All About Carbon Air Filters for Home/Window AC Units

If you’ve ever wondered why there’s a filter in your window unit air conditioner, or why your change filter light keeps randomly coming on, or whether or not you need another filter on top of your first one, this is the article for you. We’ll break down the basics of filtration and how activated carbon filters work. You’ll be able to answer questions like, "Are carbon air filters safe?" and "Do I need an activated carbon air filter for home air purification?"

Let’s start with the basics of filters. If you’ve worn a mask lately, you understand the principle of filtration. A filter, in its broadest sense, is an impediment to the flow. The Britta in your fridge filters the flow of water, your shower grate works to filter hair out of the drain, and in a larger sense, the walls of your home filter the weather out of your bedroom. 

But not all filters operate identically. In the Windmill AC, we’ve included a washable anti-microbial mesh filter. This filter stands between the intake and evaporator in your window unit, catching dust and other junk you don’t want recirculating in your home. When your change filter light comes on, that means the AC senses a buildup of crud on the filter. In some units, that means you need to toss your filter for a new one, but the Windmill’s mesh filter is reusable and easy to clean. If you ignore the change filter light and let grime build up, eventually, a standard air filter stops letting the air through. We’ll talk more about that a bit later on.

Activated carbon filters work a little differently than standard filters, through a process called Adsorption. That’s not a typo—adsorption and absorption are related, but distinct processes. When a sponge absorbs water, the water molecules mix in with the sponge. Once the sponge has reached its absorptive capacity, it can’t take on any more water. Adsorption occurs when a gas, liquid or dissolved solid adheres to a surface. Activated carbon is a great adsorbent for filtering organic substances, so it’s the ideal backup for the mesh filter in your AC. Mesh filters won’t catch odor-carrying gasses, mildews, or other volatile organic compounds, but an activated carbon filter will snare them via the magic of adsorption.

How important is it to stop these volatile organic compounds? The EPA notes that VOCs may “have short- and long-term adverse health effects.” The EPA’s Total Exposure Assessment Methodology (TEAM) studies found “levels of about a dozen common organic pollutants to be 2 to 5 times higher inside homes than outside…. Additional TEAM studies indicate that while people are using products containing organic chemicals, they can expose themselves and others to very high pollutant levels, and elevated concentrations can persist in the air long after the activity is completed.” Getting an activated carbon air filter for home use can make a huge difference.

A common misconception is that activated carbon filters are HEPA filters. HEPA filters (which stands for High Efficiency Particulate Air filters) remove at least 99.97% percent of any airborne particles that are smaller than .3 microns. A HEPA filter has a MERV (Minimum Efficiency Reporting Value) rating of 17. On the other hand, an activated carbon filter’s MERV rating is closer to an 8. That’s a lot of technical jargon to state that, no, activated carbon filters are usually not HEPA filters. With MERV scores, the higher the number, the more restrictive the filter. We’ll come back to that in a minute, but for the time being, remember that activated carbon filters are not HEPA filters.

Now, to the next question: are carbon air filters safe for your machine? The short answer is yes. The longer answer is yes, but they stop working eventually. Remember how you have to clean your mesh filter to stop it from gunking up? Adsorptive materials work similarly. Eventually, your activated carbon filter’s adsorptive surfaces will be fully covered, and just like a sponge at its absorptive limit, the filter will stop working. Once the filter can’t adsorb any more, additional substances can dislodge crud from the adsorptive layer. Basically, once an activated carbon filter burns out, it actually starts contributing bad odors and pollutants into your home!

Again, that’s not an issue if you swap in a carbon air filter replacement when your filter expires. So how long is the average activated carbon air filter lifespan? A good rule of thumb is to change out your filter at most every three months.

Depending on how heavily you use your unit, you may experience a shorter average activated carbon air filter lifespan. You should also consider the temperature and humidity in your area when using an activated carbon filter. If either temperature or humidity reaches a high enough point, the carbon in the filter will begin to desorb sooner. Fortunately, your window unit produces cool, low-humidity air, which is the ideal working condition for an activated carbon filter. 

If you notice your average activated carbon air filter lifespan is under a month, there’s a few different common issues. One, like we just discussed, is the weather. At peak humidity season, filters will wear out faster. If an activated carbon filter gets wet or is exposed to extreme heat, the carbon will be altered and the filter will have a shorter lifespan. In fact, activated carbon is flammable—you might be familiar with a type of activated carbon we call charcoal. But there’s no need to worry; as long as you don’t store your filters in a fireplace, there’s no ignition risk.

So now we can answer, “Are carbon air filters safe for my home?,” with a resounding yes. Make sure you store your activated carbon filters in a cool, dry place. If there’s ever a bad smell coming out of your window unit air conditioner, power it down and check the activated carbon filter. Most likely, that’s the culprit.

However, a bum filter can cause issues in more ways than one. Remember earlier when we discussed HEPA filters? We mentioned that, with MERV scores, the higher the number, the more restrictive the filter. That’s great if you’re putting the filter in a machine that can handle it, but what happens when you put an overly-restrictive filter in your window AC unit?

Well, every part of the machine operates at a lower efficiency. With a clogged filter, your unit’s fan has to work harder to pull in enough air. Cool air and humidity sometimes get trapped behind clogged filters, causing moisture to accumulate inside your unit, and sometimes even freezing the inner workings. Remember, moisture and carbon don’t mix well, so you’ll need a carbon air filter replacement if you notice ice on your machine.

Of course, gunky filters aren’t the only cause of frozen ACs. Before you install your air conditioner, make sure there are no curtains or furniture blocking any of the vents. It’s possible that your unit froze due to a low coolant level. If you encounter a frozen AC, power it down and check the filter. If your filter looks okay after a clean, reach out to a professional. While we believe in your DIY skills, coolant and other toxic chemicals are best left to the pros.

If you’re a Windmill customer, check out our subscription-based, activated carbon air filter replacement program. We’ll mail you four filters each year, so you’ll always have the replacement ready. Our carbon filters are built specifically for Windmill machines and fit easily into our washable mesh filters. For other brands, check with your manufacturer to see if you can use supplemental filters, and where to install them to preserve airflow through your air conditioner. You want to filter out VOCs and other pollutants, but not block off the air through your machine. 

Alright, so back to our original questions: “Are carbon air filters safe?” Yes! As long as you make sure to change them regularly and store them properly, activated carbon air filters are safe for your home. “Do I need an activated carbon air filter for home air purification?” You don’t, but it’s a good idea to grab one. Activated carbon doesn’t trap particles like a HEPA filter would; instead, the carbon lures in adsorbents and coats itself in all the junk you don’t want to breathe. Because they work via adsorption, activated carbon filters deodorize your air as they clean it. If you’re looking for low-cost and environmentally-friendly air purification, consider an activated carbon air filter for home use.