Well Pumps & Pressure

Notes

• **A 62 gallon tank does not hold 62 gallons of water.** Most of it is compressed air. At 40/60 psi you get **16.1 gallons** of usable drawdown, and that is the only number that matters when sizing.
• **Higher pressure gives you LESS drawdown, not more.** 40/60 psi yields 26% of the tank against 31% at 30/50. Turning the pressure switch up for better showers shrinks the water buffer and makes the pump start more often — the opposite of what most people expect when they make that change.
• **The one-minute rule is about the motor, not the water.** A submersible pump has to run at least a minute per start to shed the heat of starting. Pumps die from cycle count, not run hours. A 44 gallon tank here gives only 1 minutes — that is the choice that quietly destroys pumps, and it saves about a hundred dollars once.
• **Set the pre-charge to 38 psi, with the tank empty.** Two psi below cut-in, measured at the schrader valve with the pump off and a tap open. Wrong pre-charge is the most common failure in a well system: too high and the tank waterlogs, too low and drawdown collapses. Check it yearly — it is a five-minute job with a tyre gauge.
• Constant-pressure (variable speed) pumps change this arithmetic completely: they modulate instead of cycling, so they need only a small tank. If the system has one, size by the manufacturer's table rather than by the rule above.