When we service any furnace, we recommend and provide Electrostatic Filters. This will save you money and having to find the correct size in the future as these are reusable. Simply hose both sides, let dry, and put back into furnace. This is a safe, naturally occurring phenomenon in which an electrostatic charge is generated by air flowing through a maze of static prone fibers. Airborne particles are attracted and held by the static charge until released by washing.
There are various benefits to electrostatic filter: