What is the minimum clearance required to combustible construction for commercial grease duct systems serving a Type I hood?

The correct answer of 18 inches reflects the minimum clearance required for commercial grease duct systems that serve a Type I hood from combustible construction. This clearance is crucial to prevent potential fire hazards, as grease duct systems can reach elevated temperatures due to the cooking processes they serve. The International Mechanical Code (IMC) sets these distances to ensure that heat and any potential ignition sources from the duct system do not come into direct contact with combustible materials, which could potentially ignite and cause fires. Following this guideline helps maintain a safe environment in commercial kitchens where grease-laden vapors are prevalent.

This requirement is also in line with best practices for ensuring proper ventilation and air quality in kitchens, limiting the chances of grease buildup and minimizing fire risks. In contrast, shorter clearances would increase the risk of thermal damage or ignition, highlighting the importance of adhering to these specified distances for safety compliance.