What should be fitted at all high points and at each heating unit in a hot water system?

In a hot water system, fitting a manual vent at all high points and at each heating unit is essential for effective operation. As water is heated, it expands and may create steam or air pockets that can accumulate at high points in the system. These air pockets can lead to inadequate circulation, hot spots, or even system inefficiencies. A manual vent allows for the release of trapped air, ensuring that water can flow freely throughout the system and thereby maintaining proper pressure and temperature.

This practice is crucial for the overall efficiency and longevity of the hot water system. The ability to vent air out of the system also enhances the comfort levels provided by heating units, as it ensures that they operate at optimal conditions without interruptions caused by air blockages.

Other options, while important in different contexts, do not address the specific need to remove air from high points in a hot water heating system. A pressure gauge monitors system pressure, a circulator pump ensures water movement, and a backflow preventer protects against contamination, but none of these functionalities replace the necessity for manual vents to manage air accumulation.