Most people know you should keep your air filter clean to maintain proper airflow in your HVAC system. Your air filter cleans the air before it passes through the coil of your heat and air system. Something that is often overlooked is air that may be leaking around your air filter, or through gaps in your return air duct work.This air does not get cleaned, and will eventually allow your coil to become filthy, which will inhibit thermal transfer, and reduce efficiency. in many cases these problems are easily fixed. ask for recommendations on this topic at your next preventive maintenance call.