Which heating element is commonly installed in a run of ductwork?

The correct choice is the duct heater, which is specifically designed to be installed within the ductwork of an HVAC system. Its primary function is to provide supplemental heating to the air as it flows through the ducts, ensuring that the heated air is distributed effectively throughout a space. Duct heaters are often used in conjunction with other heating systems to enhance heating capacity and maintain comfort levels in larger areas.

In contrast, an electric furnace, while capable of heating the air, is typically a standalone unit not integrated directly into duct runs. Baseboard heaters, mounted along the perimeter of a room, operate independently from duct systems and are used for localized heating. A sail switch, on the other hand, is a component used in HVAC systems for safety and functionality purposes, detecting airflow and ensuring proper operation, but it is not a heating element. Therefore, the duct heater is the most appropriate answer for installation within ductwork.