How to Change a Furnace Filter. A furnace filter helps keep the air in your home clean and your heating and cooling systems running efficiently. In order to make sure that your heating, ventilation and air-conditioning system works at its highest potential, the furnace filter must be clean.
Locate your filter. In a forced air system, it will be in your air return. In a gas or electric furnace, it will be in the furnace unit.
Remove the filter from the unit carefully and hold it up to a light. If you can see through the filter easily, it does not need to be changed and you can put it back into the unit. If you cannot see through it, the filter needs to be cleaned or replaced.
Clean the filter if it is made of plastic or metal. Follow the manufacturer's instructions for cleaning and put the filter back in the unit.
Purchase a new filter if yours is dirty and made of fiberglass, as fiberglass filters cannot be cleaned. Check the measurements of your current, dirty filter and buy a new one at your local hardware or home improvement store.
Make sure the arrows on the new filter are pointed in the right direction and slide it into the unit.