Forced Air Furnace Filter Q&A

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Dear NH,

I would like to know your opinion of the permanent washable filters that may be purchased for your central a/c heating unit. I have one and my bones tell me it is not as efficient as a 3M or some other throw away pleated filter.

J from Lubbock, TX

There is a compelling logic to your argument. A washable filter, by nature, would have to be more porous and rigid... otherwise the act of washing it would destroy it. To design a filter to be washable makes it less efficient. Fiberglass, which is the media of choice for throwaways, has a fibrous network with tremendous dust "grabbing" capabilities. However, washing is ill-advised... it into a mushy mass of plastic suitable only for a rat's nest!

Lets first establish some frame of reference.  Many standalone air filtration systems are two-stage. They use a flexible plastic foam as the primary filtering medium with either a paper or HEPA (high efficiency particulate air) secondary filter. The foam grabs the larger particles, extending the life of the more expensive secondary filter which removes the finest airborne particles.

If you look at a fiberglass throwaway furnace filter, it is also two-stage. That is why there is a directional arrow on the side indicating the air flow direction.  Yes... they can be installed backwards and will not work as well! Anyway, back to the design... the first layer of the mesh is coarser and more rigid, catching larger particles of dust. The secondary layer is finer and thicker to do a more thorough cleaning of the air.

There are, of course, furnace air filtering units that use an electrostatic charge to trap dust particles in a metallic filter. These are the most efficient air cleaners. However, they are much more costly yet hardly necessary for most folks… with the exception of very dust-sensitive individuals.

In my opinion, the cost of the throwaway filters is so low that monthly replacement will probably give better results that the purchase of an expensive washable filter. Besides, you don't have to go through the ordeal of trying to wash and dry the filter! As I always preach, time is money!!