Sequence of Operation – DC Models (Standard Units)

The ON/OFF 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 (relay can be either external or part of the ignition control board).

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 (on units with the relay on the ignition control board there is only a 1 -2 second delay).

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 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.

As power is applied to the circuit board, the system closes the following:

A timing circuit allows the blower to purge the chamber (15-17 seconds).
The board supplies current to the gas valve and causes it to open.
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 (after three tries for ignition, the control will lockout for one hour unless power is removed, or the thermostat is cycled). Turning off and re-establishing power will restart the cycle.
If the system does not ignite and the thermostat remains closed, the blower will remain on until the thermostat is reset manually on units with an external relay (units with the relay on the ignition control will shut the blower off even if the thermostat contacts remain closed). If the thermostat has not been satisfied within one hour the system will try the ignition cycle again.

When the thermostat senses the desired room air temperature contact open, removing power from the ignition system and shutting off the gas valve. The blower runs until the relay opens (90 seconds) the circuit shutting off current to the motor.