Well Pressure Tank: Sizing, Air Charge, and When to Replace It

What a pressure tank does

Most homes have a bladder or diaphragm tank. Nebraska Extension says the tank stores water and gives water under pressure when the pump is not running. It builds a reserve, so the pump starts and stops less often. That makes the pump last longer. Source: NebGuide G2153.

The pressure switch, usually at the bottom of the tank, runs the system. When pressure falls to a set point, often 30 or 40 psi, the switch starts the pump. The pump fills the tank until the stop pressure, usually about 20 psi higher, often 50 or 60 psi. The gauge on the tank shows the pressure in the line. Our pressure switch guide covers the switch itself.

Michigan's Water Well Manual says the tank should reduce wear on the pump's starting parts, prevent rapid stopping and starting (short cycling), and supply water under pressure between pump cycles. Source: State of Michigan Water Well Manual (PDF).

How to size a well pressure tank

The number that matters is drawdown, the water the tank gives between pump starts. Michigan's manual says drawdown should equal or exceed the pump's capacity times its minimum running time. Use the pump maker's cycling rate first. Where none is given, the manual gives this guide:

• 10 to 20 gallons per minute (gpm): 1 minute
• 21 to 50 gpm: 2 minutes
• 51 to 75 gpm: 3 minutes
• 76 to 100 gpm: 4 minutes

The manual's example: a 14 gpm pump on a 1-minute cycle needs a tank that gives 14 gallons. On a 2-minute cycle it needs 28 gallons. Not sure of your pump's gpm? The manual's simple method sets the pump capacity in gpm equal to the number of water fixtures in the home. Our pump size estimator works it out.

A tank gives only part of its total volume. Michigan's manual says a pre-charged bladder or diaphragm tank on a 30-50 psi switch gives about 25 percent. A plain steel tank with a pre-charge gives about 20 percent when its high water level stays at 55 percent of capacity, and one without a pre-charge about 10 percent. A higher operating pressure lowers the drawdown. Tank makers publish the figure for each model.

Nebraska Extension's table gives the drawdown for bladder tanks:

Drawdown in gallons at each pressure switch range. Source: NebGuide G2153, Table I, from the MidWest Plan Service Private Water Systems Handbook.

A variable speed (VFD) pump is different. Michigan's manual says a much smaller tank can be used, and the usual sizing method does not apply. Follow the pump maker's installation specifications.

Signs of a waterlogged tank

Nebraska Extension says a pump that runs only a few seconds and then shuts off means there is not enough air in the tank. The short cycling guide checks that and every other cause in order, and the pump troubleshooter starts from your symptom.

How to check the air charge

Nebraska Extension says a homeowner may be able to check the tank's air pressure with the tank empty. It also warns that some makers void the tank warranty if anyone other than a licensed professional removes the valve cap. Here is how it describes the check a pro does:

1. The target tank pressure is printed on the lid of the pressure switch or just inside its case. It is 2 psi less than the start pressure. A 30 psi start means 28 psi.
2. Turn off the power to the pump.
3. Open faucets until the pressure gauge reads zero.
4. Tap the side of the tank to check that it is empty.
5. Put an automobile tire gauge on the air valve at the top of the tank.
6. If the reading is low, add air with an air compressor up to the target. Put the valve cap back.
7. Check the pressure again in a week.

Do not adjust the pressure switch yourself. Nebraska Extension says it is connected to electric power and could cause a serious injury, and that a licensed professional should do any work on it.

Repair or replace the tank

• Add air when the drained tank is empty of water and only low on air.
• Replace it when water stays in the drained tank, or when the air pressure has dropped again a week after it was set. Nebraska Extension says the tank may need to be replaced in both cases.
• Expect about 10 years. Nebraska Extension says a pressure tank will last about 10 years, though some have lasted 30.

Size the new tank by storage, not total volume. Nebraska Extension says the new tank should hold usable water equal to or greater than the old one at the same switch range. Its example: a 30-gallon galvanized steel tank at 20-40 psi stores 5.1 gallons, so a 14-gallon bladder tank can replace it. Choose the next larger size if the home uses more water now, or if the switch setting must go up.

Low pressure with no clicking? Read low water pressure from a well. For the pump itself, see well pump repair and the pump and pressure help page. The yearly well maintenance checklist includes a tank check.