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« Last post by Cary Austin on June 11, 2025, 11:37:08 AM »
Many customers have a large pressure tank or multiple pressure tanks in the attempt to supply extra storage for a low producing well. Many times a large pressure tank or multiple pressure tanks can be detrimental to a low producing well, in that they become another demand that needs to be satisfied, on top of the water being used for the house, restaurant, or other entity. I usually hear of air spurting out of the faucets. This is because the well is pumped dry and making air while the large tank(s) are refilled. Air from the faucets is not good, but that also means the pump is drawing air, which greatly shortens pump life. There are ways to get some storage from pressure tanks, which I will discuss further.
First, a pressure tank only holds about 25% water, the rest is the air needed to push the water out when demanded. This means to store as little as 100 gallons would take five of 86 gallon size pressure tanks. With a 50/70 pressure switch and 49 PSI air charge in the tank, an 86 gallon size tank only holds 20 gallons of water. Then it needs to be understood that the stored water in a pressure tank is used before the pump is started. Only when the tank(s) are empty does the low pressure cause the pump to start. Then all you have left is the water coming from the pump and low producing well before you are out and it starts pumping air.
Pressure tank manufacturers started the fallacy that pressure tanks can work as storage. Reading there instructions carefully you can see they recommend a much lower air charge in the tank when needing storage. Using a 50/70 switch, and only putting 30 PSI air in the tank, means the tank is not empty at 49 PSI and still produces water until the pressure drops to 30 PSI. This will make an 86 gallon size tank that hold 20 gallons with a 49 PSI air charge hold about 25 gallons instead. However, there is only 5 gallons left in the tank when the pump starts at 50 PSI. So, you really only have 5 gallons of stored water in an 86 gallon size tank that way.
Then, with a 50/70 pressure switch you have no control over how much water is in the pressure tank(s) to use before the pump starts. If by chance the pressure was still at 70 when you start using water, you will get 20 gallons out of an 86 gallon size pressure tank as the pressure drops from 70 to 50, which is when the pump starts. The 5 gallons left in the tank will be exhausted while the pump is running and the pressure continues to fall to 30 PSI. At which point the tank is empty, the volume produced is limited to the size of the pump, and only for as long as the well can supply that much. If for instance you have an 8 GPM well, and are using 12 GPM, there will only be 8 GPM coming out the faucet with spurts of air. Technically, the pump should be equipped with a device like the Cycle Sensor, which will shut the pump off when air is detected to prevent destroying the pump. But without a Cycle Sensor, the pump will just make what the well can give. Only when the demand decreases below 8 GPM or stops can the well recover, and the pump refill the pressure tanks. But the pump is a 12 GPM pump and is trying to refill the tank(s) at 12 GPM, when only getting 8 GPM to work with. This allows a lot of air to get in the pressure tanks, which will blast out of the faucets the next time they are opened.
An 8 GPM Dole valve on the line going to the pressure tank(s) will limit the tank fill rate to the well production, and keep the pump from making air while refilling the big tanks. A check valve bypassing the Dole valve is needed so the tank(s) can supply more than 8 GPM when needed. However, even without filling the tanks with air, once the tanks are empty and the demand is more than the 8 GPM the well can make, the pump will still be drawing air and spurting out the faucets. There really is no way to use a pressure tank for storage, the manufacturers just like selling multiple large tanks for any reason they can persuade you to think it will work.
A retention tank is also worthless for storage. No water comes out of a retention tank without the same amount of water entering the tank at the other end. Retention is all they are good for.
Using an SQE instead of a 50/70 pressure switch makes the matter even worse. The SQE turns on when the pressure is 7 PSI below the set point, which in this case is 60 PSI. Then the SQE will keep the pressure constant at 60 PSI for as long as water is being used. With 49 PSI air charge in the pressure tank(s), an 86 gallon size tank will only add about 10 more gallons to the volume the pump/well can supply before it starts pumping air again. Then when the demand stops, the SQE refills the pressure tank(s) to 7 PSI above the set point. This is only 14 PSI differential for the tank to work with, making an 86 gallon size tank that holds 20 gallons with a 50/70 switch, only hold 14 gallons when using an SQE/CU301/2.
On top of that, if the first demand is large enough to quickly drop the pressure, the SQE will start at its set point and not drop the 7 PSI to get any water from the tank. Only when the first demand drops the pressure slowly will the pressure fall 7 PSI below the set point before the pump starts. That means you are only getting 7 gallons of water from the 86 gallon size tank.
You would be much better off with a cistern/storage tank and a booster pump to use with the pressure tank. Even a 100 gallon cistern would supply more volume than five 86 gallon size pressure tanks. A booster pump in the cistern supplies as much volume as needed at 60 PSI constant pressure. This can be done using as small as a 4.5 gallon size tank when the pump is controlled with a Cycle Stop Valve. A larger pressure tank would be fine, just not needed as all the water needed is stored in the cistern and well. When there is a demand and the booster pump is started, the CSV will supply whatever volume is needed at 60 PSI constant. When the cistern has only dropped a foot or so, the well pump is activated, and using an 8 GPM Dole valve, refills the cistern at 8 GPM without pumping the well dry. While refilling the cistern at 8 GPM, the booster pump can be supplying say 15 GPM, and the extra 7 GPM needed will lower the level in the cistern. This means a little 100 gallon cistern can help an 8 GPM well supply 15 GPM for up to 14 minutes before the cistern runs dry. If 14 minutes is not enough time to handle peak demands like 15 GPM, then a larger cistern is needed.
This is a complicated explanation, which is why tank manufacturers can make people think a pressure tank can be used as storage. However, in my more than half a century of working with these type of systems, pressure tanks always cause more problems when trying to be used for storage. A pressure tanks only job is to limit the number of on/of cycles of the pump, not storage. When you have a Cycle Stop Valve to limit the pump cycling, not much of a pressure tank is needed at all. There always needs to be enough water either in the well or in a cistern storage tank to supply the demand required, because a pressure tank can't do it.
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