What Size of Water Pressure Tank Do I Need for My Well?

Pressure tanks come in many different sizes. This is because household needs vary depending on your well’s depth, location, type of pump, and water use. So if you’re installing a new well or upgrading your current pressure, taking time to calculate which size pressure tank will work best for your well is vital to ensure that it works at the optimal performance for as long as possible.

Pressurized well tanks can extend the life of your well pump because it prevents the pump from cycling on and off rapidly, all while maintaining the water pressure in your home. This article will discuss what pressure tanks are and how to know what size pressure tank you should choose for your home.

What is a Water Pressure Tank?

The pressure tank holds water so that it is readily available for that use so that the well pump isn’t constantly having to kick on and off. When the well pump turns off, and on too frequently, that is called “short-cycling,” which puts a significant amount of stress on your well pump and can decrease the life of your well pump.

How Does a Water Pressure Tank Work?

Pressure tanks don’t hold a lot of water. Typically, only a third of the tank holds water, and the rest of the tank is made up of air. Although that may sound strange, the air in the tank is compressed to create pressure. Then, when you need to use water, the compressed air pushes pressurized water out of the tank into your home. If your tank doesn’t have air, it won’t work.

Important Terms to Know

Here are a couple of important terms that you should know when sizing your water pressure tank.

  • Pressure Switch – Monitors the pressure in your tank. When the pressure changes in your tank, it tells the well pump when to turn off or on. A gauge on the front of the tank shows when maximum pressure has been reached, and then the pressure switch will shut off the pump automatically.
  • Drawdown – The usable water in the tank is known as drawdown. When water is removed from the tank, the tank pressure decreases. For example, if you use a 30/50 pressure switch, the pressure will decrease until it hits 30psi. When that happens, the pressure switch will turn on, and the well pump will run until the tank hits 50psi. The volume of water between 30psi and 50psi is the drawdown.

It’s important to remember that drawdown is different from your tank’s total volume. The total tank volume is the tank size required to generate the capacity of drawdown needed.

How to Size a Water Pressure Tank

To choose the proper size water pressure tank, you need to know the flow rate, pressure switch setting, and the minimum runtime of the pump. These three factors will help you determine the drawdown, which, in turn, will help you choose the correct tank size.

Flow Rate

The flow rate refers to how many gallons of water your pump produces per minute. Typically, the flow rate is tested when a new well is drilled.

Minimum Runtime

The minimum run time is how long it takes for the pump to give you the desired drawdown capacity. The guidelines below can help you determine the drawdown capacity that will best suit your needs.

  • 0 – 10 GPM flow rate = 1.0 GPM run time
  • 10 – 20 GPM flow rate = 1.5 GPM run time
  • 20+ GPM flow rate = 2.0 GPM run time

Pressure Switch Setting

The pressure switch setting is the pressure that needs to be reached for the pump to turn on to fill the tank or the pressure at which it should shut off when the tank is full. Pressure tanks have three settings: 20/40, 30/50, and 40/60. The first number represents the pressure that will turn the pump on, while the second is the number that turns the pump off. Most pressure tanks will provide a chart so that you know the drawdown capacity determined by the pressure switch settings.

How to Use These Numbers to Determine Tank Size

When you have these numbers, you can then plug them into the equation below to determine the drawdown needed from your pressure tank:

Minimum run time x Pump flow rate = Drawdown capacity

Here is an example:

20 GPM flow rate x 1.0 GPM run time = 20 gallon drawdown capacity

Pressure tanks come in 20, 30, 50, 85, and 120-gallon sizes. Here is the drawdown volume for each tank size:

  • 20-gallon tank = 6 gallons of drawdown
  • 30-gallon tank = 9 gallons of drawdown
  • 50-gallon tank = 14 gallons of drawdown
  • 85-gallon tank = 25 gallons of drawdown
  • 120-gallon tank = 36 gallons of drawdown

So, for our example, a pressure tank with a 20-gallon or more drawdown capacity is needed, meaning you would need to install an 85-gallon water pressure tank. Remember, a larger tank is better than a smaller tank.

Can My Tank Be Too Large?

There is no such thing as a pressure tank that’s too large. And, because larger pressure tanks have fewer pump cycles, it can increase the life of your pressure tank. 

However, there are some downsides to having a larger tank. Not only do they cost more, but they take up extra space. You also run the chance of the tank pumping the well quicker than it can recover.

On the other hand, you can choose a tank that is too small. When the pump cycles too quickly, your tank will wear out faster. That means that you will need to replace the tank more quickly and more frequently.

Let the Water System Experts Help You Select the Best Water Pressure Tank

A properly sized water pressure tank is vital in keeping the water supply to your home readily available and at an appropriate pressure. If you need help selecting a new water pressure tank, the water experts at Independent Water Services, Inc. of Yakima, Washington, are here for all of your tank, installation, and service needs.

We are an EcoWater dealer and specialize in water softening, water filtration, water purification, drinking water, deionization, and ultraviolet lights. We have been helping customers throughout Central Washington get the best water since 1986, and we’d love to help you too.

Contact us today to request a tank consultation.