It is absolutely essential that hydronic heating system hot water piping be kept free of air. Assuming that a system is initially properly filled with water, what is the primary source of air contamination?

Correct Answer: Option A — The introduction of air via the makeup water.

Makeup water introduced to compensate for system losses carries dissolved atmospheric gases. When heated inside a closed hydronic circuit, these gases come out of solution, making raw replenishment water the primary and continuous source of internal air contamination.