Circulators, Piping, and Hydronic Distribution
Domain 6, Circulators, Piping, and Hydronic Distribution, tests pump selection, piping layouts, air removal, expansion control, and balance. Expect questions that connect flow, head, and system behavior.
Core rules
- A circulator is selected at the intersection of system flow (GPM) and head (ft). Pump curves show that flow changes with system resistance; a wide-open zone valve or a clogged strainer changes head.
- Common layouts include series loop, one-pipe, two-pipe direct return, two-pipe reverse return, primary/secondary, and zone-valve or zone-circulator systems. Reverse return helps equalize path length.
- Air elimination requires an air separator or scoop at the hottest point, plus an expansion tank. A waterlogged expansion tank causes rapid pressure rise and relief-valve discharge.
- Keep water velocity reasonable to limit noise and erosion. Balance zones so the farthest or highest-head zone receives design flow.
Worked example
A system needs 6 GPM at 8 ft of head. Select a circulator whose curve passes near 6 GPM at 8 ft, not a larger pump chosen only by flange size. If a new zone valve adds 3 ft of head, recheck the operating point; flow may fall short.
Traps
- Oversizing the circulator, causing noise and high velocity.
- Leaving air in the system after a repair.
- Installing the expansion tank on the wrong side of the circulator or with a blocked connection.
- Using a relief valve with the wrong pressure rating or piping its discharge to a closed location.