To maintain healthy air in your home, change the filters in your furnace every three months. Keep in mind that if your comfort system also includes central air, your system is in use all year round not just during heating season. The filter is most commonly found next to the furnace’s blower; the location of the blower varies depending on the furnace manufacturer and how it is installed (horizontal or vertical). Look for a slot roughly one inch wide that runs the width or height of the duct attached to the blower; the filter slides in/out from there.