What statement is true concerning a one-pipe hydronic heating system?

Correct Answer: Option D — Each heating coil inlet temperature is different, as the hot water inlet temperature to each heating coil progressively drops as the water passes through each successive series-connected coil.

In a one-pipe system, water flows through a single main loop. As the water passes through each successive heating coil, it loses heat to the space. Consequently, the water returning to the main is cooler than the water entering, causing each subsequent coil to receive a progressively lower inlet temperature.