How Does a Furnace Filter Work?

The furnace filter is a way to filter the air in your home. Newer furnaces reuse air inside the house by heating it again, which increases their efficiency over older ones. The oldest furnaces pulled in air from the outdoors and heated it. Then, they realized it was more efficient to use air that was already warm.

Before There Were Furnace Filters

Since there was no filter in older furnaces that reused indoor air, the dirt and dust in the air collected on the motor. This would cause the motor to overheat and wear out before due time, and decrease efficiency dramatically over time. This effect can be seen clearly in the fins of an air conditioner unit if there is no filter.

How a Furnace Filter Is Constructed to Work

The furnace filter has fibers of variable thickness, made of flexible material, laid under and over each other many times over. This creates an intricate web, and there is not pattern in it. This acts to catch larger and smaller particles together. A lattice type of woven backing is applied, both for structure and strength. Each time the furnace draws air from the inside the air is sucked through the filter, and the filter catches dust and dirt particles in the process.

Filter Filters Aren't Air Purifiers

Although the furnace filter is designed to trap dirt and dust, it does not do so perfectly as an air purifier does. Nanoparticles, which are microscopic particles, do get through. A furnace filter is not necessarily intended to clean indoor air as an air purifier does, but it does keep most of the dirt and dust away from the motor. It is still a good idea to clean around the furnace motor periodically.