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How often should I replace my air filters?

I was in one of those gigantic home improvement stores the other day. You know the ones: they have a big orange or blue sign on the building and look as though you could park a fleet of 757 airliners under the roof. I was looking for light bulbs and I walked by the aisle that stocks the furnace filters.

Two massive rows of air filters stacked end to end and up to the ceiling. Wandering aimlessly up and down the aisle were close to a half dozen frustrated homeowners in various states of angst and confusion scratching their heads and rubbing their chins. They were all looking around, glancing at their phones or a scrap of paper in hand. There was a shopping cart, partially filled with random items, pushed off to the side, apparently left there by someone who had abandoned all hope and fled. I knew that feeling well.

For the most part we don’t give much thought to our furnace’s air filter until we’re wiping a film of dust off of a shelf or table, suffering from an asthma attack, or hear the cat sneeze. Then we get in the car, drive to nearest big box home store, and face-off with a wall of choices and an overwhelming, incomprehensible, variety of prices and specs. There’s the two-dollar filter that says it lasts 30 days and the $20 filter, right next to it. The $20 one is made by the same people who make tape, glue, and sticky notes and it claims to last 90 days and will “clean the air.” Who wants to come back here in a month? The $20 filter it is and back home we go. Sound familiar? Here are some tips to get started on what you’ll initially need to consider:

  • How often you’ll need to change your filter depends on a few things: the type of furnace, the type of filter, the size of the furnace, the size of your home and how dusty your home is. A large home requires the furnace to move more air and the filter will clog faster. If you have pets, you’re going to need to change your filters twice as often—not only are pet dander and pet hair considered allergens, they also clog filters quickly. So, a beginning rule of thumb is to start with a pleated 1” filter that you will change every two months. If you have a large home or pets, you’ll need to do it every month.

  • Consider buying media filters. They typically last 3 (or more) months and can save money over the long run. Filters are rated on a scale called Minimum Efficiency Reporting Value (MERV) and the higher the MERV the more dust and microbes are filtered.

  • You might want to buy your filters by the case (usually 6 to 12) and store them near the furnace. This should help you avoid putting-off changing them and will potentially save you some money. Try to get into a routine of regular filter changes, like you do with your car’s engine oil. Set a reminder on your phone if that helps.

  • Never run your system with the wrong filter size. A poorly fitting furnace filter will allow dust and other contaminants to seep past the filter and into your home. In the case of air conditioners, this could also cause clogs in the coils. Clogged A/C coils greatly reduce efficiency and compressor life and may cause expensive problems down the road.

  • Before replacing your filter, always turn off your furnace at the breaker panel, or disconnect the unit (not just the thermostat.) Some furnaces have electrical wiring close to where the filter is located that could shock you if the power is not disconnected.

To wrap up, don’t guess. If you aren’t sure about where your furnace filter goes, what direction to install it, or what size/type to get, ask a professional.


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