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Learn More About Furnace Filters

If you have a question about furnace filters that is not covered here, please e-mail us at and we will do our best to answer it.

What a furnace filter can do for you?

Traditionally, furnace filters were designed to protect the furnace and fans. With increased air quality awareness, some filters are now being installed to reduce exposure to particles which can affect your health.

What airborne particles are found in your home?

The particles you breathe in your home come from a variety of sources including:
  • dust on floors or other surfaces that is disturbed by activity in the house
  • dust generated by smoking, burning candles, cooking, doing laundry, etc.
  • hair and skin flakes from humans or pets
  • and particles from the outside air which come into your home with infiltrating air
Some particles are so small that they are inhaled and then exhaled without being trapped in your lungs. Some larger particles are trapped in your nose and throat and never reach your lungs. Still other particles are too large to be inhaled.The particles most dangerous to you are those that enter your lungs and lodge there.

You can see the particles of dust which accumulate on your television screen, shelves, and furniture. But you can't see the respirable particles. Respirable particles can be easily inhaled into your lungs and provoke respiratory illness. Although you would probably like to keep visible dust out of your home, the main health risk comes from respirable particles, which include tobacco smoke, spores, bacteria, and viruses.

The activity levels of the people in your home can affect the air you breathe. Activity such as vacuuming and cooking can create or stir up particles. On the other hand, during periods of inactivity such as the middle of the night, particle concentrations tend to be much lower.

Filter research

CMHC - Canada Mortgage and Housing Corporation, the department of the Canadian government that deals with mortgage insurance, assisted housing, and various forms of housing research - conducted a study to verify filter manufacturer claims and to determine whether good filters will significantly reduce your exposure to airborne particles.

The CMHC study first tested ten filter types in a single home and then the following filters in 5 additional homes:

Air in the houses was tested when these higher efficiency filters were in use. The results were compared to when no filter was used.

According to the test results, the ESP filter proved to be the most effective, with 1" pleated and HEPA bypass filters running a close second.

Despite being the most effective filter in the tests, the ESP produces small amounts of ozone during operation. Since exposure to elevated ozone can irritate your lungs, separate testing was done to verify whether the amount of ozone produced by the ESP could affect the occupants of the home. In the research project, a survey of fifteen homes with ESP filters showed that all ESPs created ozone in the air stream of the duct, though none of these raised ozone levels in the house air above the safe concentrations recommended by health guidelines.

This research showed that the particles in the duct air can be reduced when an upgraded filter is installed. The results also showed that this reduction will only moderately reduce indoor exposure to respirable particles. So... how do you reduce levels of respirable particles in your home?

The best current advice is to reduce dust entry by:

  • removing footwear on entry
  • keeping major dust generators (smoking, pets, etc.) out of the house
  • reducing dust collecting surfaces (open shelves, carpets, upholstered furniture, etc.)
  • diligent and frequent vacuuming with an efficient vacuum cleaner
  • reducing the entry of particle-laden outdoor air by closing windows, improving house airtightness, installing an intake filter on the air supply or using an indoor air purifier
  • using a good furnace filter
Most of these recommendations will also reduce the amount of visible dust in your house.

What is an efficiency or MERV rating?

Most filters are labeled with a MERV (Minimum Efficiency Reporting Value) rating number, which measures a filter's ability to trap particles ranging in size from 3.0 microns to 10.0 microns. This number is derived from a test method designed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) and is intended to help people compare filters.

Residential filters commonly have MERV ratings of 1-11. The higher the MERV rating, the more efficient the filter is, and the more particles it can filter.

  • A MERV rating of 6 means the filter is 35% to 50% minimum efficient at capturing particles, sized 3.0-10.0 microns.
  • A MERV rating of 8 means the filter is 70% to 85% minimum efficient at capturing particles, sized 3.0-10.0 microns.
  • A MERV rating of 11 means the filter is 85% to 95% minimum efficient at capturing particles, sized 3.0-10.0 microns.
MERV is an industry standard rating, so it can be used to compare filters made by different companies.

Choosing the right furnace filter

Listed below are some of the most common furnace filter types.

Disposable fiberglass filters are the least expensive – and also the least effective, designed to block only large dust and dirt particles to protect your furnace. Smaller particles, like pollen and mold, pass right through.

Washable "electrostatic" filters have a static charge that attracts dust, dirt and other matter. These filters are slightly more effective than disposable, but still block only 15 to 20 percent of airborne particles.

Pleated "allergy" filters use a much denser mesh material to trap particles. The pleats increase the surface area, eliminating large allergens like pollen and mold – most are 35 to 50 percent efficient.

Electronic air cleaners use electrodes to create an ionized electrical field that "magnetizes" pollutant particles and collects them on the filter material. An electronic unit can eliminate virtually all pollen and mold spores, up to 94 percent of smaller particles, and even up to 80 percent of airborne viruses.

HEPA furnace filters, which stands for High Efficiency Particulate Air filters, are the "gold standard." These filters are used in commercial applications like hospitals, research facilities and electronics manufacturing where clean air is vital. HEPA filters not only block air particles, but also air flow, so they’re not recommended for residential use.

If allergies and respiratory problems aren’t a concern for your family, a disposable or washable filter can be enough to keep your furnace clean. But it’s important to change or clean the filter every month – remember that dust and dirt are the cause of half the repair calls to service technicians!

A pleated filter is a cost-effective option for reducing allergens. These filters cost around $5-$15 each, but are designed to last up to three months.

A high-efficiency pleated air filter costs around $250 to $450.

Electronic air cleaners are expensive, ranging from $650 to $850, but they’re the best at removing harmful particles and pollutants from the air. In many cases, the initial cost can be offset with lower medical bills.

No matter what type of filter you choose, it will work best if you keep the blower fan running continuously.

According to Home Energy magazine, if the fan is set to "auto", it’s running only 20 percent of the time – meaning your furnace filter will trap only 20 percent of the 50 percent of the particles it’s designed for – resulting in only a 10 percent overall efficiency.

Setting the fan to "on" will also help keep air circulating through your home, preventing the warm air from rising and pooling near the ceiling and increasing your comfort.

Cleaning and replacing furnace filters

Has it been more than a month since you ventured into your furnace's filter area? Then it's probably time to clean or replace this dust and dirt catcher to prevent you from sneezing your way into winter mornings. A clean filter maximizes your furnace's efficiency and longevity - and minimizes your energy bills.

  1. If the floor or area near the furnace is a dust-bunny breeding area, vacuum or sweep prior to replacing the filter.
  2. Locate the service panel, usually on the furnace's lower front or side.
  3. Turn off the furnace, then gently pop open or pull down the panel door with your hands; tools usually aren't needed.
  4. Locate the filter - a framed-mesh rectangular screen inserted either horizontally or vertically near the intake-outtake blower.
  5. Slide the filter screen out.
  6. Check for brown, dusty buildup on the mesh screen (or a screen you're unable to see through).
  7. If you have a reusable plastic-frame or metal-frame filter, use a hose to rinse away the dust particles on the screen in the backyard or driveway. Let it dry, then return it to the furnace.
  8. If you have a disposable cardboard-frame filter, write down the size, then throw it away. Buy a new furnace filter of the same size (available at hardware and home supply stores).
    Tips and Warnings:
  • To keep the air in your home healthy, replace your furnace filter monthly during chilly months and at least once a season.
  • Be sure you turn off the furnace before opening the front panel.
  • Do not use the furnace until a clean filter is installed.
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