Our HVAC units often operate year-round, either by heating or cooling, and the quality of the indoor air depends on the performance of our furnace filters. This discussion describes the different kinds of particle (furnace) filters, how they work, and how we can improve their filtration. The most common material used for filters is polymer fibers or fiberglass fibers in various sizes, framed with metal, plastic, or cardboard frames. These fibers are designed to catch particles as air moves through them.
The overall efficiency of filters relies on four key items: the surface area of the filter, the thickness of the web, the particle size, and the airflow through the filter. In general, studies have shown that larger particles will impact the web fiber of the filter and stick, while electrostatic attraction/electric filters tend to perform best for collecting small particles. The costs for web filters mostly depend on whether they are flat or corrugated, and the thickness of the web.
The efficiency rating of a particle or furnace filter is measured in Minimum Efficiency Reporting Values or “MERV”. MERV ratings vary from one to sixteen; one being the lowest efficiency, and sixteen being the highest efficiency. What determines this number is how well the filter captures smaller particles. Most residential homes use a MERV filter between 8 and 10. Your HVAC contractor can tell you what is the highest MERV filter that you can use. A tighter filter may damage the motor as it pulls the air through the filter. For people who are sensitive or have allergies, the filter with the highest MERV rating allowed is recommended.
Electronic air filters are not given a MERV rating. For maintenance, the manufacturers of these filters normally recommend cleaning them every 3 weeks; they show a fast drop off in performance after about 8 weeks. The HVAC industry generally does not feel that one filtration method (web or electric) is superior to all others. The American Society of Heating, Refrigeration, and Air-Conditioning Engineers(ASHRAE) is the leading authority on furnace filters and MERV ratings. They have performed many studies that indicate using different filtration methods in combination with each other greatly decreases the particle load at the supply discharge vents.
The size of a particle ranges from micrometers (also called “microns”). Most filters will focus on particles less than 10 microns, which have the most effect on indoor air quality. Particles of a larger size will fall out of the air quickly due to their weight. Commonly found particles of this size are ash, soot from smoke, pet dander, and mold debris with spores and pollen.
How often we should change our filters depends on size, setting, and amount of use. Most residential HVAC systems use 1-inch filters. If the filters are in continuous use, you’ll likely need to replace them in 2 months. For 2-inch filters, the time is about the same according to ASHRAE studies up to 3 months. The 3-4-inch filters last the longest and are recommended to change every 6 months. Dirty filters will make the inside ductwork (both supply and return) much dirtier which can lead to an increase of dust mites, mold, and bacteria which will get worse with time.
Other items that can affect how well our filters perform are: incorrect installment, wrong size, gaps, waiting too long to replace them, and a missing cover over the filter housing. In the field, we often see the wrong-sized filter used or a missing cover over the filter housing. Both of these allow for air to move around the filter. This is severely problematic when the HVAC unit is in the basement and there is mold growth on the contents or building surfaces. These items might seem obvious, but these examples are very common and have a great impact on how well our filters work.
Air filtration in your home or building is the primary method of maintaining good indoor air quality from suspended particulate debris. Although many people resort to purchasing one or more air filtration units for areas of a home, the first task is to make sure the filtration in the HVAC unit is operating correctly and is the best equipment available.
At Farsight Management we understand that not all indoor air quality companies are created equal.
We feel that it is imperative to educate ourselves, our employees, and our customers. You can trust that we follow all the national standards in regards to indoor air quality. This includes mold remediation, lead abatement, asbestos removal, and everything that we do.