Sequence of Operation - DC Models (
switch allows power to pass to the circuit
breaker and the thermostat.
The thermostat controls the operating circuit to the
furnace by reacting to room temperature. When room
temperature is below the thermostat set point, the
contact closes to allow current to flow to the relay.
The circuit breaker limits amperage draw of motor.
The relay allows current to pass to the motor by
closing a switch within the relay. Voltage from the
thermostat activates the relay to turn the fan on. This
takes 1-25 seconds.
Current flows to the motor to operate the blower. One
end of the motor shaft is for the circulating air wheel
and the other side 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
The limit switch is a safety device that protects the
furnace from over heating. 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.
As power is applied to the 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.
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 approximately six seconds, the board
will lock out (three try for ignition, one hour lockout
and then three retry), shutting off power to the
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 relay opens the circuit, shutting
off current to the motor.
FURNACE PRODUCES HIGH TEMPERATURE
• 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 furnace.
Relay and Circuit
(Limit and Sail Switch
are reversed in 79/80 Series.)