Lennox B050A3B, Installation/Servicing Instructions And Users Information Manual, Page: 16 / 36

Page 16 of 36 506307-01 Issue 1207
THEORY OF OPERATION
Here’s How Your System Works:
Call For Heat
The thermostat calls for heat by energizing the “W”
terminal. The control checks to see the limit switch is closed
and pressure switch is open. If the limit switch is open, the
control responds per the Open Limit section following. If
the pressure switch is closed, the control will flash “3” on
the LED and wait indefinitely for the pressure switch to open.
If the pressure switch is open, the control proceeds to pre-
purge.
Pre-Purge
The control energizes the induced draft motor and waits
for the pressure switch to close. The control flashes “3” on
the LED while the pressure switch is open. If the pressure
switch does not close within 60 seconds of the inducer
energizing, the control will de-energize the inducer for 300
seconds, and then re-energized the inducer. This cycle shall
continue as long as a call for heat exists until the pressure
switch is proven.
When the pressure switch is proven closed, the control
begins the pre-purge time. If flame is present any time while
in pre-purge, the control will flash “5” on the LED and go into
soft lockout. The control runs the inducer for a 15 second
pre-purge time, then proceeds to the ignition trial period.
Ignition Trial Period
The control energizes the spark and main gas valve.
The inducer remains energized. If flame has not been
sensed within the 7 second ignition trial, the control de-
energizes the gas and spark outputs and proceeds with
ignition retires. If flame is not established after three (3)
trials for ignition, the control will flash “2” on the LED and
goes into lockout.
Blower On Delay
The control waits for 30 seconds from the time the gas
valve opened and then energizes the indoor blower heat
speed. The gas valve and inducer remain energized. The
control proceeds to steady heat mode.
Steady Heat
Control inputs are continuously monitored to ensure limit
and pressure switches are closed, flame is established, and
the thermostat call for heat remains. When the thermostat
call for heat is removed, the control de-energizes the gas
valve and begins post-purge and blower off delay time.
Open Limit
Any time the limit switch is open, the control de-energizes
the gas valve and runs the indoor blower motor on heat
speed, and runs the induced draft motor. While the limit
switch is open, the control flashes “4” on the LED. Check
for a restriction in the duct system (i.e. dirty filters, blocked
ductwork, closed registers . . . . . . ).
When the switch re-closes, the control runs the induced
draft motor through post-purge and runs the indoor blower
through the selected fan off delay. If the call for heat is still
present when the limit switch closes, the control will begin
an ignition sequence while the blower off delay continues.
NOTE: An open limit switch breaks the power (“R”) to the
thermostat. Cycling on the limit is an abnormal condition
and a corrective action must be taken. Failure to correct
this condition could damage the heat exchangers and void
the warranty.
Post Purge
The inducer output remains on for a 15 second post-
purge period after the thermostat is satisfied.
Blower Off Delay
The indoor blower motor is de-energized after a 90
second blower off delay. Blower timing begins when the
thermostat is satisfied. If the thermostat calls for heat while
in the blower off delay, the control immediately restarts the
ignition sequence while the blower off delay continues.
Should overheating occur, or the gas supply fail to shut
OFF, turn OFF the manual gas valve to the appliance
BEFORE turning OFF the electrical supply. A failure to
adhere to this warning can result in a fire or explosion
and bodily harm.
WARNING
For cooling operation, when the inside temperature
exceeds the thermostat setting, the thermostat will turn ON
the cooling system.
When the thermostat calls for cooling, power from the
transformer energizes the fan control board (for blower
operation) and the outdoor condensing unit (for air
conditioning).
The fan control board will automatically turn on the
blower and condensing unit. The air moving over the indoor
coil by the blower is cooled (and dehumidified) and passes
through the ducts to the room registers.
When the thermostat is satisfied, the fan control board
is de-energized and the condensing unit is shut-off. The
blower will continue to operate for an additional 30 seconds
for added cooling efficiency.