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Ac Models - Atwood 79 Series Service Manual

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Sequence of Operation - AC Models
Operating Circuitry 24 VAC
The transformer receives 120 VAC which it converts to
24 VAC for the operating circuitry.
The thermostat controls the operating circuit to the
furnace by reacting to room temperature. When room
temperature is below the thermostat set point, the
contacts close to allow current to flow to the relay.
The relay receives 24 VAC and energizes a heater coil
within the relay. This activates a bimetal disc which
closes the relay circuit. This takes 17-20 seconds.
Once the relay circuit is closed, 120 VAC flows to the
motor and allows the blower to run. One end of the
motor 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 to the electronic ignition system that
controls the gas valve.
Operating Circuitry 24 VAC
Black wire
120 VAC
Circuit Board
Motor Circuitry 120 VAC
(if applicable)
As power is applied to the electronic ignition circuit
board, the system does the following:
1. A timing circuit allows the blower to purge the
chamber (15-17 seconds.
2. The board supplies current to the gas valve and
causes it to open. There is an electrical switch in
line to the valve to allow power to be manually
shut off to the valve. This switch must be on for
the furnace to operate. (Switch may be separate or
combined with circuit breaker).
3. As the valve opens, the board sends a high voltage
spark to the electrode at the burner. The board
detects the presence of a flame, if the flame is not
sensed after seven seconds, the board will lock
out, shutting off power to the valve.
4. If the system does not ignite and the thermostat
remains closed, the blower will remain on until the
thermostat is reset manually.
When the thermostat senses the desired room air
temperature, the contacts open removing power from
the ignition system and shutting off the gas valve. The
blower runs until the heater in the relay cools and
opens the circuit, shutting off current to the motor.
120 volts
AC input
Circuit Breaker
or Breaker/
Switch Combination
• Locate furnace out of traffic and away from furniture and
• Do not touch or put combustibles near appliance. Hot surface
temperature may occur.
• Supervise young children in the same room as the furnace.
• Do not place clothing or flammable materials on or near the

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