ergized.
NOTE TO INSTALLER
Review warnings and the contents of USER’S INFOR-
MATION MANUAL with the homeowner when installa-
tion is complete and equipment is ready to be turned
over to the homeowner for normal operation.
CONTROL AND SAFETY SWITCH
ADJUSTMENT
LIMIT SWITCH CHECK OUT
The limit switch is a safety device designed to close the
Gas Valve should the Furnace become overheated.
Since proper operation of this switch is important to the
safety of the unit, it
must be checked out on initial
start up by the installer
.
To check for proper operation of the Limit Switches, set
the Comfort Control to a temperature higher than the
indicated temperature to bring on the Gas Valve. Re-
strict the airflow by blocking the return air or by dis-
connecting the blower. When the Furnace reaches the
maximum outlet temperature as shown on the rating
plate, the burners must shut off. If they do not shut off
after a reasonable time and overheating is evident, a
faulty Limit Switch is probable and the Limit Switch
must be replaced. After checking the operation of the
Limit Control, be sure to remove the paper or card-
board from the return air inlet, or reconnect the
blower. Refer to the Service Facts for additional in-
structions.
INDOOR BLOWER TIMING
▲
WARNING
!
DISCONNECT POWER TO THE UNIT BEFORE REMOV-
ING THE BLOWER DOOR.
FAILURE TO FOLLOW THIS WARNING COULD RESULT
IN PROPERTY DAMAGE, PERSONAL INJURY OR
DEATH
.
