If you have a 90% AFUE furnace, 90% of the fuel is used to directly heat your home. The remaining 10% is lost. High-efficiency furnaces have AFUE percentages above 90%. Think of AFUE as the gas mileage of your furnace.
This is an extremely important part. If it's not working, you won't receive any heat. Heat exchanger can also be dangerous if they develop any leaks or cracks.
This system is also known as a ducted warm-air or forced warm-air distribution system.
The energy efficiency of your furnace is dependent on its Annual Fuel Utilization Efficiency (AFUE), which basically tells you how much of your fuel is used for heating and how much is lost due to combustion (usually out of your vents).
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