Furnace Components:

The furnace components can be divided into three categories.
1. The burners, heat exchanger, draft inducer, and venting.

2. The controls and safety devices.

3. The blower and air movement.

The flame originates at the burners and is drawn into the heat exchanger by the negative pressure produced by the draft inducer. The hot gasses produced by the combustion of the flame pass through the chambers of the heat exchanger and heat the metal walls of the heat exchanger. The gasses cool as they transfer the heat to the heat exchanger and are at about 120 degrees as they exit on a high efficiency furnace. The cooled gasses then enter the draft inducer blower and are pushed into the venting pipes. The exhaust gasses then are directed out of the house through the vent pipes.

The controls include the gas valve, ignition control, ignitor, flame sensor, transformer, limit control, blower control board, and flame roll out switch. The transformer provides 24 volts of electricity to power the controls. 24 volts is applied to the thermostat that is installed in the living space. The thermostat is basically an automatic switch that closes and completes the electrical circuit when the room temperature drops below the heat setting. This then allows 24 volts to the circuit board which initiates the heat sequence. The circuit board has a relay that closes to power up the motor on the draft inducer blower. Then the circuit board ignitor relay is energized which sends 120 volts to the hot surface ignitor and makes it glow bright and get extremely hot. Next the gas valve relay in the circuit board is energized. This allows voltage to the gas valve and energizes a solenoid coil in the gas valve which opens the valve to allow gas to flow to the burners. The gas flows into the burners and is i gnited by the hot surface ignitor. The ignition control circuit board applies an AC voltage to the flame sensor which is just a stainless steel rod. A interesting thing occurs inside a burning flame called ionization. That is free electrons are produced which can conduct electricity through the flame itself. The electrons will normally flow from the flame sensor, through the flame when present, and back to ground through the grounded burners. The ignition system must prove that a flame is present to continue the gas flow or if no flame shut off the gas flow through the gas valve to prevent a possible explosion. It also must not be fooled into thinking there is a flame present by a flame sensor that is touching the ground from being broken or bent. The way it does this is by a diode effect where the sensor surface area is less than 10% of the ground surface area. This produces a half wave of electrical current out of each full wave. The ignition control circuit detects the half wave to determine if the sens o r is merely touching ground. If the ignition control receives this half wave signal from the flame sensor then combustion will continue. Now the circuit board timer counts a determined amount of time and energizes the blower relay. This relay powers up the blower motor and air is then pushed through the heat exchanger where it removes the heat from the hot metal and enters the ductwork to go to the various rooms in the house. The limit control is a safety device that will open the electrical circuit to the ignition control and stop the gas flow if the furnace over heats. The flame roll out switch does the same thing if the flame was rolling out of the heat exchanger instead of being completely induced into it by the draft inducer.

The blower creates a negative pressure on the intake side which draws air into the ductwork return air system and blows the air out through the heat exchanger and then into supply air ductwork to distribute throughout the home.

source: http://en.wikipedia.org/wiki/Furnace