Low-pressure situations are usually desirable, but pressure is a touchy subject when it comes to your well water. It’s an important part of maintaining proper water pressure to your sink, shower, and other water fixtures while also preventing premature wear and tear to your well pump. Since factors like flow rates and pressure switch ranges can vary, choosing well pressure tank sizes isn’t a one-size-fits-all situation. Check out the steps below for how to size a well pressure tank.
How To Size a Well Pressure Tank for Your Home
The pressure is on, but don’t sweat it
Photo: Altin Osmanaj / Adobe Stock
Photo: Altin Osmanaj / Adobe Stock
Written by Alannah Koene
Contributing Writer
Updated November 9, 2023
Difficulty
Easy
No experience? No problem.
Time to complete
30 minutes
Depending on if you need to do manual tests.
Cost
$0
Roll up your sleeves!
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What you'll need:
TOOLS
- Two 5-gallon buckets
- Garden hose
- Calculator
SUPPLIES
- Pen or pencil
- Paper
Identify Flow Rate
The flow rate of your pump refers to how many gallons per minute (GPM) it delivers. This is typically tested when you or a professional drill a new well and may be in your records, but you can test it later if you don’t have the information handy.
Pressure tanks are filled with one part water and two parts air to create pressure in your water fixtures, so measuring the flow rate takes some calculations. To perform a simple test, get a few buckets that have either measurement lines or a standardized size, such as 5 gallons. Without any other water sources running, fill your bucket(s) for 60 seconds at full blast from a garden hose. The number of gallons at the end of the 60 seconds represents your flow rate—for example, if you have 10 gallons at the end of the test, your flow rate is 10 GPM.
Calculate Drawdown Capacity
The drawdown capacity of your pump refers to the usable amount of water stored between a high- and low-pressure range. When water flows out of the tank to your water fixtures, the tank pressure decreases until it hits the low end of the range, such as 30 PSI (pounds per square inch), then turns on the pressure switch until it reaches the high end of the range, such as 50 PSI.
When multiplied by the flow rate, the minimum runtime describes how long the pump takes to give you the appropriate drawdown capacity. The standard guidelines for GPM and minimum runtime are as follows:
0–10 GPM: 1 gallon of drawdown per 1 GPM of flow
10–20 GPM: 1.5 gallons of drawdown per 1 GPM of flow
20+ GPM: 2 gallons of drawdown per 1 GPM of flow
As an example, a pump with a 10 GPM flow rate should have about a 10-gallon minimum drawdown capacity. A pump with a 16 GPM flow rate multiplied by 1.5 should have a 24-gallon minimum drawdown capacity.
Determine Pressure Switch Setting
Photo: galinast / iStock / Getty Images Plus / Getty ImagesThe pressure switch setting refers to the low and high pressure ranges that determine when your well pump should turn on or off. Settings vary, but most pumps are either 30/50 or 40/60. The lower of the two numbers represents the PSI that triggers the pump, and the higher number is the PSI that turns off the pump. Most pressure tank manufacturers provide a chart with the unit indicating the drawdown capacity in reference to the pressure switch setting.
Calculate Tank Size Needs
Once you’ve determined the flow rate, drawdown capacity, and pressure switch setting, you can calculate your tank capacity needs. Pressure tank sizes are standardized by size and associated drawdown volume:
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
Let’s say your pump has a 10 GPM flow rate and 1X GPM runtime for a 10-gallon drawdown capacity. According to these standards, you would need a 50-gallon tank. This is where the pressure switch setting comes in: The higher the pressure switch setting is, the lower the drawdown capacity is. If you’re operating with a higher pressure in this example, you may need to upgrade to an 85-gallon pressure tank.
DIY vs. Hiring a Pro
A plumber can install a well pressure tank for between $125 and $200 in labor costs, and they can help you select the appropriate size for your home as part of the cost of well pressure tank replacement.
If you plan to install the well pressure tank yourself, determining the size on your own might be more convenient than calling a local well pump company. Sizing your own well pressure tank requires minimal time and resources, and you can usually get input from the supplier as you start shopping around for the unit.
Frequently Asked Questions
Oversizing a well pressure tank won’t hurt your well system. Larger tanks result in longer and slower pump cycles and consequently fewer startups and longer runtimes, which means the tank may have better longevity. The only drawback to oversizing is efficiency since larger tanks tend to cost more and take up more space.
Pressure tanks can last anywhere from 5 to 15 years, but the lifespan of your tank depends on a few factors. Lower-quality tanks tend to wear out more quickly than high-end options, and the quality of water and the amount of sediment or rocks from your well can cause premature damage. Overworking an undersized pressure tank can also limit its lifespan.
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