A duct that has an operating temperature of 150 deg F shall be insulated to limit the exposed surface temperature to how many degrees Fahrenheit?

A duct that has an operating temperature of 150 deg F shall be insulated to limit the exposed surface temperature to how many degrees Fahrenheit?

• A. 100 °F
• B. 120 °F
• C. 140 °F
• D. 160 °F

Answer: (B)

Insulation is used to limit heat transfer from hot ductwork and keep the exposed surface at a safe, touchable temperature. When a duct carries hot air, the surface without insulation can get very hot, so the goal is to lower that surface temperature to a practical limit. A common design limit for exposed duct surfaces is about 120°F, which provides a safe contact temperature for typical conditions. With a duct at 150°F, insulating enough to bring the surface down to 120°F achieves this safety target. A surface temperature of 100°F would require more insulation than necessary and isn’t the standard target, while 140°F or 160°F would remain hot enough to pose a burn hazard.

Insulation is used to limit heat transfer from hot ductwork and keep the exposed surface at a safe, touchable temperature. When a duct carries hot air, the surface without insulation can get very hot, so the goal is to lower that surface temperature to a practical limit. A common design limit for exposed duct surfaces is about 120°F, which provides a safe contact temperature for typical conditions. With a duct at 150°F, insulating enough to bring the surface down to 120°F achieves this safety target. A surface temperature of 100°F would require more insulation than necessary and isn’t the standard target, while 140°F or 160°F would remain hot enough to pose a burn hazard.