How Does a Well Pressure Tank Work?

The Basic Function

A well pressure tank serves one primary purpose: to store pressurized water so your pump doesn't have to run every time you open a faucet. Without a pressure tank, your pump would start and stop hundreds of times per day — every time someone washes their hands, flushes a toilet, or runs the dishwasher. That kind of short-cycling destroys pump motors in months.

The tank works by using compressed air to maintain pressure in your water system between pump cycles. When the pump runs, it fills the tank with water, compressing the air inside. When you open a faucet, the compressed air pushes water out of the tank — without the pump running. The pump only starts when the tank pressure drops to the cut-in pressure (typically 30 or 40 PSI) and stops when it reaches the cut-out pressure (typically 50 or 60 PSI).

Bladder vs. Diaphragm vs. Air-Over-Water Tanks

Modern pressure tanks use either a bladder or a diaphragm to separate the air charge from the water. This prevents the air from being absorbed into the water over time — a problem that plagued older "air-over-water" tanks, which required periodic air recharging.

Bladder tanks have a rubber bag (bladder) inside the tank. Water fills the bladder; air surrounds it. When the bladder fails, water and air mix — the tank becomes waterlogged.

Diaphragm tanks have a rubber membrane that divides the tank into two chambers. They work the same way as bladder tanks and fail the same way.

Air-over-water tanks (older style) have no separation — air sits on top of water. These require periodic air recharging and are prone to waterlogging. Most have been replaced with bladder or diaphragm tanks.

What Is a Waterlogged Pressure Tank?

A waterlogged pressure tank has lost its air charge. The bladder has failed, and the tank is completely full of water with no air cushion. When this happens, the tank can no longer store pressurized water — it becomes just a pipe fitting.

The result is pump short-cycling: the pump starts and stops every few seconds, every time any water is used. Each start puts enormous stress on the pump motor — the startup current is 5–7 times the running current. A pump that normally starts 10–15 times per day may start 300–500 times per day with a waterlogged tank. Most pump motors fail within weeks to months under these conditions.

How to Check Your Tank's Pre-Charge Pressure

The pre-charge pressure is the air pressure in the tank before any water is added. It should be set to 2 PSI below the cut-in pressure of your pressure switch. For a 30/50 PSI switch, the pre-charge should be 28 PSI. For a 40/60 PSI switch, it should be 38 PSI.

To check the pre-charge: turn off the pump, drain all water from the tank (open a faucet until no more water flows), then check the air valve on the tank with a tire pressure gauge. If the reading is significantly below the target, the bladder may have failed or the tank needs recharging.

Important: If water comes out of the air valve when you press the pin, the bladder has failed and the tank needs to be replaced — not recharged.

How Long Does a Pressure Tank Last?

A quality pressure tank (Amtrol WellMate, Pentair, or similar) lasts 10–15 years under normal conditions. In Maryland, where well water often has high iron content and low pH, tank life can be shorter — acidic water accelerates corrosion of the tank exterior and can degrade the bladder material.

Annual pre-charge pressure checks are the best way to extend tank life and catch bladder failure early — before it destroys the pump.

Sizing Matters

A properly-sized pressure tank allows the pump to run in long, efficient cycles — typically 1–2 minutes per cycle. An undersized tank causes short-cycling even when the bladder is intact. The correct tank size depends on the pump's flow rate (GPM) and the desired cycle time.

For most residential well pumps (1/2 to 1 HP), a 44-gallon tank is the standard recommendation. Smaller tanks (20 gallon) are often installed to save money but cause premature pump failure.