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How to Change your Furnace or Air Conditioner Filter

changing furnace filter

All furnaces are equipped with an air filtration system designed to trap dirt, dust and other pollutants. Maintaining a clean furnace filter not only helps keep your furnace's interior components clean and in good operating order, but also contributes to improved air quality by reducing the amount of allergens and harmful particles in the air.

While service technicians may replace your furnace filter during an annual inspection, these filters are working constantly to keep airborne pollutants out of the air you breathe and away from the operating parts of your furnace. With everyday debris constantly piling on, changing or cleaning your furnace filter frequently is essential to maintaining the unit's optimal operating efficiency -- and thereby keeping your monthly bill low. But how do you replace your filter? And how often is often enough?

Understanding the Need for a Furnace Filter

First, let's take a step back to understand how your filter operates and why it's such an important part of your HVAC system. The furnace operates through a cold and hot air return process. The process starts in the cold air vents as cold air travels through the return air ducts and passes through the furnace filter to start the heating cycle. As the cold air moves through the system, hot air return begins at the furnace, travels through the heat exchanger and then out through the vents into the home. When the warm air is forced into a room, it replaces the cooler air and the cold air moves back through the return vent to begin the process again. As the air passes through the system, dust and dirt particles in the air are trapped by the filter, which prevents these tiny particles from building up on the interior components of the unit and slowing their operation. If your filter isn't changed regularly, it becomes more difficult for air to pass through these returns, which in turn means your furnace is performing overtime to keep air moving.

Timing Your Replacement

Generally, air filters should be changed or cleaned every few months, before the filter is full. Whether that should take place every month or every three months will depend on the filter manufacturer's directions; be sure to carefully read all directions supplied with your new filter. You may also want to visually inspect your filter more frequently to see if a sooner replacement is necessary. If you have pets or smoke, for example, you may find your filter turning black with pollutants sooner than you might otherwise expect.

The exact location of the filter varies depending on the furnace's make, model and manufacturer, but they generally are found near the fan motor, which is kept in the blower chamber. Filters also may be near the system's ductwork. Some may be exposed and easily located from the exterior of the unit, while other furnaces are manufactured with the filter inside the fan enclosure. These units require removal of an access panel to uncover the filter. Homeowners should consult their owner's manual for factory instructions on how to change their individual unit's filter.

Still have questions about timing? See our Frequently Asked Questions page.

Choosing a Replacement Filter

Not sure which filter you need? Find your filter.

Furnace filters come in a variety of types, including fiberglass, HEPA, polyester, electrostatic and pleated filters. Each type has different capabilities and performance based on the size, density of the material and amount and size of particles it's designed to filter. If your furnace doesn't mandate the use of a specific type of filter, you can refer to the Furnacecompare.com air filter guide to find the style that best fits your needs.

In addition, filters are available in reusable or disposable options. Reusable filters still require replacement every few years, but on a monthly basis simply need to be cleaned with water or a vacuum, depending on the level of pollutants captured. Disposable filters require a homeowner to purchase a new filter every few months, but eliminate the potential for exposure to contaminants during the cleaning process. This filter type is regarded as more efficient at trapping smaller debris. Although reusable filters may seem more environmentally friendly, they generally are less effective at trapping small dust particles, which may reduce the overall efficiency of the unit. Regardless of the type, air filters are most effective when new and clean.

Once you determine the type of filter you want, it's important to ensure you purchase the correct size. The simplest way to determine what size air filter to buy is to look at the old filter being replaced. The size is typically printed on the filter frame and is presented with the dimensions measuring width by height by depth (for example, 20" x 25" x 1"). If the size of the filter is not on the frame, it can be found by measuring the height, width and depth with a tape measure or reading your furnace's owner's manual.

What's in a (Brand) Name?

Both brand name and generic filters can be used in heating and cooling units. Generic filters often have the same specifications as brand name filters and may cost less. Buying a filter in the right size will have a greater impact on performance than the brand. Installing the wrong size filter can allow air to bypass the filter and leak through to the blower fan, causing excessive dirt build-up, slow air flow and a soiled blower fan. Depending on the severity of the build-up, this can lead to more costly repairs, cleaning and service. If you are concerned about purchasing the wrong furnace filter, don't hesitate to talk to your maintenance technician about their recommendations for keeping your unit running in top shape year round.

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