Question 67 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C. **Explanation for Option C (Correct):** A 4-pipe hydronic heating/cooling system utilizes four main pipes: a supply and return pipe dedicated to carrying hot water (for heating), and a separate supply and return pipe dedicated to carrying chilled water (for cooling). Because both hot and chilled water are simultaneously available to all terminal units (like fan coils), different zones within the building can independently choose to heat or cool based on their individual thermostat settings and load requirements at any given time. This capability for simultaneous heating and cooling in different zones is the primary operational advantage of a 4-pipe system. Conversely, a standard 2-pipe system uses only two pipes (one supply, one return) which must carry either hot water or chilled water, but not both simultaneously. Therefore, the entire system must be switched to either heating mode or cooling mode. **Explanation for Incorrect Options:** * **A) A 4-pipe hydronic heating/cooling system requires one-half the amount of piping as compared to a 2-pipe hydronic heating/cooling system serving the same number of zones.** This is incorrect. A 4-pipe system inherently requires twice the amount of main distribution piping (four pipes instead of two) compared to a 2-pipe system serving the same area or number of zones. * **B) A 4-pipe hydronic heating/cooling system can serve twice as many zones as a 2-pipe hydronic heating/cooling system.** This is incorrect. The capacity to serve zones is determined by pump capacity, piping size, and terminal unit selection, not by the number of pipes defining the system architecture (2-pipe vs. 4-pipe). Both systems can theoretically serve a large number of zones, but the 4-pipe system offers greater control flexibility. * **D) A 4-pipe hydronic heating/cooling system requires double the amount of piping as compared to a 2-pipe hydronic heating/cooling system serving the same number of zones.** While this statement accurately describes the increased physical piping requirement of a 4-pipe system compared to a 2-pipe system, it is a statement of **disadvantage** (increased cost and complexity), not the operational **advantage** for which the system is typically chosen. The true functional advantage is the ability to provide simultaneous heating and cooling (Option C).