How To Fix It

How To Tell Which Way Air Flows in an AC Unit?

Before you can properly install the air filter in your air conditioning unit, you need to know the direction of the airflow. The filter protects the operating system of your air conditioner from dust and dirt and prevents dust, dirt, pollen, and pet hair from continuing to circulate through the air in your home. So, how can you tell which way air flows in an AC unit?

The easiest way to tell which way air flows in an AC unit is by holding a strip of paper where the filter goes. Turn on the AC unit and watch the direction it blows. Another way to determine the direction of airflow is to look for an arrow in or around the filter slot.

Read on to learn how to ensure that your AC air filter is always properly installed. I’ll also answer how often you should replace your filter and how to choose the right one. We’ll also discuss how a properly installed filter improves your air quality and the negative effects of an improperly installed filter. 

How To Ensure Your AC Filter Is Always Properly Installed?

Here’s how to ensure that your AC filter is always correctly installed: 

  1. Before removing the old filter, look to see which direction the arrow points. 
  2. Take a permanent marker, and draw a diagram on the AC unit showing the direction of airflow. 

When the time comes to change your AC filter again, it’ll be much easier to determine which way the filter should face.

Read: How To Choose A Garage HVAC Unit?

Why You Should Install the AC Filter Correctly?

Here are a couple of good reasons why you should pay attention to the direction of airflow in your AC unit when installing a new air filter.

Incorrectly Installing the Air Filter Lowers the AC’s Efficiency

At a glance, both sides of the filter look the same. However, the side of the air filter that faces the operating system is denser than the side meant to face the air intake. When the filter is installed backward with the dense side facing the air intake, the fans and operating system must work harder to pull air through the system. 

As with a dirty filter, an improperly installed filter reduces the efficiency of your system. It forces your unit to use more energy and increases the wear and tear, leading to more repairs and an earlier failure. When the filter is dirty or not installed properly, it won’t clean your home’s air as effectively as it should. It’ll allow dust, mold, lint, and other contaminants to build up in the ductwork of whole house HVAC systems.

Read: What To Do When Your AC Unit Freezes Up Inside?

A Correctly Installed Air Filter Improves Air Quality

Properly installed air filters improve the air quality in your home by removing contaminants and particles that can affect your family’s health. It’s essential to know which way air flows in your AC unit to install it correctly. Air filters also make it easier to breathe for those with respiratory illnesses and conditions like asthma, emphysema, and allergies. 

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Air filters can remove: 

  • Dirt and dust 
  • Pollen 
  • Lint and fibers from upholstery, carpets, and rugs 
  • Human and pet hair and dander 
  • Mold and mold spores 
  • Bacteria and other microorganisms 
  • Wood and plaster particles 
  • Other undesirable contaminants 

When To Install a New AC Filter?

You should install a new AC filter every one to three months. Change fiberglass filters and wash electrostatic filters every one to two months. Change pleated filters every three to five months. Pet owners, smokers, and people living in polluted areas should change filters more often. 

Signs That You Need to Replace Your AC Filter

Signs that your filter needs replacing include: 

  • Decreased energy efficiency leading to higher utility bills
  • Reduced airflow into the air intake duct
  • AC shutting off before cooling to the temperature set on the thermostat
  • Inability to see through the filter

A dirty filter blocks the air flowing through your whole HVAC system. It forces your fans and AC unit to use work harder and consequently use more energy to draw air through the filter. The strain can damage your system if it continues for extended periods, causing frequent breakdowns and the need to replace your AC’s fan motor sooner. 

Read: How To Unfreeze An AC Unit?

How to Dispose of a Disposable Filter?

Here’s how to contain the dust, pollen, and other contaminants trapped in your old filter: 

  1. Have a plastic trash bag with you when you replace the filter. 
  2. Carefully place the old filter into the trash bag while avoiding banging or shaking it. 
  3. Tape or tie the garbage bag shut and carefully carry it through your house. 
  4. Put the bag into an outdoor garbage can. 

Read: Why HVAC Maintenance Is Important?

How To Choose the Right AC Filter?

There are four factors to consider when choosing an AC filter: 

  • Size 
  • MERV Rating 
  • MPR Rating 
  • Materials 


AC filter sizes are listed in the order of length x width x depth (L x W x D). The easiest way to determine the filter size is to check the dimensions of your current one. You can also measure the filter slot. Round up the numbers you get to the next largest whole number to determine what size to buy. This gives you the nominal size — the size listed on new filters. 

