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 is the primary source of air contamination in closed hydronic systems. Fresh water contains dissolved gases that are released as the water is heated. Proper system maintenance requires the use of air separators and automatic vents to remove these gases and prevent corrosion and circulation blockages.