Short-Cycling Submersible in a Low-Yield Bedrock Well
A 6-inch bedrock well is 300 ft deep and has a tested yield of about 1.2 gpm. Static water is 45 ft; the 1.5-hp submersible pump is set at 280 ft and is rated at 12 gpm at that head. The pressure system has a 20-gallon diaphragm tank with about 5 gallons of drawdown and a 40/60 psi switch. A family of four plus summer lawn irrigation uses the well. The pump now cycles about every 40 seconds during irrigation and the motor overload trips after 20–30 minutes. The homeowner has already replaced the pressure switch and tank, and asks whether a larger tank or a more powerful pump will solve the problem. A neighbor's well in the same bedrock unit yields 4 gpm, so the owner believes the well is underperforming.
1. Explain the cycling and overload using the well and pump numbers.
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The pump's 12-gpm capacity is ten times the well's 1.2-gpm yield. During irrigation, the pump draws the water level down to the pump intake faster than the well can refill. The 5-gallon tank drawdown is emptied in about 25 seconds at 12 gpm (5 gal ÷ 12 gpm × 60), so the pump starts and stops frequently. The well may recover only 1.2 gpm, so continuous irrigation cannot be sustained. The overload trip is consistent with low-flow overheating or cycling, not necessarily a failed pump. A larger tank would lengthen each cycle but would not create water; a more powerful pump would make drawdown and cycling worse. The correct diagnosis requires a pump test and water-level measurement, not parts replacement.
2. Recommend a sustainable system design and the data needed to size it.
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For a 1.2-gpm well, storage is the key. Install a cistern or storage tank sized for peak daily demand (for example, 500–1,000 gallons) filled by a low-yield pump or a pump controlled by a float, then use a booster pump and pressure tank to serve the house. Alternatively, a cycle stop valve can reduce cycling, but it does not increase yield and may not satisfy irrigation demand. Before sizing, run a step-drawdown test and a constant-rate test to determine safe yield and drawdown, measure recovery, and confirm the pump setting is above the worst-case pumping level. Compare peak demand (house plus irrigation) with 24-hour well recovery. If the well cannot sustain the demand, reduce irrigation or add storage; do not simply increase pump horsepower.
3. What immediate operational changes would you make while the permanent fix is designed?
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Stop lawn irrigation and any simultaneous high-demand use until the system is stabilized. Lower the pump setting only if the measured pumping level shows the intake is being drawn dry; otherwise, the problem is storage and cycling, not pump depth. Install a temporary cycle stop valve or adjust the pressure switch differential cautiously, but recognize this only reduces cycling and does not increase yield. Verify the tank precharge and drawdown; a waterlogged tank worsens cycling. Measure static and pumping levels during household use to confirm the well is not being over-pumped. Warn the owner that repeated dry-running will damage the motor. Schedule the yield test and storage design before restoring irrigation.