A furnace filter’s primary job is to keep dirt and debris out of the furnace, where it can damage parts and shorten the lifespan of the unit. Its second function is to filter the air circulating throughout your home to reduce dust and allergens that could be harmful to your health.
There are many filter designs and rating systems. Basic filters can remove larger, heaver particles from the air but allow smaller dirt particles to pass through. Large particles include airborne lint, pollen, and mold spores. Medium particles include dust and animal dander. The smallest particles include smoke and smog that are under 0.3 microns. By comparison, a human hair ranges from 3 to 200 microns.
The American Society of Heating, Refrigeration, and Air Conditioning Engineers has devised a rating system for filters. It uses numerical values ranging from 1 to 12, with the higher number capturing more of the dust.
Manufacturers often use uses the Filter Performance Rating (FPR), which is based on the ability of the filter to capture particles from 0.3 to 1.0 microns. These sub-micron particles are most likely to be inhaled, where they can cause problems in the lungs. This rates filters in a range from 300 to 1600.
Types Of Furnace Filters
Fiberglass or cellulose pad – This type of filter is usually designed with a cardboard frame. It’s capable of protecting the equipment by catching most of the larger dust particles which tend to block the heating and cooling coils. While inexpensive, they are the least effective at removing small particles.
Washable/reusable filters – These use a flat plastic or metal foil pad which can be washed off with a hose and reinstalled. Some of these can be sprayed with a tacky coating material to increase their ability to catch smaller particles. Washable filters should be washed monthly and may last 3 to 5 years.
Pleated polyester filter – Provides more filtering capacity than a flat filter; many are made with electrostatically charged fibers that attract small particles; lasts up to 3 months.
Deep-pleated, high-efficiency air filter – About the same size as the ordinary filter but 4 to 6 inches thick, they do not fit in standard filter holders and require a special box be installed inside the duct system. Electrostatically charged fibers are often with the filtering media.
Electronic air filter – About the same size as the box for a deep-pleated filter and requiring electricity to operate, electronic air filters work by directed air through a high-voltage grid which applies a positive charge on particles in the air stream; particles are attracted to a negatively charged element.
While they have a high initial cost and can require more maintenance, electronic air filters can effectively remove most indoor pollutants, including smoke particles.
How effective are different kinds of furnace filters?
The effectiveness of various filter types in removing sub-micron particles is:
- Fiberglass up to 2%
- Washable/reusable up to 6%
- Thin pleated up to 11%
- Deep pleated up to 25%
- Pleated electrostatic up to 49%
- Electronic up to 94%
Have questions about furnace filters and indoor air quality? Call Superior Plumbing and Heating. We’re here to help answer all your home heating and cooling questions.