Is A 20 Gallon Pressure Tank Big Enough? (Find Out Now!)

Jessica Stone
by Jessica Stone

Your home’s pressure tank is an essential piece to maintaining your well. These pressurized well tanks work to extend the life of your well pump by maintaining water pressure throughout your home and thwarting rapid on/off cycling of the pump. A pressure tank that is appropriately sized ensures that the water needs of your household are met and your pump is safeguarded against short-cycling.

If you currently have a 20-gallon pressure tank, you may find yourself wondering if this size is big enough. Many professionals recommend a larger tank so that the pump has less start and stop cycles.

However, in order to properly size your well pressure tank, you need to know the flow rate, minimum runtime, and the pressure switch setting. From these factors, you can determine the drawdown needed, which will tell you the necessary tank size to produce the preferred drawdown capacity.

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What Is a Well Pressure Tank?

In homes with a well, an electric pump extracts water from the well and distributes it throughout your home at a pressure high enough to service all of your fixtures with a sufficient amount of water. If the pump starts and stops too frequently, its lifespan can be depleted due to the constant stress of the startup process and preliminary current surge.

The purpose of a well pressure tank is to sustain the water pressure and protect the life of the well pump. It stores several gallons of water under pressure, permitting the tank to supply the fixtures in your home with water after the pump has shut off. With the tank, the pump does not have to cycle every single time water is needed, effectively extending its life.

When a pump turns on and off too frequently, this is referred to as “short-cycling.” Short cycling places an enormous amount of stress on your well pump and can even cause it to fail prematurely. By prolonging the time between the on and off cycles, a well pressure tank keeps your well pump from experiencing unnecessary wear and tear. It also stabilizes the water pressure in your home, that way your fixtures don’t spit or sputter as they are waiting for the pump to catch up.

With the properly sized well pressure tank, you can successfully avoid any expensive replacements of your well pump.

How Does a Pressure Tank Work?

A well pressure tank is filled with a large volume of compressed air, whose purpose is to push pressurized water out of the tank and throughout your home. The tank’s inlet allows water into the base of the tank. Since air is compressible and water is not, the water that enters the tank will compress the air, pressurizing it.

Inside the pressure tank is a diaphragm, or bladder, which separates the water from the chamber of air. When the well pump hits maximum pressure and turns off, generally 50 to 60 PSI, this newly compressed air will push the water out of the tank when a fixture needs it.

The pressure tank does this without needing to turn the pump back on. Then, the pump will only turn back on to supply fixtures and refill the tank when the air pressure in the tank decreases to the minimum water pressure for the system, generally 30 to 40 PSI.

What Is a Pressure Switch and How Does It Work?

Your well pressure tank is designed to work in union with a pressure switch. The pressure switch’s purpose is to monitor the pressure within the tank. It will then notify the well pump when to turn on and off based on the increase and decrease of pressure inside the tank.

The majority of households are set up to work with a pressure tank that engages the pump at 30 pounds of pressure and then shuts it off at 50 pounds of pressure. There is a gauge located at the front of the pressure tank which indicates when the maximum pressure has been reached. At this point, the pressure switch will automatically shut off the pump.

What Is Pressure Tank Drawdown?

Before you start sizing your well pressure tank, it’s important that you understand pressure tank drawdown. Drawdown refers to the amount of stored water that is available in the tank between the time that the pump turns on and off. As water exits the pressure tank, the pressure inside the tank drops.

For example, if you are using a 30/50 pressure switch, the pressure will continue to decrease until it reaches 30 PSI. At this point, the pressure switch will engage and activate the pump until the tank returns to 50 PSI. The volume of water that exists between 50 PSI and 30 PSI is the pressure tank drawdown.

Put simply, the drawdown describes the length of time that the pump is protected from shutting on and off. Not to be confused with the tank volume, as the total tank volume is a measure of the tank size that is needed to produce the desired drawdown.

Types of Pressure Tanks

Although there are a variety of different pressure tanks, they all do the same thing: rely on compressed air to provide pressure between pump cycles. The most common types of pressure tanks include:

  • Bladder Tanks: With bladder pressure tanks, the water enters into a balloon-like bladder. The bladder is surrounded on all sides by air that takes of the remaining volume of the tank. As the bladder swells with the water, the air becomes compressed. As the water exits the tank, the air condenses the bladder back to its original size.
  • Air-Over-Water Tanks: These pressure tanks consist of a single chamber, with the air rising naturally to the top and compressing as water fills the tank. With no separation between the air and water, air has the ability to dissolve into the water. However, this does require that the air be checked and refilled often. To minimize this issue, some models come with a wafer or float between the air and water.
  • Diaphragm Tanks: In diaphragm pressure tanks, a rubber or vinyl disc-shaped diaphragm exists to separate the air and water into their own compartments. As water enters the tank, it extends the diaphragm upwards toward the top of the tank, resulting in compressed air.

How to Size a Well Pressure Tank

If you’re wondering whether your 20-gallon pressure tank is big enough, you should gather all the necessary information to size your tank appropriately. To do this, you need to know the pressure tank drawdown. In general, the larger the tank, the bigger the drawdown (or the actual amount of stored water). A larger drawdown is correlated to a longer run time and fewer cycles.

To properly size your pressure tank, you need to know the following factors:

  • Flow Rate: The flow rate refers to the gallons per minute (GPM) that your well pump produces.
  • Minimum Run Time: For efficient motor cooling, many manufacturers recommend a minimum run time of one minute. However, larger pumps and one horsepower will necessitate longer run times.
  • Pressure Switch Cut-In and Cut-Out PSI: As previously mentioned, your pressure switch will turn on the pump when the system reaches the low-pressure cut-in and it will turn it off when the high-pressure cut-out is reached. The three pressure switch settings available for pressure tanks are 20/40, 30/50, and 40/60. The first number represents the pressure that your pump must reach to turn back on and the second is the pressure that the pump turns off at. The number that turns off the pump is going to have a direct impact on drawdown capacity. For instance, with a 40/60 switch, you will have much less of a drawdown capacity than you would with a 30/50 switch.

Fortunately, many pressure tank manufacturers will supply you with a chart that indicates your drawdown capacity based on the pressure switch setting. With the above factors, you can easily determine the drawdown needed by multiplying the flow rate by the minimum run time.

As a general rule of thumb, any pump that operates at 10 gallons per minute (GPM) or less should produce one gallon per minute of run time. Anything above 10 GPM, should produce 1.5 gallons per minute of run time and if your flow rate is about 20 GPM, you may need multiple pressure tanks. These calculations would look something like this:

  • 10 GPM flow rate x 1 = 10-gallon drawdown capacity
  • 16 GPM flow rate x 1.5 = 24-gallon drawdown capacity

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Drawdown Capacity vs. Total Tank Volume

In order to determine whether or not your 20-gallon pressure tank is big enough to service your household and protect the life of your pump, you need to know its actual volume of water drawdown. The table below outlines rough estimates of drawdown capacity for internal bladder water pressure tanks and how it related to the actual tank volume.

Pressure Tank Size (volume in gallons)Volume of Water Drawdown (in gallons)
20-gallon tank6 gallons
30-gallon tank9 gallons
50-gallon tank14 gallons
85-gallon tank25 gallons
120-gallon tank36 gallons

Keep in mind that the exact volume of your pressure tank and the anticipated drawdown volume will vary between manufacturers and models.

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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