Your Furnace Filters and Allergies

Sneezing, itchy eyes, runny nose? Maybe you are getting a cold or maybe your house air is full of irritants and allergens. Dogs, cats, guinea pigs, and even hamsters and birds can trigger an allergic reaction in their owners, let alone airborne dust, dust mites, smoke, mold, lint, and pollen. Since most of us can’t live without our pets or in a sterile environment, we need to find ways to cope.

Advice for dealing with allergens includes seeing a doctor for allergy shots or medications, using a vacuum with a HEPA filter, running room air filters, mopping floors, bathing or wiping pets to remove dander, and having your air ducts cleaned. In addition, your furnace filter can help keep the air clean if you change it on schedule or when dirty. Consider running your furnace fan continuously instead of setting to ‘auto’ mode in order to keep the air moving through the furnace filter but be aware that, if you have a single stage furnace, this will increase your energy bills. If you have a variable speed or modulating variable speed furnace (where the blower fan is on even when heating or cooling cycles are not occurring), you’ll actually see savings in your electric bill as the air keeps flowing through the furnace and filter.

When it’s time to replace your furnace filter, buying a new one a new can be a bit confusing due to all the choices out there. It is always safe to replace with a filter like the one your furnace came with. You can refer to your owner’s manual or the sticker on your furnace to find that information. If you are interested in finding a filter that removes certain particles better, it is helpful to understand the filter rating system created by American Society of Heating, Refrigerating, and Air-Conditioning Engineers. This rating system is called MERV – Minimum Efficiency Reporting Value – and ranks the ability of a filter to trap particles on a scale from 1 to 16. . The higher the value, the better the filter is at cleaning air, although usually at the expense of impeding air flow through the furnace. Typically, you’ll want a filter that is rated at 7 or 8. (Note that values from 17-20 aren’t officially part of the MERV standard, and reflect the performance of HEPA filters.) Home Depot uses its own rating system called FPR scaled from 1 to 10.

What I’m going to say next is probably the most important point of this article: It is not necessarily true that the best furnace filter for your home has the highest MERV or FPR rating! You must make sure that the filter is appropriate for your system in that it provides the necessary air flow. Your furnace was designed with a specified air flow requirement in order to keep it working efficiently and to maximize the life of the blower motor and heat exchanger. Often, the higher rated filters impede the air flow. In fact, true HEPA filters (MERV 16-20) are not appropriate for use in a typical furnace, and would definitely require some retrofitting of your furnace because of the air flow issues. (Read the fine print on your warranty if you are tempted to use HEPA filters!) As you may know, our furnace of choice is Trane. Trane recommends filters with MERV rating of 7 or 8 because these offer a optimal balance between capturing particles and allowing enough air to flow through the filter to not impact furnace operation.

After you make sure your filter complies with the air requirements of your furnace, you need to consider what level of air filtration you need. Furnaces need protection from the larger particles, so they are happy with around a MERV 2 filter. But you and your family may want the protection that a higher MERV gives you. The table below is from Wikipedia, and can help you decide what level of filtration you want.

There are three broad categories of furnace filters – nonpleated, pleated, and washable electrostatic. Nonpleated filters are usually made from spun fiberglass, are rated MERV 4 or lower, and trap particles at least 10 microns or bigger. These protect your furnace from gunk in the air, and provide some protection to you by filtering out larger particles such as pollen and dust mites Pleated air filters are made from fiberglass or synthetic, vary in pleat depth from 1 inch to 4 or more inches, and are available in a range of MERV or FRP ratings. The deeper filters means more surface area to trap particles, so it takes longer for the filter to get clogged. Your furnace may require modification by a professional to handle the deeper filters, and the depth may impede air flow. Washable electrostatic filters are made of layers of metal mesh that basically use static electricity to trap particles. They are usually in the range of MERV 4, and can be a pain to clean.

Believe it or not, someone is always trying to invent a better filter that may or may not be compatible with your furnace. For example, Home Depot sells a high air flow, triple core tacky media that traps particles that are 5 microns or greater and is good for a full year, rated 8 on their FPR scale. Consult your furnace manual or sticker, or better yet, call KJ Thomas Mechanical at 303-435-8141 or contact us here to get a recommendation for your particular situation. And remember to keep on top of changing your filter.