Most home heating and cooling systems, including forced air heating systems, do not mechanically bring fresh air into the house. Outdoor air enters and leaves a house by:

• natural ventilation, such as through open windows and doors
• infiltration, a process by which outdoor air flows into the house through openings, joints and cracks in walls, floors and ceilings, and around windows and doors.
• mechanical means, such as through outdoor air intakes associated with the heating, ventilation and air conditioning (HVAC) system; outdoor-vented fans that intermittently remove air from a single room, such as bathrooms and kitchens

For more information see Improving Indoor Air Quality - www.epa.gov/indoor-air-quality-iaq/improving-indoor-air-quality.