man changing dirty air filter

Why Are My AC Vents So Dirty?

Good question, but to answer that question we must first ask an even more frequently asked question.

How Often Should You Change Your Air Filter?

  • If you have standard 1” pleated filters you should be changing them once every 30 days at least
  • 4-5” pleated filters should be changed every 3-6 months
  • Electrostatic filtration systems (less common) should be changed and cleaned according to manufacturers' recommendations. Electrostatic filtration systems usually have a contamination meter to indicate build up.
  • Usually, pre-filters should be cleaned or changed every 30 days and internal components should be cleaned every 3-6 months

Why Do My AC Filters Get Dirty So Fast?

Have you noticed that your HVAC filters get dirty more quickly than what seems normal? There are many things that can cause this problem. Here are some of the most common culprits:

  • Your air ducts are leaking. Cracked or damaged air ducts can allow contaminants to enter your system and cause your filter to clog up. They can also significantly increase your energy bills.
  • You're using the wrong type of filter. Make sure you're using the correct size and thickness of filter. If you're using 1-inch pleated filter, you may want to consider spending a little more on a thicker pleated filter or simply switch to a cheap disposable fiberglass filter.
  • You have pets. If you have cats or dogs, then you know that their dander gets everywhere - and that means that your air filter will clog up with their hair quickly.
  • You smoke indoors. Smoking indoors is one of the easiest ways to ruin your indoor air quality and quickly blacken your air filters. 
  • You live in an area with high amounts of air pollution. Many people who live near major highways or in urban areas with poor air quality notice that their homes are dustier and their HVAC filters don't last very long.

A Note on Return Air Grills

If you find that your filters are clean at normal change intervals with normal use, that may indicate an airflow related issue within your system usually caused by undersized intake grills. Undersized grills will most certainly lead to dirty coils and will redistribute dirt and dust through your supply air system.

How To Size a Return Air Grill

The general rule of thumb for grill sizing is 200 sq. inches per ton of cooling capacity. For example, if you have a 4-ton system you should have 800 sq. inches of return grill space equal to two 20x20 intake grills.

  • To find out if you have enough return air grill space measure the height and width of the louver space (the open-cut space in the grill)
  • Then multiply them together to get the total square inches of the grill. For example, a 12x12 return air grill will equal 144 sq. inches of return space

In conclusion, having the proper amount of return air grill space is essential to getting proper air filtration.

To learn more about what could be contributing to your dirty AC vents or to schedule an AC maintenance service in the San Antonio area, give Zimmermann Air Conditioning & Heating a call today at (210) 880-0885.

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