How to Know When to Change Out Your Air Filters
Overview
Your air filters are designed to capture particulate of a certain size range and will load up with dirt over time until they reach the end of their useful life. It is possible to estimate how long that life cycle might be, but only if you know certain details about the system they are installed in, the amount and composition of the particulate they will see, and at what rate they will see the dust load. Most filter manufacturers will state an initial airflow resistance (either in inches of water column or in pascals) and a final recommended resistance at which point they suggest changing out the filters. In order to know what the pressure drop is in your system, you should install filter devices such as a Magnahelic or Photohelic gages across the bank.
For example, a Pleated prefilter size 24x24x2 MERV 8 Standard Capacity may have an initial resistance of 0.20 w.c. @ 500 FPM and may have a recommended final resistance of 1.0” w.c. So, the factory recommends that filter be changed out at 1.0” w.c. The same type of filter in a high capacity version has an initial resistance of 0.19” w.c. @ 500 FPM and may have the same recommended final resistance of 1.0” w.c. The difference between the two is that the High Capacity version has more media area per square foot of filter, holds more dust and will therefore take longer to reach its final resistance and will last longer.
AirIQ ANALYSIS
There are some rules of thumb when setting up filter changeout intervals for prefilters and secondary filters across the industry, but before you adopt them, you should consult a NAFA Certified Technician or a NAFA Certified Air Filtration Specialist to help you assess your system. For prefilters, in standard applications, a changeout frequency of 3-4 times per year is a good starting point. For higher efficiency filters such as MERV 13, 14 or 15 Rigid Box Filters or Pocket Filters intervals can often be every 6-12 months and in some instances longer. The longevity of your secondary filters is very dependent on how well you maintain your prefilters to protect the higher efficiency media.
HEPA filters, carbon filters and other industrial types need to be handled in a different way as regards changeout intervals. HEPA filters, depending on the application, should be leak tested and pressure drop tested in place to monitor their effectiveness. Because they are designed for removal of sub-micron particles, they are often used in critical systems and the leak testing will confirm that they are achieving the efficiencies required. Carbon filters are designed to adsorb gas phase components such as odors, VOCs and other contaminants. It is more difficult to determine changeout intervals for these filters but you can use devices such as Air Quality meters, Breakthrough indicators and your olfactory senses to help in knowing when they need changing.
In Summary
It is important to look at all the factors when establishing a proper changeout interval for your air filters. As a filter loads, it takes more energy to force air through the filters. You may find that you can keep your air filters in place longer than the final recommended resistance if you are not seeing any particle bypass and there is no significant reduction in airflow. This will get more life out of the filter, but keep in mind the increased energy cost may be higher than the cost of replacing the filter. Labor hours are another factor to consider, as are stock carrying costs and the potential for equipment failure.
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