When should I change my furnace filter?

Change 1-2 inch filters every three months, 4 inch filters every six months, and 5 inch filters every 12 months. The air filter is dirty and needs to be changed. The material of the filter should be replaced if you can't see it.A good rule is to change filters 1-2 inches every three months, filters 4 inches every six months and filters 5 inches every 12 months. The signature of the air filter must change the filter is visible dirty. Of course, if you cannot see the filter material itself, it must be replaced.