How does my private well pressure tank work?

When well water enters your home, it is pumped into a pressure tank. A pressure tank serves two basic purposes: First, it provides water storage and water under pressure to your home when the pump is not running. In times of high demand, it also provides a reservoir of additional water to help the pump keep up. Pressure tanks operate by maintaining a range of pressure in the tank that forces water out into the line when a faucet or other source is opened.

When the pressure in the line drops below the low-end preset level, generally 20 to 40 PSI, it engages automatic controls and preset gauges like the pressure switch in Figure 1. This causes the pump to kick on and begin pumping water into the tank and home distribution lines. The pump continues running until the usage stops and or the pressure in the line reaches the preset high-end level, generally 40 to 60 PSI. The minimum pressure has to be high enough to lift water to the highest and farthest point in the line.

The original pressure tank was a single tank of pressurized air that could have water pumped into it. Because air compresses, but water doesn't, pumping water into a pressurized air tank compresses the air even further. The compressed air creates the pressure needed to push the water throughout your home. Many of these tanks are still in use today. However, over time, the air can dissolve into the water, lowering the air pressure in the tank and reducing the amount of air in the tank. When this happens, the tank is effectively waterlogged and more air has to be added to keep the tank functioning properly, thus requiring more maintenance.

When your tank is waterlogged, the reduced amount of air means that using water from the tank will quickly lower the pressure in the tank so that the pump will kick on to re-pressurize it. With less air in the tank, your pump cycles on and off much more often, which both wastes energy and is hard on the pump. Because of this issue, these tanks are rarely installed anymore. To help deal with the waterlogging problem and to help the tank maintain its air for a longer period of time, a wafer or a float can be installed in a standard air tank. This separates the water and air with a flexible barrier that keeps them in their own compartments. The advantage is that the tank operates more efficiently and will require less maintenance.

Today, most new pressure tanks are bladder tanks. As the name implies, a bladder tank contains a flexible bladder that permanently separates the air and water. The water goes into the bladder, which expands against the air pressure in the tank. As water is used, the air in the tank pushes against the bladder, so that water in the bladder empties into the distribution line. As the bladder empties, the pressure in the tank drops to a point that the pump kicks on to begin refilling the bladder again.

There are a number of factors influencing tank size. A low yielding well, for example, would require a larger tank for water storage. That way the pump can provide water at a much lower rate to fill the tank when no one is using water. The pressure tank stores the water and has it available during times of higher demand. Without a large pressure tank, water use would quickly empty the tank — requiring the pump to kick on and try to keep up. If the demand were high (like during a shower) and your well only produced a few gallons a minute, it could cause a supply interruption.

If well yield isn't a concern, then it might be possible to use a variable speed pumping system and controller to help maintain water pressure. The pump maintains the pressure in the system and changes speeds to match the changing pressure requirements. For these installations, the pressure tank is only a few gallons in size and helps keep the pressure constant on pump start-up.

The Private Well Class is a collaboration between the Rural Community Assistance Partnership and the University of Illinois through the Illinois State Water Survey and the Illinois Water Resource Center and funded by the United States Environmental Protection Agency. The views expressed here are solely from the class authors and not endorsed or reviewed by USEPA. For more information on private wells, sign up for our free 10-week email course, at www.privatewellclass.org.