Filters Play Essential Role in HVAC Systems

By Bill Ivey

Principal Consultant

Filters are an essential component of an HVAC system and have several purposes. One of these purposes is to remove particles from the air, making for cleaner and healthier buildings. Filters also protect HVAC equipment like heating and cooling coils, which get plugged up with dust, lint and other particles, if the air is not properly filtered.

Types of Filters

HVAC filters are typically placed in the return air stream and are constructed from various types of materials. Filter types range from simple fiber disposable products to multi-layer, folded media filters, capable of trapping the smallest contaminants.

Filter Ratings and What They Mean

HVAC filters are rated based on how much particulate the filter is capable of removing. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 52.2 defines the filter’s ability to remove contaminants with a standardized value known as MERV or “minimum efficiency reporting value”. MERV ratings range from 1 to 16.

With the recent uptick in wildfires in the U.S., there is more emphasis on using higher rated filters in buildings. (Previously the standard for buildings was MERV-8 to-11 filters.) The wildfire smoke particles that are especially hazardous to the respiratory system are known as PM2.5 or particles smaller than 2.5 microns. A micron is one one-thousandth of a millimeter in size. MERV-8 filters only remove 20 percent of PM2.5 particles, while MERV-13 filters remove as much as 85 percent.

The California Environmental Protection Agency and other state and local health departments recommend MERV-13 filter retrofits for schools and other public places. The California Energy Code (starting with the 2019 code cycle) now requires MERV-13 filtration for most occupancies, including residential.

However, substituting MERV-8 filters with MERV-13 filters is not a “drop-in” replacement. MERV-13 filters have higher resistance to airflow, both when clean and as they fill up with particles. The fans of the air handling equipment may not have enough capacity to handle the additional pressure. The result is less air delivery, which then reduces the efficiency and performance of the ventilation system. Before upgrading filters on existing units, building owners or their HVAC technician should review the fan capacity..

Using a filter higher than a MERV-13 rating results in a sizable increase in pressure drop. While MERV-16 filters remove most contaminants efficiently, their very high pressure drop makes them practical only in systems where they are absolutely necessary, such as semiconductor manufacturing and pharmaceutical buildings and some surgical operating rooms.