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The purpose of a well pressure tank is two-fold. First, by having a tank of mostly air pressurized to the level of your fresh water system, you have a reserve of pressure that allows the water pump to only run intermittently. If your electricity goes off, you will be able to draw a few gallons of water before the system goes dry. Secondly, the air in the tank absorbs the surges of water that pumps characteristically move, smoothing out the flow from your faucets.
There are two types of tanks. Older homes used a simple tank which held the cushioning air. These tanks have a standard automotive-type air valve for adding air. In newer homes or for tank replacement, a tank with an internal bladder is used. This bladder is like a balloon filled with air, which keeps air and water separate.
Over time, the air in a simple well tank will be absorbed by the water. As the amount of air in the tank decreases, the tank looses its ability to hold pressure, and the well pump on-off cycle time decreases until the pump cycle causes surging at the faucets. This continuous running is potentially damaging to the pump and can literally take years off its life. A years worth of wear and tear can occur in a matter of weeks as the pump wildly cycles on and off trying to maintain pressure in your water system.
To recharge a bladderless well tank, the tank must be completely drained of water. You may need to rent a powerful air compressor to blow the air out of the tank if your tank in below grade without a convenient drain. The procedure is simple... first turn off the well pump switch or flip the circuit breaker. Open a faucet above the level of the tank to relieve all pressure in the system. Attach the compressor hose to the air valve on the tank and blow air into it. When air begins to come through the open faucet, disconnect the compressor.
Turn on your well pump and the tank will be automatically pressurized. You may close the faucet after all air is exhausted from the system. Be aware that you may experience sudden spurts of air from faucets and toilets for a day or so as the system relieves itself of air introduced by the partial draining of the system.
If you have a bladder-type tank, the tank should be pre-pressurized to the same as the low pressure setting on your pump. Usually, the bladders do not leak, so the only reason for adding air to the tank would be if you decide to increase the water pressure in your home. Conceivably, if the pressure is the tank is too low, you could overstretch or break the bladder if you were to increase the system pressure. You would not necessarily have to replace the tank, but you would lose the benefits of a bladdered tank.
To adjust the pressure in a bladdered tank, first decide on the LOW pressure you want for your system. If you are not changing the pressure, but wish to check to be sure your tank is properly charged, look at the pressure gauge near the tank and taking note of the pressure level at which the pump turns on.
To perform the charge, the pressure in the system must be released by turning off the well pump and opening a faucet. Use a compressor or a hand pump to increase the pressure in the tank to the desired level. You don't need as powerful a compressor as you do for vacating all the water from the tank. A small electric tire pump will do the job nicely. You can even use a hand pump if you want to, though it may take an eternity to increase the pressure even a small amount!