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How to Size a Well Pressure Tank

Posted by John Woodard on August 07, 2019

A pressure tank is an integral part of maintaining your well. Pressurized well tanks extend the lifespan of your well pump by preventing rapid on/off cycling of the pump and maintaining water pressure throughout your home. A properly sized pressure tank ensures your household water needs are met and your pump is protected against short-cycling. 

Join John Woodard, our Master Water Specialist, as he discusses what well pressure tanks are, how to size them, and what accessories you need to make sure the tank functions properly. 

What is a well pressure tank? 

A well pressure tank maintains the water pressure in the household and more importantly, protects the life of the well 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 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. 

What is pressure tank drawdown?  

Drawdown is the amount of useable water in the pressure tank. It is the volume of water drawn from the tank between the time the pump takes to turn on and off. As water comes out of the tank, the pressure of the tank decreases. If you are using a 30/50 pressure switch, the pressure will continue to drop down until it reaches 30psi. At this point, the pressure switch will turn on and activate the well pump until the tank reaches 50psi. That volume of water between 50psi and 30psi is the drawdown. Essentially, the drawdown is the length of time the pump is protected from shutting on and off. The drawdown is not to be confused with the total tank volume. Your total tank volume is a measure of the tank size required to produce the desired drawdown capacity. 

How do I 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.

Most well pressure tank manufacturers will provide you will a chart to tell you what your drawdown capacity is based upon the pressure switch setting. 

What is a tank package?

When purchasing a new pressure tank, it is important to also purchase a corresponding tank package, also called a t-pack. The t-pack will attach to the outlet of the well tank and then hook up to your inlet line from the wall and onto your household. These come with many of the necessary parts to operate and monitor your well pressure tank, including the pressure switch, pressure gauge, relief valve, tank cross, and ball valve. 

Explore more on water storage tanks: What is a Water Storage Tank and How Does it Work?

Comments 1-10 of 8

I just switched the holding tank in my house from a 5 gallon to a 50 gallon. My pump is set up to 40/60 and it’s a ½ horsepower. Is that ok?

Allen Mooneyhan jr on February 23, 2023

How much for a 30 to 50 under ground tank

Robert Downey on August 25, 2022

I just bought a new build in Lebanon Maine. My well, I just learned is 400’ down and draws a half gallon per minute. Is this normal to be so slow?

Diane on June 08, 2022

I live in a small town/village with a set town pressure of 38lbs out of the village water tower. I’m building a new home and considering to put a 200gal holding tank in the crawl space. Filling up with city water, with a float on the holding tank and a shallow pump coming out to increase household water pressure to about 45lb. I’m looking for a measure twice cut once type suggestion.

Jeff on May 20, 2022

if your not in a rush to fill your 350 tank, then any economical pump would work.. on our farm we like pumps in tank, pushing water.. sometimes that isn’t an option.. 1/2 hp deep well pumps with 1/25” line pump lots of water
our friend just bought a pump that does NOT require a tank, sounds about right for your needs. you would have to explain the system for a guaranteed answer

randy swanson on April 11, 2022

You need a pump able to flow on average your demand you plan to meet and at a head of the pressure of the Amrol tank setting.

Joe on May 20, 2022

Having a difficult time getting an answer to my question. Seems no one knows.
Scenario: 5000 gallon reserve cold water below grade tank, running water line from 5000 gallon tank to my new Amrol WX-350 tank.
Question: What size of pump do I need to fill my WX-350, from my 5000 gallon fresh water tank?

Mark Miller on February 25, 2022

We ha e a flotec 82 gal. Tank . Is this too LG for shower,and washing machines?

Rodney E. Moos on May 20, 2022
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