There are many different kinds of HVAC filters. The most common filter is a disposable type known as a pleated filter. To the left is an example of a disposable pleated filter. The surface of these filters are not flat. In fact they are quite rigid. In the picture to the left you can see ridges or peaks on the surface of the filter, these are referred to as pleats. A pleat is a double or multiple fold in a garment. The pleats in the filter help to catch the pollutants in the air.
Within the pleated filter category there are various options available. The biggest difference between most filters is size. HVAC filters have three different dimensions: height, length, and width. Also, filters are usually measured in inches. Some common sizes for filters are: 16 in x 20 in, 16 in x 25 in, 20 in x 20 in, 20 in x 25 in. The width of the filters could range anywhere from one inch to five inches with any of the previous sizes mentioned. Therefore, the filter will have three dimensions, H x L x W. It is important to note that these sizes are not the exact size of the filter and are actually rounded. For example, a 16 in x 20 in x 5 in filter could be 15 1/2 inches by 19 3/4 inches by 4 7/8 inches. However, the HVAC industry simply rounds these numbers when referring to filter sizing in order to eliminate confusion.
Pleated filters are categorized not only by size but also by MERV rating. MERV stands for minimum efficiency reporting value. This rating refers to how effective the filter purifies the air. The higher the rating the more effective the filter. In addition, the higher the MERV rating means that the filter has a larger surface area. Essentially, the more surface area available the more effective the filter will be at purification. A MERV 16 filter will have a much greater surface area than a MERV 8 filter. Consequently, the MERV 16 filter will also have many more pleats than the MERV 8. Further, the higher the MERV rating the longer the filter will last. For illustration, a MERV 16 filter is good for up to one year. On the other hand, a MERV 8 filter is only good for one month.