This should always be the very first thing you do when you perform routine winter HVAC maintenance. Air filters are designed to remove all the dust and allergens from the air. When your building is heavily used, they need to be replaced regularly. When your air filters are dirty and clogged, then the system has to work harder to maintain a proper air flow through all the returns and ductwork. A dirty air filter can cause permanent damage to your system as well. Filters in commercial HVAC systems should be changed every month, at the very least, to keep a healthy, optimal air flow throughout the system.