The filter in your HVAC system has two jobs. It stops undesired particles from entering the ductwork and passing over the internal components, namely the coils. The second job is to clean the air by removing dust and airborne particles which improves the indoor air quality. Modern buildings are constructed to minimize natural ventilation because they seal all the construction cracks and gaps. Have you noticed a lot of dust in your house or office? Maybe smells and odors linger longer than you want them to? Let’s talk filters.
Any filter you buy will have something called a MERV Rating. MERV stands for Minimum Efficiency Reporting Values. The MERV rating is an industry-standard system for rating the minimum efficiency of air filters. The MERV rating is a scale from 1 to 20, with 1 being the least effective designation and 20 the most effective. A very economical fiberglass filter would have a MERV rating of 3 or 4, and only traps the largest of airborne particles which generally is bad for the HVAC system and the indoor air quality. A more efficient filter with a MERV rating of 20 will trap almost all particles. As the MERV rating increases, so does the filter effectiveness. If you’re into the more formal definition, here you go:
MERV 20 Particle Size Filtered = < 0.3 microns Used in Industrial Cleanrooms
MERV 16 Particle Size Filtered = 0.3 – 1.0 microns Used in Operating Rooms
MERV 12 Particle Size Filtered = 1.0 – 3.0 microns Used in High Performance Residential
MERV 8 Particle Size Filtered = 3.0 – 10.0 microns Used in Average Commercial
MERV 4 Particle Size Filtered = >10.0 microns Filters out small rocks
Most home HVAC systems tend to use the lower end filters, MERV 4 or so. While these are the most economical, they actually do little more than stop pollen, dust mites and carpet fibers. Perhaps you have noticed dust accumulating on the horizontal surfaces of your home. Unless you keep your windows open a lot, the dust is a side effect of poor HVAC filtration. If you want to reduce the amount of dust, step up to a MERV 10 or so.
An easy way to pick your filter is by knowing what you want to filter out of the air. Some examples are:
*MERV 5 to 6 – Lint & Heavy Dust Particles
*MERV 8 or 10 – Settling/Average Dust Particles, Mold Spores, Dust Mite Debris, Pet Dander, Pollen
*MERV 11 – Suspended/Fine Dust Particles, Mold Spores, Dust Mite Debris, Pet Dander, Pollen, Auto Emissions
*MERV 12 – Suspended/Fine Dust Particles, Mold Spores, Dust Mite Debris, Pet Dander, Pollen, Auto Emissions, Bacteria
*MERV 13 – Suspended/Fine Dust Particles, Mold Spores, Dust Mite Debris, Pet Dander, Pollen, Auto Emissions, Bacteria, Sub-Pollen Particles, Pollution, Most Viruses
Resist the temptation to upgrade your filter beyond MERV 12. The supply fan for your HVAC system may not be able to overcome the resistance (pressure drop) these filters require. Remember that the resistance of the filter will steadily increase as it gets dirty which will decrease your systems air flow. A good test is to measure the air temperature at one of the supply registers before you upgrade the filter. Then remove the filter and measure the temperature while there is no filter installed. Compare it to the first measurement. The two temperatures should be very close. The last part of the test is to measure the temperature after installing your new filter. If there is a change beyond 3 degrees, you will want to monitor the HVAC systems performance and note if it still heats or cools your home adequately.
Types of Filters
The size of the fan in your HVAC system determines the optimum filter type you can install. The proper filter for your furnace will maximize the particles removed from the air and minimize the loss of efficiency. Below are the most popular types of air filters and a brief description.
* HEPA Filter: Normally not used in residential applications due to the pressure drop, HEPA filters are among the most efficient filters on the market. They operate at an efficiency rate of over 99 percent and remove almost all of the bacteria, most tobacco smoke, cooking oil and sneeze droplets in the air.
* Charcoal Air Filter: This is a filter with carbon deposited inside the filtering media. Also known as activated carbon filter. Charcoal filters are ideal for the removal of odors caused by tobacco smoke, chemicals, and gases.
* Pleated Filter: The pleated filter is the best filter for most residential applications. It is a good option for efficiently removing particles from the air. The filter material is pleated, or folded, in order to increase its overall surface area and consequently trap more particles. The pleats will also increase the life of the filter.
* Electrostatic/Electronic Filter: Electrostatic filters are typically provided as an upgrade when you purchase a new air handling unit. They are a great way to improve air quality without decreasing the efficiency of your fan. Basically an electrostatic field is produced across the filter which attracts airborne particles. This is similar to the bug zapper you might have in your back yard. Since the filter material is relatively thin, airflow reduction caused by electrostatic filters is minimal.
* Bag Filters: This type of filter is not used in residential applications. It is installed in a rack which houses multiple bags of filter media. The bags come in various MERV ratings which makes this a good choice for hospitals or high-end commercial applications.
Value Added Features
You can also find filters with baking soda (Arm & Hammer), antimicrobial properties, pre-filter pad or odor control properties. While these features do not effect the MERV rating, they can be used to enhance the filter’s effectiveness.
About the Author
I am a professional engineer with over 30 years of design experience relating to plumbing and mechanical systems. Visit the Pro Energy & HVAC site.