How to size a well pressure tank?
Sizing a well pressure tank is a three-step process of knowing the flow rate, the minimum runtime of the pump, and the pressure switch setting. These three variables must be calculated together to find what size pressure tank works for a system. Follow these steps to properly size your well pressure tank:
1. Flow Rate
Your flow rate is how many gallons per minute your pump produces.
2. Minimum Runtime
Multiplying the flow rate by the minimum runtime of the pump will give you the drawdown capacity. The minimum runtime rule of thumb is any pump operating at 10 gallons per minute (GPM) or under should be producing one gallon per minute of runtime.
(For example, 10 GPM flow rate x 1 = 10 gallon drawdown capacity.)
Anything above 10 GPM should be producing 1.5 gallons per minute of runtime.
(For example, 16 GPM flow rate x 1.5 = 24 gallon drawdown capacity.)
If your flow rate is above 20, you may require multiple tanks.
3. Pressure Switch Setting
The pressure switch setting is the pressure at which the pump turns on to fill the tank up, and the pressure at which the pump shuts off, knowing the tank is full. The three pressure switch settings for pressure tanks are 20/40, 30/50, and 40/60. The first number is the pressure at which your pump turns back on and the second number is the pressure at which your pump turns off (i.e. 20psi on, 40 psi off). The number that turns off the pump pressure is going to have the most direct effect on your drawdown capacity. For example, if you have a 40/60 switch, you are going to have less drawdown capacity than you would have on a 30/50 switch.