A furnace filter is located where a home's air-intake ductwork meets the furnace, the place where air circulating the house is brought back to be reheated.
Furnace filters can range in price from less than a dollar to more than $50. Generally speaking, the higher prices also come with improved performance (capturing smaller particles) and longer time span between replacements.
For a furnace filter to be at its peak effectiveness, the furnace fan would have to be running 24/7. If you require a high rate of particle removal, consider that you'll be adding several hundred dollars to yearly energy costs by having a continuously running fan.
Minimum Efficiency Reporting Value (MERV) is the current rating standard for furnace filter efficiency. Developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), MERV ratings follow a scale of 0 to 20. The rating indicates a filter's ability to remove particles one to 12 microns in size. A higher rating means the filter can remove both smaller particles and more particles in total. The system was brought in to replace vague and often misleading percentage efficiency claims made by manufacturers. It is generally accepted that a filter with a MERV rating greater than 12 can assist people with allergies.
Filters are rated when new, but as particles build up, spaces between filter fibers become smaller, allowing smaller particles to be trapped. So, filters actually work better the longer they're used, up to a certain point. Eventually they become too blocked and must be replaced.
* Some filters with high MERV ratings can reduce the amount of air passing through the furnace and negatively affect its performance. Talk to a furnace technician before installing a high-efficiency filter.
From the basic woven fiberglass filters to further-performing pleated filters or even electronic furnace filters, they all come with their own benefits. The basic models are cheaper but need to be changed often, whereas high-efficiency filters can last up to a year and grab a lot more particles. For those who like to get the most bang for their buck and don't mind a little extra work, reusable furnace filters may be the answer. They can last a long time provided they are cleaned regularly.
Make sure you buy a furnace filter that is sized for your furnace. If you are unsure, take out the current filter and measure it.