If you have well water, a well pressure tank is a water storage reservoir that can help keep your water pressure at a constant level.
If you’re putting in a new well or replacing an old well pressure tank, you might have some questions about which size well pressure tank you need.
Several factors determine the pressure tank size you need and which size is best.
What Size Pressure Tank Do I Need?
When you know the flow rate, minimum run time, and pressure switch setting, you can determine the drawdown. Knowing the drawdown will help you determine your tank size.
Pressure tanks come in 20-, 30-, 50-, 85-, and 120-gallon sizes. Size matters when it comes to well pressure tanks, so you’ll need to know a few things before you choose one:
- Flow rate: What is your pump flow rate in gallons per minute (GPM)? The flow rate is usually initially tested when you drill a new well.
- Minimum run time: How long will your pump run?.
- Pressure switch setting: The pressure switch setting is the pressure at which the pump turns on and shuts off. The pressure switch setting can change the tank drawdown.
There are three pressure switch settings for pressure tanks: 20/40, 30/50, and 40/60.
The first number is the lowest pressure (at which the tank turns on), and the last is number is the highest pressure (at which the tank turns off).
Most well pressure tanks come with a chart to tell you the drawdown capacity based on your pressure switch settings.
When sizing a pressure tank and determining the drawdown capacity you need, you can use these guidelines:
- 0 to 10 GPM pump flow rate: 1.0 GPM of run time
- 10 to 20 GPM pump flow rate: 1.5 GPM of run time
- 20+ GMP pump flow rate: 2.0 GPM of run time
Once you know the pump flow rate and minimum run time, you can use the following equation to determine how much drawdown you need in your pressure tank:
Drawdown capacity = Pump flow rate x Minimum run time
Here’s an example of how these calculations might look:
15 GMP pump flow rate x 1.5 GPM minimum run time = 22.5-gallon drawdown capacity
In the above scenario, you would need to use a well-pressure tank with a 22.5-gallon or more drawdown capacity, which would be an 85-gallon pressure tank (more about that later).
Can a Well Pressure Tank Be Too Big?
You can’t have a well pressure tank that’s too big. A larger tank would have longer and slower cycles, but the runtime would be the same.
In fact, bigger is better when it comes to well pressure tanks because they wear out more slowly with fewer startups and longer run times. Having fewer pump cycles increases tank longevity.
The only downsides to having a larger tank are that they cost more and take up more space. There’s also a chance it may pump the well faster than it could recover (although unlikely).
Thus, if space and cost aren’t an issue, it’s in your best interest to size up.
However, it is possible to have a tank that is too small. If your tank is too small, the pump will cycle too quickly, and you can wear your tank out faster, requiring a costly replacement.
The average person uses 101.5 gallons per day of water. The larger the number of people using your well water, the more work your well pressure tank has to do.
A correctly-sized well pressure tank will last an average of 15 years. Some cheaper tanks only last five years, while high-quality pressure tanks can last up to 30 years.
A larger tank will last longer because it doesn’t have to work as hard. On the other hand, choosing a too-small tank will result in needing to replace it more quickly.
Is a 20-Gallon Pressure Tank Big Enough?
A 20-gallon pressure tank is only big enough if your pump flow rate is below seven gallons. It will only work below seven gallons because a 20-gallon tank only has six gallons of drawdown.
It’s important to understand that the volume of water that a well pressure tank can hold is not the same as the volume of its drawdown.
To determine if a 20-gallon pressure tank is big enough to both service your usage needs and protect pump longevity, you need to understand how much drawdown volume a tank has:
- 20-gallon pressure tank: 6 gallons of drawdown
- 30-gallon pressure tank: 9 gallons of drawdown
- 50-gallon pressure tank: 14 gallons of drawdown
- 85-gallon pressure tank: 25 gallons of drawdown
- 120-gallon pressure tank: 36 gallons of drawdown
With the 6-gallon drawdown capacity of a 20-gallon pressure tank in mind, let’s look at the drawdown equation again:
6 GPM flow rate x 1 GPM minimum run time = 6 gallons of drawdown capacity
A pump that has a flow rate above 7 GPM will need more than 6 gallons of drawdown. Thus, a 20-gallon pressure tank is only useful if you have a flow rate at or below 6 gallons.
Even if you don’t need the capacity of a tank that’s bigger than 20 gallons, you may still want to invest in one.
Keep in mind that the larger the pressure tank, the better the tank’s overall life. So, even if your pump flow rate is only 0 to 6 GPM, a larger tank will serve you better for longer.
Will Installing a Larger Pressure Tank Increase Water Pressure?
Getting a larger well pressure tank will not increase your water pressure beyond the pump’s pressure control setting.
Thus, getting a larger pressure tank won’t improve your water pressure.
However, a larger pressure tank will cause the water pressure to decrease more slowly since it has a longer drawdown time. So, with a larger tank, water pressure will be higher for longer.
The best way to improve water pressure is to add a booster pump to improve water pressure and flow, including water pressure in buildings high above the waterline and on upper floors.
Related Articles:
- Expansion Tank vs Pressure Tank: What are the Differences?
- Bladder Tank vs Pressure Tank: What are the Differences?
What Is Well Pressure Tank Drawdown?
Well pressure tank drawdown is the amount of water that a pressure tank stores and makes available between the times when the pump turns on and off.
The tank’s drawdown capacity is the minimum amount of water the pressure tank can store and/or deliver between the time the pump shuts off and restarts.
Here’s an example of the action of a well pressure tank with a 30/50 pressure switch:
- The water pressure starts at 50 PSI.
- The water pressure gradually decreases down to 30 PSI.
- Once the pressure gets down to 30 PSI, the pump will activate again until it reaches back up to 50 PSI.
The volume of water that the tank contains when going from 50 PSI down to 30 PSI in this scenario is the pressure tank drawdown.
You shouldn’t confuse tank drawdown with tank volume. The tank volume is the tank size you need to get the drawdown you desire.
The bigger the well pressure tank, the more water you can store. If you have a larger drawdown, your tank can run longer and will go through fewer cycles.
How to Calculate Drawdown Capacity
You can use the following equation to determine how much drawdown you need:
Drawdown capacity = Pump flow rate x Minimum run time
Note: You should not use the above equation if your system has a variable frequency drive.
