Choosing filters for forced-air HVAC systems

June 15, 2017

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Multifamily buildings with decentralized heating and cooling systems often rely on forced-air systems in each unit. To keep these systems operating at their most efficient — and to keep the occupants comfortable — the air filters should be changed at the start of each heating and cooling season.

Filters may need to be changed more often if they become dirty sooner. Clogged filters lead to stress on the HVAC equipment, especially the unit's blower motor, as occupants adjust the thermostat to maintain a comfortable temperature.

Types of filters

The basic filter is a disposable fiberglass or polyester flat filter that keeps dust and other relatively large particles from reaching the working parts of the system's motor. It is usually located just before the end of the return duct where the return air enters the cabinet containing the heat exchanger and blower motor.

In addition to the standard filters, there are other types of filters that remove smaller contaminants, such as mold spores, bacteria and microscopic allergens. Some of the most efficient filters can even remove odors.

The purpose of these types of filters is to improve air quality by removing contaminants that can affect sensitive individuals. These types of filters include:

  • Pleated. These filters have a larger surface area that can capture smaller contaminants. There are both single-use disposable models and reusable models that can be cleaned with soap and water or vacuumed. Some pleated filters can remove pollen, mold spores and bacteria from the air.
  • Electrostatic. As with pleated filters, these tend to have a large filtering area, and they are electrically charged. The charge attracts the smallest particles, including viruses, microscopic allergens and odors. They are also available in disposable and reusable types.

Filter rating systems

To help sort out the effectiveness of the different filters available, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) developed the Minimum Efficiency Reporting Value (MERV) system for filters.

MERV applies a value from one to 20 for each type of filter, with 20 being the most effective at removing pollutants. Filters above 15 are used in hospitals pharmaceutical manufacturing and clean rooms. Filters in the seven to 13 range provide the best protection for residences. They are likely to be nearly as efficient as high efficiency particulate air (HEPA) filters at removing airborne particles, according to the Environmental Protection Agency.

A typical disposable fiberglass filter usually has a rating of four.

Before switching to more efficient filters, experts recommend that building owners and managers discuss plans with an HVAC technician. Some filters can reduce airflow in the system, which could lead to complaints from occupants and more stress on the equipment, resulting in the possible failure of the blower motor.

The technician may be able to boost the blower speed to compensate for the filter. These filters need to be changed or cleaned as often as any other type of filter.

By Fran Donegan, who writes for several digital publishers and The Home Depot. He regularly shares DIY tips on topics that range from choosing the right replacement air filter to installing hardwood flooring. 


Topics: Building Owners and Managers, Energy Saving Products, Healthy & Comfortable Buildings, HVAC - Heating, Cooling, and Ventilation, Indoor Air Quality (IAQ), Interiors, Multifamily / Multiunit Residential, Ventilation


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