In a hydronic heating system, what is the terminal device commonly used?

In a hydronic heating system, the terminal device used to distribute heat throughout a space is a radiator. Radiators work by circulating hot water from the heating system through metal pipes and into the radiator units, which then radiate heat into the room. This method is efficient for distributing warmth, as water can retain heat well and the radiators effectively transfer that heat into the air.

The other options serve different functions within heating systems. A boiler generates heat by boiling water, while a heat pump can either heat or cool a space by transferring heat, and a furnace generally uses air as a medium for heating rather than water. Thus, in the context of hydronic systems specifically, the radiator is indeed the appropriate terminal device utilized for heat delivery.