How a Well Pressure Tank Stores Water Between Pump Starts

A well pressure tank uses compressed air to push stored water into your plumbing while the well pump is off. When the pump runs, it forces water into the tank and compresses the air. When you open a faucet, that air expands and pushes water back out. As pressure falls, a pressure switch starts the pump again. This stored-water buffer keeps the pump from starting for every small water use. Water Worker’s tank guide illustrates this cycle.

The tank does not generate pressure independently: the pump supplies the energy, the compressed air stores it, and the pressure switch controls when the pump runs.

What is inside the tank?

In a precharged well tank, a flexible bladder or diaphragm separates the water from the air charge. Water occupies one side of the barrier; compressed air occupies the other. The barrier moves as water enters and leaves.

The air is essential because it compresses readily, whereas water is nearly incompressible at household operating pressures. Without a useful air cushion, withdrawing a small amount of water would cause pressure to fall sharply and the pump to restart. The Private Well Class explains this mechanism and the separation used in modern tanks.

The surrounding components have distinct jobs:

• Well pump: moves water from the well and raises its pressure.
• Pressure tank: stores a limited volume of water under pressure.
• Pressure switch: starts and stops the pump at its pressure settings.
• Pressure gauge: displays water-system pressure where it is connected.

The operating cycle: a 40/60 PSI example

A conventional fixed-speed system with a 40/60 PSI pressure switch starts the pump at approximately 40 PSI and stops it at approximately 60 PSI.

1. Pump off, tank charged: At cut-out—60 PSI in this example—the tank contains water and compressed air. It is not completely full of water; air still occupies part of its volume.
2. A faucet opens: The expanding air pushes water from the tank into the plumbing. Tank pressure gradually falls.
3. Pressure reaches cut-in: At about 40 PSI, the switch starts the pump. The pump supplies the plumbing and, when its delivery exceeds demand, refills the tank.
4. Pressure returns to cut-out: As water enters the tank, it compresses the air. At about 60 PSI, the switch stops the pump.

These steps follow the operating cycle in Pentair’s Pro-Source tank manual.

If water use continues, the pump may remain running until demand falls enough for pressure to reach cut-out. Water does not necessarily pass through the tank first: in a typical tee-connected arrangement, the pump can supply the house directly while the tank accepts or releases water.

Tank gallons are not usable gallons

Drawdown is the water delivered between pump cut-out and the next cut-in. It is smaller than the tank’s total volume because the air cushion takes up space and some water remains at cut-in.

For example, Water Worker lists its 20-gallon HT-20B tank with 5.35 gallons of drawdown at 40/60 PSI, versus 6.18 gallons at 30/50 PSI. Those are model-specific ratings, not a rule for every 20-gallon tank. See the manufacturer’s drawdown table.

That distinction matters when sizing: usable drawdown, pump delivery and the motor’s required run time matter more than the gallon label alone. Our guide to sizing a bladder tank by drawdown covers that calculation.

Precharge is different from operating pressure

Precharge is the tank’s air pressure with the water side empty and depressurized. It is not the pressure shown on the plumbing gauge during normal operation.

For conventional systems covered by its instructions, Water Worker specifies precharge 2 PSI below cut-in—38 PSI for a 40/60 system—with the pump off and the tank empty. Follow your tank and controller manuals rather than assuming that setting applies to every installation. Adding air alone is not a way to increase household water pressure. Water Worker’s FAQ addresses both points.

Before checking or adjusting the air charge, disconnect electrical power to the pump and drain the tank’s water side completely, as directed in Pentair’s manual. For the checking procedure and setting distinctions, see precharge versus operating pressure.

What changes when the air cushion is lost?

Older air-over-water tanks have no bladder or diaphragm. Air contacts the water directly and can gradually dissolve into it, so these systems need a means of maintaining their air charge. Modern separated-air tanks reduce that problem, but air loss or barrier failure can still reduce effective storage.

With too little usable air volume, pressure rises and falls quickly, causing frequent pump starts—often called short cycling. That is a reason to investigate the tank and controls, not proof that the pump itself has failed. The Private Well Class explanation describes why waterlogging increases cycling.

A functioning tank buffers demand; it cannot correct an inadequate well supply or make a pump deliver beyond its capacity.