HOW OFTEN DO I NEED TO CHANGE MY FURNACE’S FILTER?
The short answer is as often as they get dirty. However, the size of your filter and your home environment both play a significant role in how often you change your filter. The performance of a filter is related to its thickness, regardless of outside dimensions. There are 1 inch thick, 2 inches thick, 4 inches thick, and 5 inch thick filters. Your furnace is designed to accept only one size (possibly 1 or 2 may work in one furnace).
The next consideration is environmental. Suppose you live on a gravel road and have ten indoor cats shedding constantly. In that case, you will need to change your filter more frequently than someone with no pets, no kids, no husband, and who never opens their windows.
So the best way to determine your frequency is to evaluate your environment and then begin checking your filter. Shut off the furnace at the thermostat and remove your filter. If the filter is dirty, change it. If not evident, hold it up to the window, and if you can see daylight through it, it is still good. If you cannot see the light, then it is time to change it.
Rules of thumbs are 1-inch filters change at least every three months if not sooner, 2-inch filters four months, 4-inch filters can last 6 to 8 months, and 5-inch filters can last as long as a year depending on the quality.
If you can see dirt on your filter, it is about 50% plugged.