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How the Furnace Works

The furnace itself has several key components; the burner, draft inducer, heat exchanger, blower, gas valve, flame sensor, ignitor, and limit thermostat.

When the thermostat switches on, current is supplied to the furnace controls. The sequence of events then is the draft inducer blower comes on followed by the ignitor. The ignitor heats up and then the gas valve coil opens and allows gas to flow. The gas ignites and is drawn into the heat exchanger by the draft inducer. The flame sensor now must confirm that a flame is present in order for the gas valve to remain open. As long as the flame sensor senses a flame and the thermostat calls for heat, the burner will continue to fire. Once the thermostat reaches the preset temperature it will cut power to the heating controls. The blower will remain on for a preset time to continue to draw the remaining heat off of the exchanger.

The flame is drawn into the heat exchanger by the draft inducer. The hot gases heat the metal surfaces of the heat exchanger. The gases cool down as they give up their heat to the heat exchanger and they ultimately pass through the draft inducer blower and out of the home through the flue vent.

Two safety mechanisms are also employed, a limit thermostat to guard against overheating and a flame roll out switch to cut power if the burner flame is not fully drawn into the heat exchanger by the draft inducer.

How the Ducts Work

The air handling duct work carries "cool" air to the furnace through the cold air return. The cool air typically passes through a filter before passing over the heat exchanger. As the air passes over the steel surface of the heat exchanger, it absorbs heat from the metal and carries it away in the ducts to the rooms of a home. A blower connected to the plenum moves the air through the ducts and over the heat exchanger.

How the Flue Vent Works

The combustion of natural gas, propane or fuel oil results in the creation of deadly combustion gases. The flame, heat and gases from the furnace are all contained separately and pass through one side of the heat exchanger while the home's air is passed over the hot metal on the other side of the heat exchanger. In this way, combustion gases are kept safely away from the air circulated through the home. The hot gases are vented away from the home, usually through a rooftop flue vent.

What is a High-Efficiency Furnace?

A high efficiency furnace is one that operates at a high AFUE rating, generally 90% or greater. AFUE is the "annual fuel utilization efficiency." What it means is that for every dollar spent on fuel, the AFUE percentage is converted into useful heat and the rest is waste. So, with a 95% efficient furnace, 95 cents of every dollar is converted into heat for the home and 5 cents is wasted, usually as heat going up the combustion gas flue vent.

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