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How To Size A Well Pump Pressure Tank

What is a well pump pressure tank?

A well pump pressure tank maintains the water pressure in the household and more importantly, protects the life of the well water pump. Every time a faucet gets turned on, a toilet flushes or someone runs a bath, water is demanded from your well. The pressure tank holds a quantity of water to draw upon to meet these needs before asking the well pump to kick on. This extends the time between the pump turning on and turning off. If that on/off cycle is fast, every time you open a faucet the pump will turn on, and when you close the faucet, the pump shuts off. When the well pump is turning on and off too frequently, it is known as “short-cycling”. Short cycling puts enormous stress on your well pump and can cause it to prematurely fail. This requires an expensive replacement that could have been avoided.

By extending the time between the on and off cycles, a pressure tank keeps your well pump from undue wear and tear. It also stabilizes your overall household water pressure, so your faucets don’t spit and sputter as they wait for the pump to catch up.

How does a well pressure tank work?

A well pump pressure tank uses compressed air to push pressurized water out of the tank and into your home. Pressure tanks have a diaphragm, also called a bladder, that separates a chamber of air from the water. As the tank fills with water, it compresses the air chamber. When you turn your water on, the compressed air pressurizes the water and pushes it back out of the tank.

How does a pressure switch work on a well pump?

Pressure tanks are set up to run in conjunction with a pressure switch. The pressure switch monitors the pressure in the tank. It informs the well pump when to turn on and when to turn off based on the rise and fall of the pressure within the tank. Most households are set up to run with a pressure switch that turns the pump on at 30 pounds of pressure and turn the pump off at 50 pounds of pressure. The gauge on in front of the tank indicates when the maximum pressure has been reached and the switch then automatically shuts off the pump.

How do I size a well pressure tank?

Sizing a well pump 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.

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.

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