Commissioning a Drainback System: No Flow, Sensor Wiring, and Handover
A new drainback system with two 4 ft x 8 ft flat-plate collectors, a 120-gallon storage tank, and a 1/6 hp AC pump is being commissioned. The differential controller has an on setpoint of 8°F delta-T and an off setpoint of 4°F delta-T. The collector sensor is a 10k thermistor wired with 18 AWG stranded cable in the same conduit as the 240V pump feed for 40 ft. The tank sensor is clamped to the outside of the tank’s lower boss, not in a well. At startup, the pump runs 90 seconds, stops for 3 minutes, and repeats. The collector sensor reads 15°F higher than the tank sensor immediately after the pump stops, and the flow meter shows 0 gpm during the run. The installer plans to lower the on delta-T to 4°F to keep the pump running longer. The drainback tank is filled to the mark, and the pump inlet strainer is clean. The controller is mounted indoors, 25 ft from the tank.
1. Diagnose the short cycling and zero flow. What is the primary fault, and why is lowering the delta-T setpoint a poor fix?
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Zero flow is the primary fault. The pump runs but the flow meter reads 0 gpm, so the controller is reacting to a hot collector with no heat removal. Likely causes: the pump has not primed (air lock in the drainback loop), the pump inlet is blocked, the check valve is stuck, a valve is closed, or the pump is too small for the head. Short cycling follows: the collector heats, the controller turns the pump on, no flow means no heat transfer, and after the pump stops the sensor readings drift. Lowering the on delta-T to 4°F is a poor fix because it will make the pump run more often without moving water and may overheat the pump. Fix flow first: verify drainback tank level, open all valves, vent the pump, check rotation, inspect the check valve and strainer, and confirm the pump curve at the required head. Only after flow is established should you tune setpoints; typical on/off delta-T is 8–12°F on and 4–6°F off.
2. Evaluate the sensor installation and wiring. What corrections are needed?
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The collector sensor cable is run in the same conduit as the 240V pump feed for 40 ft. That is a noise problem: the controller may see erratic resistance and cycle the pump. Separate low-voltage sensor wiring from line-voltage conductors, or use shielded cable with the shield grounded at the controller end only. The tank sensor clamped to the outside of the lower boss is slow and reads the tank wall, not the water; install it in a thermowell with thermal paste, or insulate the clamp-on sensor heavily. The collector sensor should be on the absorber or outlet pipe, protected from sun and water. Verify the thermistor resistance at ambient (a 10k thermistor is about 10kΩ at 77°F) and check the controller’s sensor calibration. Correct wiring and sensor placement before changing controller settings.
3. What commissioning tests and handover items would you complete before calling the job finished?
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Commissioning: confirm 8 gpm total (4 gpm per 4x8 collector) with a flow meter; measure a 10–20°F temperature rise across the collector; verify pump pressure and no cavitation; test drainback drain-down and refill; check controller on/off setpoints and minimum run time; verify freeze protection and relief valve; check glycol concentration; leak-test all joints; confirm electrical connections, GFCI, and disconnect. Handover: label pipes and valves, provide a schematic, controller settings, maintenance schedule, and glycol test records. Show the homeowner how to read the controller, check flow, clean the strainer, and recognize a leak or stagnation event. Record the actual flow and delta-T in the commissioning report. Do not sign off while the system short cycles or the flow meter reads zero.