Hopefully, you know that you should change you furnace and A/C filter every couple of months. But what kind of air filter should you buy?

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If your HVAC system came with a user manual, then it probably says what kind of air filter you should use, but if you’re like most homeowners, you probably have no idea where the user manual is for your HVAC system (if you even still have it in the first place).

Here are some things you should know about your HVAC’s air filter, and which one you need for your particular system.

The Furnace Filter and A/C Filter Are the Same Thing

First of all, you may think that your heating and cooling systems use their own separate air filters, especially since air filters are sometimes colloquially known as “furnace filters”—it’s easy to think that the furnace has its own filter, while the A/C also uses a separate one.

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However, both your furnace and A/C share a single filter. With forced-air systems, air gets sucked in through the return vents from all around your house, goes through the air filter, and then through the HVAC system where the air is either heated or cooled, depending on what you have your thermostat set to.

This is why it’s important to change your air filter year round, since both the heating and cooling units take advantage of that air filter.

Where Does the Filter Go?

Knowing what kind of air filter to get is good, but it’s useless information if you have no idea where the air filter goes in the first place.

In almost every traditional forced-air system, the air filter is placed between the return duct and the HVAC unit itself. There will usually be a cover with a handle on it that you can remove to gain access to the air filter.

Take off that cover and you’ll see the air filter in the slot. From there, you should be able to grab onto it and easily slide it out.

If your HVAC system is in your attic, the air filter may be in a vent in the ceiling. You’ll find a few levers that you can move to open the vent.

The filter is easy to grab—in fact, it may fall out of the vent if it isn’t snug, so be ready for that.

Which Direction Does the Filter Need to Be Facing?

You might think that you can insert the air filter into its slot in any orientation, but they actually need to be placed in a specific direction. Luckily, it’s pretty easy to figure out which way it needs to go.

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First, you need to figure out which direction the air flows in your HVAC system. Sometimes it will tell you right on the outside of the unit, either with an official marking or someone just drawing an arrow.

If there’s no marking, you can simply look at the overall setup of the HVAC system and know from that. Locate the return duct and the main unit. Air flows from the return duct into the main unit. So in my case, air flows to the left.

Next, take a look at your air filter and you should see an arrow located somewhere along the edge of the filter. This arrow needs to point in the direction of your HVAC’s airflow.

If you have an attic vent in your ceiling like the one pictured further above, the arrow should point up toward the HVAC unit.

Find the Right Filter Size

Air filters come in all different sizes and range from about an inch thick to a few inches thick. It’s important to know the size of the air filter you need. Otherwise, well…it won’t fit.

Usually all you have to do is pull out the old filter and note the dimensions that are labeled on the edge of the filter. However, if that’s not possible and you have no idea how big of an air filter to get, it’s time to get out the measuring tape.

Start by measuring from the bottom of the air filter compartment to the top. Then measure the thickness of the slot. After that, extend the measuring tape and stick it into the compartment until it touches the back wall—measure from that point to the entrance of the compartment.

You’ll end up with three numbers (in inches), which is the air filter size you need (usually denoted as something like 16x25x1 on the side of the filter). The numbers you end up with may not be exact, but generally the filter size closest to your numbers is the one to get.

Check the MERV Rating

Air filter size is not the only important factor, but also its MERV ratingwhich stands for Minimum Efficiency Reporting Value. This is technical speak for how good the air filter is at catching dust particles and keeping them from recirculating into your house. A MERV rating of 1 is the worst rating, while a MERV rating of 16 is the best. What this means is that a MERV 16 air filter will catch more dirt, dust particles, allergens, etc. than a MERV 1 air filter.

You might think that a MERV 16 air filter is the one to get without question, but if your HVAC system isn’t capable of handling such an air filter, you’re in for some trouble. Thicker air filters are great at catching dust particles and allergens, but they also heavily restrict airflow from passing through, so you need to make sure that your HVAC system has a powerful enough fan to handle something like a MERV 16 filter.

You can usually find this information in the owner’s manual, but if not, do some experimenting with different MERV levels, starting with lower ratings and working your way up until you think your HVAC system starts to struggle. You can also call a HVAC technician to inspect your system and have them recommend an air filter.

Some air filter brands won’t have MERV ratings marked on the filter, with 3M’s Filtrete being a good example. Instead, they use their own numbering system called MPR (Micro-Particle Performance Rating). This web page provides a basic conversion from MPR to MERV, which should help you out if you happen to switch air filter brands that use different rating systems.

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Craig Lloyd is a smarthome expert with nearly ten years of professional writing experience. His work has been published by iFixit, Lifehacker, Digital Trends, Slashgear, and GottaBeMobile.
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