Step 1: Find your air filter. With a forced air system, it should be inside the air return and in a gas furnace, it should be inside the furnace enclosure.
Step 2: Take out the air filter from the enclosure and bring it up to the light. If while you have it held up to the light and you are able to see through to the other side of the filter then it does not need to be changed and you are able to place it back into the furnace enclosure.
Step 3: You are can manually clean the air filter only if it's made of a plastic or metal material. Remember to follow the manufacturer's directions for cleaning. Once the air filter is cleaned you can return the filter back into the furnace enclosure.
Step 4: Buy a fresh air filter whenever yours is filthy and constructed of fiberglass, as fiberglass air filters can't be cleaned. Determine the height and width of your old filter and purchase a fresh one at your local shop.
Video Tutorial
