In the past, gas and oil furnaces relied primarily on gravity to move combustion byproducts away from your home. These gases are extremely harmful, so it's important to keep them safely away from living spaces. With old, standard-efficiency furnaces, so much heat energy remained in the exhaust stream that the gases could naturally rise through a flue pipe and exit the home.
However, modern furnaces extract more heat from the combustion process.
Any system that moves air through your home will inevitably collect dust, hair, allergens, and other small particles along the way. If you've ever replaced the filter in a small room-sized air purifier or cleaned the top of a ceiling fan, you know how quickly these problems can develop. Your home's HVAC system uses a filter to help protect its components from dust, but it's not a foolproof solution.
Contaminants can still make their way past the filter, allowing dust and other particles to pass over your evaporator or heat exchanger and, eventually, travel into your home's HVAC system.
