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
Your Return Air Grill Could Be the Issue
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.