Like most filters, disposable filters usually measure 1 to 2 inches (2.5 to 5 cm) in depth. Filters can range up to 5 inches (13 cm) in depth. While thicker filters need replacing less often, you may need to have your unit modified by an HVAC professional to use them. 

Read: Why HVAC Not Working?

MERV Rating

MERV is an acronym that stands for Minimum Efficiency Reporting Value. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) developed the rating in 1987 as a more detailed way of measuring the efficiency of air filters. MERV ratings range from 1 through 16. 

The higher the MERV, the higher the level of filtration due to an increase in density. As with a clogged filter, a filter that’s too dense can impede airflow in some HVAC systems, causing the fans and system to work harder, break down more frequently, and wear out more quickly. 

Here’s a table that will give you a good idea of what MERV rating to look for:

MERV RatingMinimum Trapped Particle SizeMaterialsTypes of Contaminants TrappedComments
1–4(60 to 80% effective)>10 micronsFiberglassDisposable filtersWashable synthetic or metal filtersPassive self-charging filtersTextile and carpet fibersDust and dust mitesPollenCockroach debrisDust from sanding Spray paints Commonly used in window AC units. Standard filters provide basic, low-cost filtration.
5–8(80 to 95% effective)10–3 micronsPleated fabric and clothCubesMedia panelsMold and mold spores Dust mite debrisPet danderPudding mixSprays such as fabric protectors, dusting aids, and hairsprayUsed for better residential air filtering and in commercial and industrial spaces. Typical for use in residences. Provides good filtration. 
9–12(efficiency greater than 95%)3–1 micronsExtended pleated fabric and cloth filtersNebulizer dropletsLegionellaAuto emission particlesdust from milled flour, humidifiers, and leadUsed for superior residential air filtering. Better filtering in commercial spaces. For hospital laboratories, mid-range filters with comparatively high efficiency 
13–16 (efficiency greater than 98%)1–0.3 micronsElectronicand HEPA filtersBacteriaDroplets from sneezingMost smokeCooking oilFace powderPaint pigmentsDust from insecticidesUsed in hospitals and in areas where general surgery is performed. The best standard filters available with the highest levels of efficiency. 

MPR Rating

The MPR rating was developed by 3M. It stands for the Microparticle Performance Rating. This rating measures how well air filters remove particles smaller than 1 micrometer (micron). Ratings between 1,500–1,900 are among the best. 


AC filters can be made of fiberglass, pleated paper, or cloth. Fiberglass is a common material for AC filters — they provide a reasonable amount of protection at a low cost. They also offer the least resistance to airflow, allowing your system to operate at a high degree of efficiency. 

Pleated paper or cloth filters are denser than fiberglass filters. They remove more contaminants from the air in your home, but they’re also more expensive and produce more resistance to airflow. As mentioned above, anything that increases the resistance to airflow through your system increases the strain on your unit unless it is designed or modified for use with denser filters. So, consider the tradeoff carefully. 

If you have someone living in your home who has severe allergies or breathing problems, a HEPA or electronic filter may help. While these filters are the most efficient at removing almost all types of pollutants and contaminants from the air in your home, they’re also the most expensive. Generally, they aren’t needed in most homes. 

This two-pack of Filtrete Healthy Living Air Filters by 3M is an Amazon’s Choice filter. The filter has a MERV of 13 and an MPR of 1,900. They’re available in a range of sizes and come in 1-inch (2.5 cm), 4-inch (10.2 cm), 4-inch (10.2 cm) slim fit, and 5-inch (13 cm) widths. 

The 1-inch (2.5 cm) filters last for three months, and the 4 and 5-inch (10.2 and 13 cm) filters last for one year. Users report the number of pleats allow these filters to combine low airflow resistance with high filtration levels. 

Final Thoughts

It’s important to know which direction the air flows in your AC unit so that you can be certain you’ve installed your filter properly. Whichever filter you choose, it’ll work most efficiently and provide the highest air quality when it’s installed with the arrow pointing away from the air intake and toward the operating system of your AC system. 

Improperly installed filters place the same strain on your system as dirty filters, so always properly replace yours on schedule.