Four-Borehole Reverse-Return Header with Air Lock
A four-borehole vertical loop serves a 6,000 ft² office. Each borehole is 300 ft deep with a 1.25 in U-tube; the vault contains 2 in supply and return headers piped reverse-return, a flow center, and isolation valves for each circuit. At start-up, the flow meter on circuit 3 reads 3 gpm while the others read 6 gpm. The entering water temperature is 42°F; circuit 3 leaving water is 56°F versus 51°F on the others. The sight glass at the pump shows intermittent bubbles, and the vault sump contains 6 in of water. The installer proposes opening the circuit 3 valve fully and raising pump speed to blow through the air. The building operator is concerned about uneven loop temperatures and electrical safety in the wet vault.
1. Diagnose the likely causes of the circuit 3 symptoms and evaluate the proposal to raise pump speed.
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The pattern is low flow on circuit 3 (3 gpm versus 6 gpm), high delta-T (14°F versus 9°F), and bubbles at the pump. That points to an air lock or restriction in circuit 3, not a need for more pump speed. Raising pump speed may increase velocity but will not necessarily move a trapped air pocket and can worsen cavitation or noise. Check the circuit 3 isolation valve, flow meter, and U-tube for a kink or debris; verify the reverse-return headers are actually piped so each circuit has equal length and that no valve is partly closed. Purge circuit 3 individually before balancing.
2. Outline the correct purge, start-up, and balancing sequence for this four-circuit loop.
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Use a purge cart with a high-flow pump, connect to the purge ports, and flush each circuit individually to a high velocity while isolating the others. Purge until the sight glass shows no bubbles and the discharge is clear, then repeat on the return side. After purging, pressurize the loop, start the circulation pump, and adjust flow-balancing valves to design flow. Record flow and temperature for each circuit. Reverse-return helps equalize path length but does not replace purging or balancing.
3. What should be done about the water in the vault before the system is energized?
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Water in the vault is an electrical and freeze hazard and can corrode headers. Before energizing, pump the sump, dry the vault, and inspect conduit seals, penetrations, and the sump pump. Determine whether the water is groundwater, condensate, or a loop leak; pressure-test if a leak is suspected. Seal penetrations, provide drainage away from electrical components, and do not operate the flow center with standing water around connections. Correct the source, not just the symptom.