This 16x25 MERV-13 filter in a ceiling filter grille keeps the resistance low and the indoor air quality high.

Load calculations, equipment selection, and duct design, done according to Manuals J, S, and D, respectively, are often required by code and sometimes even enforced. But one thing that’s not required anywhere is retrofitting the air filtration so that it not only protects the equipment from dirt but also does a good job improving the indoor air quality.

The coronavirus spreading COVID-19 is an airborne contaminant, so all the hullabaloo around ventilation and filtration is warranted. But what can you do about filtration in an existing home, especially if you’re not doing anything else to the HVAC system?

You can’t just swap out a cheap fiberglass filter for a pleated, high-quality filter. You have probably heard this warning before—and it is justified, because most HVAC systems are not designed to handle the extra resistance to airflow that a high-quality filter will introduce.

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