Sequence Of Operation - Dc Models (2- Stage Excalibur™ Furnaces Only) - Atwood AFS Series Training Manual

Sequence of Operation – DC Models (2- Stage Excalibur™ Furnaces ONLY)
The digital thermostat controls the operating circuit to the furnace
by reacting to room temperature. When the room temperature is
below the thermostat set point by 2°F a heat demand signal will
be sent to the controller module).
The ON/OFF switch is an agency safety power shut off to the
furnace ignition and gas valve systems.
The circuit breaker limits amperage draw of the motor.
Current flow to the controller module and during the first seconds
the micro-processor confirms inputs and verifies correct operation
of safety redundancies. This module will perform the following
diagnostic checks of the system.
a. Sail Switch is open
b. Internal Microprocessor faults
c. Voltage inputs
d. Ignition
e. Open Limit Switch
In the event of a failure, an LED on the controller module will flash
a code. On newer control a diagnostic error code light has been
added by counting the flashes an error code can be determined.
See diagnostic information for code failures.
The motor receives current from the controller module and will run
at high speed or low speed depending on the demand signal the
digital thermostat sends to the controller module. One end of the
motor shaft is for the circulating air wheel and the other end is for
the combustion air wheel.
Circulating air blows against the sail switch and closes the
contacts, completing the circuit. The sail switch is a safety device
that insures air flow before ignition.
The limit switch is a safety device that protects the furnace from
overheating, the contacts in the limit switch open at a given
temperature setting, shutting off power. This activates the open
limit switch diagnostics and the LED on the controller module
flash shutting down the gas valve. See Chart.
As power is applied to the circuit board, the system does the
following:
a. Timing circuits allow the blower to purge the heat
chamber for 15 seconds.
b. When current is supplied to the gas valve it opens to
high burn stage. (The controller module activates the
low burn operation on the valve.)
c. As the valve opens, the ignition module sends a high
voltage spark to the electrode at the burner. The
ignition module detects the presence of a flame. If the
flame is not sensed after 7 seconds of sparking a signal
is sent to the controller module that there is no ignition
and shuts off the valve. After another 24 second purge,
it will try again. After a third try, the controller will go into
"soft" lockout, timing for one hour and the diagnostic
LED will flash a code, See Chart. After the timed hour,
the controller will initiate (3) more tries for ignition. If
there is no ignition, the timing sequence begins again.
d. If the system does not ignite and the thermostat is still
calling for a heat demand, the blower will for 90
seconds as a post purge then shut off.
When the thermostat senses the desired room air temperature, a
signal is sent to the controller module to shut down operation of
the gas valve and run the blower for 90 seconds as a post purge
of heat from the furnace heat chamber.
Two-Stage operation in automatic mode, when temperature is
within 1°F of the set point of thermostat the furnace will start in
low fire mode. If temperature is above 1°F of the set point of
thermostat the furnace will start in high fire mode. Thermostat can
also be set to manual for either high or low modes this will not
allow the unit to switch automatically with temperature changes.
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Thermostat
Circuit
Breaker
Blower Controller
Sail Switch
10 VDC Motor
Limit
Electrode
Ignition Control
Number of Flashes
1
2
3
4
5
- 12VDC
+ 12VDC
ON/OFF Switch
Valve
Diagnostic Information
Low Input Voltage
Ignition Failure
Open High Limit
Stuck APS (sail switch)
Module Fault