Quick answer: A well pressure tank stores water under compressed air pressure so your pump doesn’t have to run every time you open a faucet. As the pump fills the tank, trapped air compresses and pressure rises until it reaches the pressure switch’s cut-out point, which stops the pump. As you use water, that compressed air pushes the water back out, and pressure drops until it hits the cut-in point, which restarts the pump. The tank is what turns a pump that runs in short bursts into a system that delivers steady pressure.

The core mechanism: compressed air doing the work

A well pump alone can’t deliver smooth, on-demand water pressure. Left to run only when a faucet is open, it would start and stop constantly, which wears out the pump and the pressure switch quickly. The pressure tank solves this by acting as a buffer: it stores a reserve of water under pressure so the pump can run in longer, less frequent cycles instead of short bursts every time someone turns on a tap.

Inside the tank, a fixed charge of air sits separated from the water (in a bladder or diaphragm tank, a rubber membrane keeps the two apart). Water is not compressible, but air is. When the pump pushes water into the tank, the air pocket shrinks and its pressure rises. That rising air pressure is what pushes water out to your fixtures between pump cycles, and it’s also what the pressure switch measures to decide when to start and stop the pump.

The full pump cycle, step by step

  1. Pump starts (cut-in). When pressure in the system drops to the switch’s lower setting (for example, 40 psi in a 40/60 setup), the pressure switch closes its contacts and starts the pump.
  2. Tank fills, air compresses. Water flows into the tank, compressing the air charge. System pressure climbs steadily.
  3. Pump stops (cut-out). Once pressure reaches the switch’s upper setting (60 psi in the example above), the switch opens and the pump shuts off.
  4. Water is drawn from storage. As you run faucets, showers, or appliances, the compressed air pushes stored water out of the tank, and pressure gradually falls.
  5. Cycle repeats. Once pressure falls back to the cut-in point, the pump restarts and the cycle begins again.

The amount of water delivered between cut-in and cut-out is called drawdown, and it’s what actually determines how long the pump runs and rests each cycle. We cover how to size that correctly in our well pressure tank sizing guide.

Why the air charge needs to be set correctly

Before the tank is ever connected to water, it needs a specific amount of air pressure added on the air side, called the pre-charge. Both major tank manufacturers (Amtrol and Flexcon) confirm the same rule for setting it: pre-charge should be set 2 psi below the pressure switch’s cut-in setting. On a 40/60 system, that means a pre-charge of 38 psi.

If the pre-charge is wrong, two different problems show up:

  • Pre-charge too low: the bladder over-expands, water fills more of the tank than it should, and the tank delivers less usable drawdown than it’s rated for. In severe cases the bladder can be stretched and damaged over time.
  • Pre-charge too high: the tank fills with less water before reaching cut-out, so the pump receives less run time per cycle. This directly causes short cycling, which we cover in our guide to diagnosing well pump problems.

Pre-charge should be checked with the system fully depressurized and no water in the tank, using a standard tire-style pressure gauge on the tank’s air valve.

Bladder, diaphragm, and plain steel tanks

Not all pressure tanks separate air and water the same way:

  • Bladder tanks use a flexible bag inside the tank that holds the water, with air filling the space around it. This is the most common design in current residential tanks (Amtrol Well-X-Trol and Flexcon Well-Rite both use this approach).
  • Diaphragm tanks use a fixed membrane that divides the tank into two permanent chambers, one for air and one for water.
  • Plain steel tanks (no bladder) are the older style, where air and water sit in direct contact inside one bare steel tank. Over time, water absorbs the trapped air, a problem known as waterlogging. A waterlogged tank has little to no usable air cushion left, so the pump short cycles heavily. This is the main reason bladder and diaphragm tanks replaced plain steel tanks in most modern installations.

Signs your pressure tank isn’t working correctly

  • Rapid, repeated pump cycling (on and off every 10 to 30 seconds) usually points to a lost air charge, a waterlogged tank, or a tank that’s undersized for the pump.
  • Noticeable pressure swings between pump cycles, more than the switch’s rated spread, can mean the tank isn’t holding its air charge.
  • Water hammer or banging pipes right after a faucet closes can indicate a failed bladder or incorrect pre-charge.

For a full breakdown of causes and fixes, see our guide on well pump troubleshooting.

FAQ

Do I need a pressure tank if I have a constant pressure system?

Constant pressure systems use a variable-speed pump controller instead of a large tank to maintain steady pressure, though most still use a small tank as a buffer. This works differently from a traditional tank and switch setup; we cover it separately in our pressure systems guide.

How do I check if my pressure tank still has the correct pre-charge?

Shut off the pump, open a faucet to drain all pressure from the system to 0 psi, then check the air valve on the tank with a standard gauge. It should read 2 psi below your pressure switch’s cut-in setting.

Can a pressure tank be repaired, or does it need to be replaced?

A lost air charge from a slow leak at the air valve can often be corrected by re-pressurizing the tank. A ruptured bladder or diaphragm cannot be repaired and requires tank replacement.