When should you change filters for your HVAC system?

How much will I really save?
By Arnie Katz
When should you change filters for your HVAC system?

Q My husband was very religious about changing the filter for our central air conditioner every month. He always said it would help the system last longer and would also save energy and improve our air quality in the house. He recently passed away, and with my arthritis it's impossible for me to climb up on the ladder to change the filter, which is in the hallway ceiling. I can hire a neighbor's son to do it for me, but my budget is really tight. How much will I really save?

—Beth, Robersonville

There are so many things in our homes that were not designed for those of us who are losing some of our physical capacities. The short answer to your question is that it's highly unlikely that changing your filter every month will save enough to pay for the new filters and to pay someone to change it for you. Most of the research I've seen on this hasn't documented any energy savings from changing the filters monthly.

So what is a person to do? The first thing to understand is that the purpose of the filter on your central cooling and heating system is to protect the coil from dirt and debris — leaves, twigs, gerbils, etc. It was not designed to improve the quality of the air in your house. The coil is where heat is exchanged, and if it gets dirty it moves the heat less efficiently. So having a filter in place is important.

In extreme cases — either because the filter hasn't been changed in a long time, or because a lot of stuff is loading the filter, or you just got three long-haired shedding dogs or there's a construction project next door generating a lot of dust — the dirty filter can slow down the air flow and create enough pressure in the unit to seriously strain the motor and cause premature failure. Sometimes, the filter gets so clogged that it gets sucked into the unit, and air just goes around it, along with the dirt and debris.

The key is to find the balance: change the filter often enough to prevent strain on the motor, but not more often than you need to, since that will be a waste of money. In most houses, changing it every few months will probably be about right. This should be often enough to prevent pressure build-up and damage. But pay attention to the filter. Change it after two months and look at it. If you can't see through it, you need to change it more often. If it looks almost brand new, you can go longer.

If you want to improve your air quality by using the furnace/AC filter to remove pollens, dust and other particles from the air in your house, you can get filters designed to do this. But it's important to make sure the better filter won't increase the pressure on the motor. One way to think about this is to picture a very dirty filter. There's so much stuff on it, that it becomes much more "efficient" at collecting stuff, and will collect more particles than a "clean" filter. Until it gets so clogged that the pressure builds up and it slows down the air flow and strains the motor.

Similarly, a "high efficiency" filter will collect more stuff, but it may reduce the air flow and put strain on the motor. It's important to get a qualified technician to certify that the more efficient filter won't hurt your equipment.

Virtually every website you go to recommends changing the filter often and claims that will lead to energy savings. Unfortunately, there's no research to back that up. It's possible that your husband developed the monthly schedule based on his experience in your home over the years, and that changing the filter monthly is really needed. It's also possible that he was simply following the advice from sources that seemed credible. Now it's up to you to learn what makes the most sense in your house.

About the Author

Arnie Katz is the former building science consultant for Advanced Energy in Raleigh. advancedenergy.org

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