When was the last time you changed your furnance filter? Depending on the type of furnance filter you use, it should be replaced either every 30 or 90 days. The blue fiberglass filter (usually the most inexpensive) is for basic removal of large particles of dirt only. They should be replaced every 30 days. The next step up in filters remove dirt, pollen, and lint. There are additional filters available that remove more items that can affect the air quality inside your home. Do you have allergies? Are there pets in your home? The higher quality filters can filter mold spores, dust mites, smoke, pet dander and more. These better filters should be replaced every 90 days except the first time you make the switch. The better filter is going to work overtime the first 30 days catching up.
Installation of a furnace filter is key. Look at the side of the package of the filter and look for the direction of air flow. If you questions regarding proper installation, fell free to ask someone for help.
Consult your local hardware store or home center for help selecting the proper filter for your home. If you still have questions, consult a HVAC specialist.