Understanding HVAC Filters

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How to improve air filtration in your central air conditioning unit or furnace without impacting cooling or fan efficiency.

The filters in your furnace or air handler remove dust and debris from the air stream that passes through your heating, ventilation, and air conditioning (HVAC) system. Keeping such particles out of the unit improves air quality and protects the cooling coil and heat exchanger inside the air handler. However, if filters are not selected or installed properly, a furnace filter will actually block the flow of air or allow dust and other allergens into your home environment. To avoid this, it's important to change your filter regularly if they are a disposable type, or clean them if they are permanent filters.

Types of Furnace Filters

Electrostatic - Often, electrostatic filters are permanent, washable filters that carry an electronic charge that attracts particles much like clothing clings together from static.

Permanent Electrostatic Air Filter

Pleated - The pleats in this type of filter increase the surface area of the filter media where particles can be captured.

Pleated Filter

HEPA - Although HEPA filters are the useful in air purifiers and vacuum cleaners, they create too much air flow resistance to be used in residential air handlers.

Activated Carbon - An activated carbon component in a furnace filter enables it to absorb chemicals, fumes, and odors as air passes through your HVAC system including formaldehyde, ozone, and VOCs. These can be useful if you are sensitive to chemicals or suffer from multiple chemical sensitivity (MCS), or if you are renovating or painting your home’s interior.

Changing Furnace Filters

Typically, filters should be changed every 3 months, but can vary depending on usage, weather conditions, pets, and occupant sensitivities. Permanent or electronic filters should be cleaned according to the manufacturers’ instructions.

Filter Selection and Indoor Air Quality (IAQ)

Despite the fact that the central air conditioning/heating system is the place to begin any residential IAQ improvement, the throw-away filter supplied with most residential HVAC systems has a typical Minimum Efficiency Reporting Value (MERV) rate of between1 and 4. These low quality filters capture less than 20 percent of the contaminants in the 3-10 micron range. These filters are jokingly referred to as “rock catchers” in the industry. Consequently, the evaporator coil functions as a filter and inevitably becomes excessively clogged. Additionally, these filters do nothing against smaller particles from contaminants such as smoke and pollen, which can lead to problems for families with allergies or asthma. Reduced airflow can also lead to a frozen evaporator coil or damage to the compressor. See my article on How to Fix Frozen Air Conditioning Coils for more information. https://knoji.com/how-to-fix-frozen-air-conditioning-coils/

Throw-Away Fiberglass Filter

Increased Filtration Effect on Cooling Capacity

The first and most logical step is to install a better filter with a higher MERV rating. A higher MERV improves the filtration effectiveness by trapping more particles via the brute force of a tighter weave, which is the case for a pleated filter. This is an acceptable solution if the system blower can tolerate the increased static head on the overall air circulation system.

Unfortunately, there is no simple answer to determine whether the existing system can handle the additional pressure drop of the improved filtration since the total pressure drop of the indoor coil, supply and return ductwork, registers, and the filter must be accounted for and accommodated by the blower.

If the pressure drop caused by the filter is significant, then the airflow rate over the evaporator coil is reduced. Head (or pressure drop) x Flow (or air volume) is equal to a constant, which is the power of the blower. Therefore, if the pressure drop across the filter goes up, the flow rate across the evaporator goes down, meaning that the evaporator has to produce colder temperatures to provide the same cooling capacity.

If the airflow rate is lowered, the temperature drop must be increased to maintain the same cooling capacity. Generally, the system is 10 percent less efficient for every 10? colder the evaporator must run. Therefore, using a filter with too much pressure drop can decrease cooling efficiency.

Without knowing the specifics of any system, however, it is safe to assume that moving from a MERV 1-4 filter to a tighter weave MERV 6 or 7 rating should not impose a significant problem. This is true especially when considering that there isn’t much difference between an excessively dirty evaporator, due to the poor performance of the less efficient filter, and the airflow resistance of a MERV 7 filter. Changing to a tighter weave of a MERV 12 filter, however, might not be tolerated by the system, without impacting efficiency.

Note: Dirty filters actually perform better than clean filters because particles accumulate into areas that were once open spaces. Unfortunately, dirty filters function similarly to filters with a tighter weave; the increased pressure drop of a dirty filter increases operating costs, and in very extreme cases, could cause evaporator icing.

If you are looking to increase the filter efficiency in your forced air system, consider having the evaporator coils cleaned professionally or you can clean them yourself to remove the years of dirt clogging the cooling fins and decrease the static pressure through the coil. See my article on cleaning air conditioning coils at https://knoji.com/how-to-clean-your-hvac-system-with-coil-combs-chemicals-or-ultraviolet-light/

Additional Information

The measure of a filter’s effectiveness is the Minimum Efficiency Reporting Value (MERV). Most filters are labeled with a MERV rating number, which measures a filter’s ability to trap particles ranging in size from 3 to 10 microns.

Residential filters commonly have MERV ratings of 1-12. The higher the MERV rating, the more efficient the filter is, and the more particles it can filter. MERV is an industry standard rating, so it can be used to compare filters made by different companies.

MERV Rating Numbers

A MERV rating of 1 through 4 means the filter is less than 20 percent minimally efficient at capturing the measured particles.

A MERV rating of 5 means the filter is 30 percent minimally efficient at capturing the measured particles.

A MERV rating of 6 means the filter is 35-50 percent minimally efficient at capturing the measured particles.

A MERV rating of 8 means the filter is 70-85 percent minimally efficient at capturing the measured particles.

A MERV rating of 11 means the filter is 85-95 percent minimally efficient at capturing the measured particles.

The other approach is to create an electrostatic filter by coating a conventional inexpensive metal, disposable fiber, or reusable filters with an electrostatic coating such as PuraClean® filter spray, a patented product originally developed for NASA to keep spacecraft ventilation systems cleaner. Independent testing has shown that electrostatic filter sprays provide a 200 and 1,200 percent filtration efficiency improvement for 3- and 7-micron particles, respectively.


Patrick Regoniel
Posted on Jul 27, 2010
lisa leverton
Posted on Jul 23, 2010
Gayle Crabtree
Posted on Jul 22, 2010