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Sizing a Pressure Tank Correctly - More Protection for your Pump


Would you like to prolong the life of your pump?

Would you like to save money by reducing energy use?

If the answer is yes to either of the questions above, it is important to have the correct sized pressure tank for your system. Choosing the proper tank for your pumping system will greatly reduce the risk of premature pump failure. If you fail to invest in the correct size pressure tank, you can short cycle your pump. Short cycling is a term used to describe when your pump is turning on and off too frequently. It increases energy use and can cause premature failure of your pump as it builds up excessive heat. When it comes to pressure tanks, bigger is almost always better.

A pressure tank creates water pressure by using compressed air to bear down on the water. When a valve is opened, water is pushed out by the compressed air in the tank. The water is pushed throughout the plumbing in the house until the pressure drops to a preset low on your pressure switch. When the low setting is reached the pressure switch tells the water pump to turn on and more water is pushed into the tank and house.

The bigger the tank you have the larger the drawdown (actual amount of water stored) you will have. A larger drawdown means longer run time and therefore less cycles. Most manufacturers recommend a minimum run time of one minute for efficient motor cooling. One horsepower and larger pumps require longer run times.

To determine the proper size of the pressure tank, there are three factors to consider:
1. Pump flow rate in gallons per minute (GPM)
2. Desired run time of the pump
3. Cut-in and cut-out psi (pounds per square inch) of the pressure switch. The pump turns on when the system reaches the low pressure cut-in and off when the high pressure cut-in is reached.

From these factors you can determine drawdown needed with the following equation:

Tank drawdown capacity required = Pump flow rate multiplied by run time

Tank drawdown capacity is the minimum amount of water stored and/or delivered by the pressure tank between pump shut-off and pump re-start. This should not be confused with “tank volume.” For example, a pre-charged tank with a tank volume of 20 gallons has only five to seven gallons drawdown capacity depending on the cut-in / cut-out (on/off) setting of the pressure switch. Pumps with flow rates up to 10 GPM should have a tank with a minimum of one gallon drawdown capacity for each GPM delivered by the pump.

Note: The above calculations should not be used if you have a variable frequency drive in your system. Consult your manual for manufacturer recommendations on tank size.



Looking for the benefits of having a properly sized pressure tank in your home? Call us today at RC Worst & Company to help you choose the right tank.


1 comment(s)
Caroline June 5, 2018 2:31 AM reply
Thanks for sharing the importance of sizing a pressure tank. Caroline

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