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 B — The introduction of air via the makeup water.

Hydronic systems are closed loops; however, makeup water introduced to compensate for leaks or maintenance contains dissolved gases. When this water is heated, the solubility of air decreases, causing it to release and accumulate at high points, which can lead to air binding, corrosion, and reduced heat transfer efficiency.