The answer to 'why is my air duct greasy' is that a buildup of dust, debris, and grease in your air ducts is a common issue. This can happen over time as air circulates through the ductwork, carrying particles that accumulate on the interior surfaces. Greasy air ducts are often caused by cooking fumes, pet dander, or other airborne contaminants that get pulled into the HVAC system.
Greasy ducts can restrict airflow, reduce energy efficiency, and even contribute to indoor air quality problems. It's important to have your air ducts professionally cleaned on a regular basis, typically every 3-5 years, to maintain proper system performance and prevent these issues.