Maximum Depth to Water for Pumps

A shallow well pump won't suck water from 29 ft because it is at or beyond the limit of the physics and mechanics of the system.

"Sucking" water doesn't actually describe how it works. A shallow well pump removes air from the suction line and allows atmospheric pressure to push the water up. Because atmospheric pressure is limited (about 14.7 psi at sea level, but only about 14.2 psi in Michigan) that is the limit to which it can push the water. A pump can't produce a perfect vacuum because the water evaporates at perfect vacuum, but the real limit is a characteristic of the pump that requires a minimum Net Positive Suction Head, often abbreviated NPSH Required. It requires that minimum pressure (NPSH) to avoid cavitation at the inlet of the pump.

Because of those factors, the limit for reliable operation of a shallow well pump is usually given as 25 ft of lift from top of water in the well to the inlet elevation of the pump. That is further diminished if the line from well to pump is too small or too long so that it causes additional pressure loss in the pipe.

The alternative is to use a deep well jet pump that involves putting a venturi/ejector in the bottom of the well with a supply pipe and a lift pipe; or make the well big enough to install a submersible pump, which is usually a 4" minimum diameter well. Deep well jet pumps are so inefficient that most new deep wells are drilled large enough for a submersible pump.

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