How to Size a Well Pressure Tank? A Complete Guide

A properly sized pressure tank is essential for maintaining the efficiency and longevity of your well system. Well pressure tanks help extend the lifespan of your well pump by minimizing rapid cycling and ensuring consistent water pressure throughout your home. Choosing the right tank size is crucial to meeting household water needs while protecting your pump from premature wear.

Whether you are installing a new system or replacing an old tank, understanding how to size a well pressure tank correctly is key. This guide will walk you through the process of selecting the appropriate tank size based on your well's output.

What is a Well Pressure Tank?

A well pressure tank is a specialized storage unit used in private well systems to regulate water pressure and protect the well pump. It consists of several key components that work together to maintain steady water flow:

• Tank: Holds both water and air, typically in separate sections.

• Bladder or Diaphragm: A flexible rubber bladder or diaphragm prevents air and water from mixing.

• Air Chamber: Compressed air creates the necessary pressure to push water through your plumbing system.

• Pressure Switch: Monitors the pressure and signals the well pump to turn on or off as needed.

By holding a reserve of pressurized water, the pressure tank reduces how often the well pump cycles on and off. This helps prevent premature pump failure and ensures a steady water supply whenever a faucet is turned on, a toilet is flushed, or a shower is used.

How Does a Well Pressure Tank Work?

A well pressure tank regulates water pressure using compressed air, ensuring a consistent flow throughout your home. Here’s how it operates:

1. Water Enters the Tank
   The well pump draws water into the tank. As water fills the tank, it compresses the air inside, creating pressure.

2. Pressure Builds Up
   As more water enters, the air pressure inside increases. The pressure switch, typically set at 30/50 psi or 40/60 psi, manages this process. When the upper pressure limit is reached, the switch turns off the pump.

3. Water is Supplied to the Home
   When a faucet or appliance is turned on, the compressed air pushes water out through the plumbing system, providing water without needing the pump to activate immediately.

4. Pressure Drops & the Pump Reactivates
   As water is used, pressure decreases. Once it reaches the lower limit (e.g., 30 psi), the pressure switch signals the pump to turn on, refilling the tank and repeating the cycle.

How to Size Your Well Pressure Tank

Properly sizing a well pressure tank is essential for maintaining stable water pressure, reducing pump wear, and preventing short cycling. Here’s how to determine the right size:

1. Find Your Pump’s Flow Rate (GPM)

The pump flow rate, measured in gallons per minute (GPM), determines how much water your well pump can deliver. A tank that is too small for your pump will cause frequent cycling, while an oversized tank may lead to inefficiencies.

How to Measure Your Pump’s Flow Rate:

• Fill a 1-gallon container with water from your system.

• Time how long it takes to fill the container (e.g., 10 seconds).

• Use the formula:
  Flow rate (GPM) = 60 seconds ÷ fill time
  Example: If it takes 10 seconds, your flow rate is 6 GPM (60 ÷ 10 = 6).

2. Calculate Minimum Runtime & Drawdown Capacity

The drawdown capacity is the amount of usable water stored in the tank between pump cycles. Preventing short cycling extends the pump’s lifespan. Use this formula:

Tank size (gallons) = Pump flow rate (GPM) × Minimum runtime (minutes) × Multiplier

• Pumps with 10 GPM or less should run for at least 1 minute.
  Example: 10 GPM × 1 minute = 10 gallons drawdown capacity.

• Pumps over 10 GPM require 1.5 minutes minimum runtime.
  Example: 16 GPM × 1.5 minutes = 24 gallons drawdown capacity.

• Pumps over 20 GPM may require multiple tanks for optimal performance.

3. Determine Your Pressure Switch Setting

The pressure switch controls when the pump activates. Common settings include 30/50 psi and 40/60 psi:

• Cut-on pressure: The pressure at which the pump starts (e.g., 30 or 40 psi).

• Cut-off pressure: The pressure at which the pump stops (e.g., 50 or 60 psi).

A 30/50 psi switch provides more drawdown capacity than a 40/60 psi switch. Most manufacturers offer charts to help match the correct tank size with the appropriate pressure switch setting.

By following these guidelines, you can ensure your well pressure tank is properly sized for your home, improving efficiency, reducing maintenance costs, and extending the lifespan of your well pump.