Using a 4-inch-thick air filter for a system that's made for a 1-inch-thick filter can be detrimental to efficiency and may even damage components. However, you can use a 1-inch thick filter in compartments that can accommodate a deeper filter. Thick air filters may seem better, as they usually mean more air filtration and last longer due to their increased surface area. Most modern HVAC systems have no problem operating with higher MERV filters, which is why millions of homeowners rely on them.
The main risk of high-efficiency air filters comes from the fact that they are not modified for long periods of time. You should expect to replace the filter every three to 12 months of use, depending on the size of the filter. Be sure to install the new filter facing the correct direction and look for arrows on the filter frame that indicate the direction of the air flow. Thicker filters, known as multimedia filters, are usually 4 to 5 inches thick compared to the ubiquitous 1-inch filters.
Low-efficiency filters are usually within the MERV 1-4 and the high-efficiency filters are those of the MERV 13 and higher. These multimedia filters have much stiffer cardboard frames, and some filter filters even have plastic “rails” on the top and bottom so that they can be slid inside the oven with a very hermetic seal. Ironically, the effectiveness of low-efficiency air filters increases as dirt and dust accumulate on the filter. However, in most heating, ventilation, and air conditioning (HVAC) systems, you should be able to install a media filter cabinet under the oven or on the side.
Choose the thickest air filter you can for maximum efficiency, but never try to force an air filter to fit in a space it's not designed for. If you use one of the best heating companies in your area to service your boiler, you can be sure that the technician will place an appropriately sized filter in your furnace at all times. For comparison, a 4-inch-thick filter will have approximately twice the surface area of a 2-inch-thick filter.
