When I first moved into my home, my father-in-law ran through several things I'd need to address as upkeep. Flush the boiler once a year, close water supply valves so pipes don't freeze and explode in the winter (no biggie), and replace the HVAC filters every three to six months.

That latter tip seemed excessive, but I had never lived in a home with central air as an adult, so I nodded and went to Home Depot, bought a filter sized to my return air vent, and installed it. Three months went by and I started wondering, “Do I just change it now?” I looked at the filter and it didn't look dirty, so it seemed a bit wasteful.

This confusion is exactly what Woosh's Smart Filter wants to solve. Founded by ex-Googler Winston Mok, this system is designed to give you a better idea of when to replace your filter. When paired with the company's Air Quality Monitor, it can double as an air purification system provided you have a compatible smart thermostat. Frying up some bacon? It'll automatically kick on your HVAC's fan circulation to push out the volatile compounds and keep your indoor air clean, like an air purifier but for your whole home.

I've had Woosh's smart filter system in my home for nine months now, and I like the visibility I get with my home's air quality. I vastly prefer seeing my filter's remaining lifespan in the app rather than opening up the grille, checking how dirty it is, shrugging, and running to Home Depot.

Smart Frame

First, you need to measure your return air vent to identify what size frame to buy. Woosh sells several sizes, but if the frame isn't available in your vent size you can sign up to be notified, as the company claims it's adding new sizes. The frame comes with a gateway, which magnetically sticks to the edge of your vent grille; it's what helps connect the system to the slick and well-designed Woosh app (androids, ioses).

The Smart Air Filter frame monitors the air flowing through by measuring the differential pressure across the filter. Basically, if you've barely used your HVAC because it's the shoulder season, the system will know that and factor it into your filter's lifespan. Woosh recommends changing the filter one year from installation, but if you use your HVAC a lot, you can see the percentage of your filter's remaining lifespan in the app and know more precisely when to replace it earlier.

After nine months, my filter has a 35 percent estimated remaining lifespan, and I'll probably swap it out soon. Conveniently, Woosh lets you order a filter refill through the app whenever—these 3M filters cost $22, $2 more than the filter I paid for at Home Depot when I first moved in.

Filters vary in thickness and have different MERV ratings (minimum efficiency reporting value). This rating indicates how well a filter can capture particles. A MERV 1 rating isn't doing much, but MERV 13 is the top recommendation for most residential HVAC systems, according to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. (HEPA filters have a MERV 17 rating or higher.) Woosh uses MERV 13 3M filters that are 1-inch thick, though thicker filters are in its roadmap.

When you pair the company's air quality monitor to the filter frame and your smart thermostat—I connected it to my Nest Learning Thermostat 4th Gen but it also works with brands like Ecobee, Sensi, and Honeywell—the system will trigger your HVAC fan circulation if indoor air quality is poor. This will run the HVAC fan until your air is clean again. I've seen this in effect so many times—when cooking, when I was fixing some drywall, and when I was painting. I like seeing my indoor air quality go from orange to green after a few minutes. These same situations triggered my freestanding air purifiers too.

Filter Folly

Here's the funny part. Nearly every time I talked to someone about Woosh's filter, they scoffed at the MERV 13 rating and told me to use a lower rating, as it would be better for my HVAC system. That includes my father-in-law and an HVAC technician (when he was flushing my boiler). Look around online and you'll also find widespread recommendations on Reddit and the like to use a MERV 8, especially if you can only use a 1-inch filter.

The reasoning is that the higher-rated filters can reduce airflow to the HVAC system, which makes it work harder and could put more strain on the system. This is a little out of my depth, so I contacted Amy Li, assistant professor in the Department of Environment and Civil Engineering at the University of Waterloo. She researches smart ventilation and filtration systems.

“It’s a helpful approach to monitor the pressure drop across the filter—this is a good way to know when it's the right time to change the filter because otherwise, there is a lot of guessing work,” Li says. She says the typical three-month replacement recommendation is based on the assumption of a 20 percent runtime of your HVAC system (about five hours a day). Much of this depends on your climate, but Li says a recent study found the median HVAC runtime of 20 homes over a year in Toronto was 10 percent, meaning those folks would be able to replace their filters less often.

As for the verdict on the MERV 13, Li says that a higher flow resistance will make the HVAC fan work harder. Depending on your system, the fan could stop working as hard, decreasing airflow and potentially damaging the cooling coils. However, a higher MERV rating does not equal a higher flow resistance. It depends on the filter quality, as Li says she has seen MERV 13 filters with lower flow resistance than a MERV 8. Ultimately, Li says the main factor is the age of your HVAC system.

“If this is a relatively new system in good condition, and you have been using MERV 8 filters with no problem, then I don't think there will be any trouble using MERV 11, MERV 13, or even MERV 14 filters just because even if there is a difference in the flow resistance, it's relatively small,” she says. “I think most systems will be able to handle that, especially if you don't change your filter for the entire year.”

Folks with older systems should be careful, as they may not be able to handle high-efficiency filters at all. She suggests listening to the sound and feeling the flow coming from the cooling and heating vents in your home.

The good news is that Woosh's smart filter can detect problems with your HVAC system. The company says it can identify early signs of coil freezing, which if left unaddressed can cause irreparable damage. It manages this by checking pressure drop and flow rate along with the temperature and humidity levels.

Thankfully, using a MERV 13 filter doesn't seem to cause any adverse effects on my system and I haven't received any notifications about problems with my HVAC. My only concern is that, according to the app, I still don't need to change my filter even though it's been nine months since I installed it. My wallet is happy, but everything I've heard and read about filters suggests I should have changed it by now.

Li also says she is surprised to hear that Woosh's 3M filters have a baseline replacement recommendation of a year, though she says it depends on the filter technology. The company says its filters outperform those in the market, and compared to the MERV 13 filter I previously bought, Woosh's filter has more pleats per inch, and this makes it more efficient, allowing it to trap more particles without a huge pressure drop.

Woosh's CEO Winston Mok tells me filters need to be replaced when the pressure has increased by 50 percent from when it was new, and my filter's data shows that its pressure has gone up by 28 percent of that 50 percent allowance (as of November 2024). I just have to trust the data and the 3M filter, but this is precisely the selling point of Woosh's Smart Air Filter—save some cash and replace your filter only when you need to, and still get the benefit of cleaner air indoors.