Well pressure tanks create water pressure by using compressed air to push water down. When the valve opens, compressed air in the tank pushes the water out. Water is pushed through the pipe until the pressure drops to the preset low value on the pressure switch. Once the low setting is reached, the pressure switch communicates with the water pump, telling it to turn on in order to push more water into the tank and house. To determine the right size well pressure tank needed, you need to consider pump flow, pump run time and cut-in/cut-out psi.

What is pressure tank drop capacity?
The drop capacity is the minimum amount of water that the pressure tank can store and deliver between pump shutdown and pump restart. Do not confuse drop capacity with tank volume size. The bigger your tank, the bigger drop (actually stored water) you will have. A larger drawdown means a longer run time and fewer loops. Manufacturers generally recommend a minimum run time of one minute for the motor to cool down. Larger pumps and higher horsepower pumps require longer run times.

Factors in choosing the right tank size
• The first thing you need to know is the flow rate of the pump. How fast does it pump? This is based on gallons per minute (GPM).

• Then you need to know the minimum run time of the pump. If the flow rate is less than 10 GPM, the run time should be 1 GPM. Any flow rate greater than 10 GPM should be run at 1.5 GPM. The formula for determining your drawdown power is flow x elapsed time = drawdown power.

• The third factor is the pressure switch setting. Standard options are 20/40, 30/50 and 40/60. The first number is the back pressure and the second number is the shutdown pump pressure. (Most manufacturers will have a chart that tells you the number of drawdowns based on the pressure switch.)

Does Home Size Matter?
When sizing a tank, the square footage of your home is less important than flow and pump run time. This actually has to do with how many gallons per minute you use in your home at a given time.

The right size tank
Your correct size tank is based on flow rate multiplied by run time (which equals drop capacity), then your pressure switch setting. The higher the flow rate, the larger the tank you can use.