A well pressure tank creates water pressure by using compressed air to push down on the water. When a valve is opened, the compressed air in the tank pushes out the water. The water is pushed through the plumbing until the pressure drops to a preset low on the pressure switch. Once the low setting is reached, the pressure switch communicates with the water pump telling it to turn on so that more water can be pushed into the tank and house. To determine the proper size well pressure tank needed, you’ll need to consider pump flow rate, pump run time, and cut-in/cut-out psi.
What is Pressure Tank Drawdown Capacity?
Drawdown capacity is the minimum amount of water stored and delivered by the pressure tank between the pump shut-off and pump re-start. Do not confuse drawdown capacity with the tank volume size. The bigger your tank the larger the drawdown (actual amount of stored water) you will have. A larger drawdown means that the run time is longer and will cycle less often. It is common for manufacturers to recommend a minimum run time of one minute for motor cooling. Larger pumps and pumps with higher horsepower require longer run times.
Factors in Choosing Proper Well Tank Size
• The first thing you need to know is your pump’s flow rate. How fast does it pump? This is based on gallons per minute (GPM).
• Then, you’ll need to know the minimum run time of the pump. If it’s less than 10 GPM in flow rate, it should be 1 GPM of run time. Anything greater than 10 GPM in flow rate should be 1.5 GPM in run time. The formula for determining your drawdown capacity would be Flow Rate x Run Time = Drawdown Capacity.
• The third factor would be Pressure Switch Setting. Standard options are 20/40, 30/50, and 40/60. The first number is to back on the pressure, the second number is to turn off the pump pressure. (Most manufactures will have a chart telling you what the drawdown number is based on the pressure switch.)
Does Home Size Matter?
When it comes to well tank sizing, the square footage of your house is less important than the flow rate and pump run time. It’s really all about the gallons per minute that you’re using in your home at a given time.
The Proper Size Well Tank
Your properly sized well tank is based on the flow rate times run time which equals the drawdown capacity, then your pressure switch settings. The higher the flow rate the larger tank you can use.