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The Best Way I Know to Slash the True Cost of Dirty HVAC Filters

Do I have a story to tell you—another lesson I’ve learned the hard way so you won’t have to. Truth be told, if my faux pas helps you avoid a huge expense, I’m happy to have suffered it.

While researching to help a reader solve the mystery of black grimy lines around the edges of his home’s carpet, I casually asked my husband if he’d replaced the filter in our heating ventilation air-conditioning (HVAC) system recently. I got one of those blank stares I could easily translate: Nope, didn’t even think about it.

When we bought this house, we had the HVAC system inspected, serviced and the filter replaced. Then we got busy with leasing it while we planned and executed our big move a year later. HVAC filter? Completely forgot about it. By the time I brought up the subject, it had been more than 2.5 years!

A close up of a sign

The scariest thing ever was to open that door on the HVAC system, dutifully labeled “Filter.” I cannot adequately describe it but I can tell you that it was nearly black and covered in what looked like fur. So gross. I’m surprised the entire system didn’t just blow up out of sheer rebellion for lack of attention.

I have since learned about the true cost of a dirty HVAC (heating, ventilation, air-conditioning) filter and it’s not pretty!

The True Cost

High energy bills

While the heat, ventilation or air-conditioning is running, 100% of the air in your home passes through a filter, typically twice every hour. When the filter is clean, the air passes through easily. But if the filter gets clogged up with all the stuff it’s filtering out, the system has to work much harder to keep air moving.

The dirtier the filter, the less efficient the system can be—and more likely to develop problems. According to the Department of Energy, regular filter changes can reduce your energy bill by 5% to 10%. Conversely, a dirty filter will send that bill through the roof!

Family health

A quality filter captures the harmful bacteria typically found in sneezes, coughs, viruses and molds, as well as pollutants like dust and car fumes that come in through doors and windows.

If you have allergies or asthma, indoor air pollution can trigger your symptoms. Most people don’t realize that indoor air pollution levels are actually much higher than those outdoors—two to five times higher, according to medical professionals.

When particles become airborne, you can breathe them in and experience an allergic reaction. The heating, ventilation, and air conditioning (HVAC) system in your home can harbor and then spread mold and other allergens.

Grimy, dark carpet stains

It’s called filtration soiling and shows up as dark, shadowy, dirty lines on the carpet along baseboards, under doors, beneath draperies and along the edges and in the crevices of carpeted stairs.

A close up of a door

Filtration soil is as ugly as it is gross and comes from airborne pollutants passing through the carpet as the air is drawn through the crack between the carpet and the baseboard, around the drapes or under a closed door.

It’s an accumulation of soot from dirty ducts, smoke from candles and the fireplace; tobacco, kitchen grease from the oven and cooktop; smog, auto emissions, and pollutants from outdoors.

The way it’s supposed to work is that all of that stuff in the air gets trapped in the system’s air filter. But once the filter is full, the system will send the air pollutants back into the house through the ducts where all of that icky mess gets lodged into corners and crevices.

MORE: Grimy Black Filtration Stains and How to Get Rid of Them


The Simple Solution

Change the filter often

Check at least once a month, more often during heavy use months (winter and summer). Keep in mind that some systems have multiple filters. Take some time to research your system to discover exactly what and how many filters you have.

If the filter looks dirty after a month (it is no longer white) change it. Homes with pets, multiple occupants or when located in regions with general air pollution, may require more frequent changes. At an absolute minimum, the filter must be changed every three months,

A dirty filter will slow down air flow and make the system work harder to keep you warm or cool, wasting energy. A clean filter will also prevent dust and dirt from building up in the system leading to expensive maintenance and/or early system failure.


Mark your calendar. Set an alarm. Do something that will trigger your memory to check your filter so you can get to the store to buy a new one. Or a bunch so you have them on hand.

Don’t assume you’ll just remember. You probably won’t until one day, 2.5 years from now you notice your utility bill has shot through the stratosphere, you have this weird cough or you see odd, dark shadows forming along the edges of your carpet. Don’t let that happen.


My filthy filter wake-up call prompted me to get online to do some research. That’s how I found Second Nature (formerly FilterEasy), a company that delivers the exact filter we need, just when we need to change it—at a comparable cost of the very same filter at the home improvement center store near our home. Wow! What a great service for those of us who need reminders. I don’t have to think about when to replace the furnace filter.

I set up an account online, identified the exact filter we need (they’re listed by size—including custom sizes if the filter you need is not standard) and the frequency of delivery, which took all of about five minutes. Now I can forget because I’ve put Second Nature in charge of remembering.

We received our first shipment. Having that sizable box show up at the door was the reminder we needed to replace the filter—right then. Done.

You can probably beat the FilterEasy price if you buy filters when they’re on sale and are diligent in sticking to your own rigid maintenance schedule. For some of us, a subscription plan with Second Nature  is a real bargain given the high cost of failure to stay on top of this important home maintenance issue.

The folks at Second Nature are offering Everyday Cheapskate readers their first set of filters free (you’ll just pay to ship) when you use this LINK. Of course, you can cancel or make changes to your subscription at any time.

Some products displayed in this post may earn us a commission. Why trust us?


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  1. Mrs. Potts says:

    Our heat pump was replaced last year and a new thermostat was installed. On the first of the month, it lights up red to remind me to change the filter. So my filter was changed March 1. I will admit that there were several months over the past year that I did not replace the filter because the system had been used less than ten days that month.

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