Pressure Tank FAQ
What is the purpose of the pressure tank on building water supply systems?
Private well and pump systems include a well (the water source), piping from the well to the building, a water pump, and a pressure tank to which building water supply plumbing is connected. Building plumbing fixtures (sinks, toilets, showers, tubs) are supplied with water from the building water supply piping, and drain into the building drain-waste-vent (DWV) system. When water is turned on at a fixture in the building, compressed air in the pressure tank pushes water out into the building water supply piping and thus to its plumbing fixtures. As water leaves the pressure tank, pressure in the tank drops. A pressure control switch, usually mounted on or near the pressure tank, senses the pressure drop, and at a pre-set pressure (typically 20 or 30 psi) the switch turns on the water pump. The pump, located at the tank or perhaps in the well, pumps water to the Building from the well, simultaneously re-pressurizing the water tank and providing water to the building.
If many fixtures are being run at once in the building, or if the water flow rate produced by the pump and piping and controls is a modest one, the pump may run continuously all while the fixture is being operated. More typically, if only one fixture is running and if the pump and well can deliver a high water flow rate, the pump may come on and off several times while the fixture is being run.
Air in the pressure tank acts like a spring or cushion which serves to smooth the delivery of water into the building as the pump cycles on and off. That's why I call this the water pressure tank rather than a water storage tank - though for most people these are the same device. This air cushion effect prevents the water from surging (much) at the plumbing fixture as the pump turns on and off as water is running during use of the fixture (such as a sink or shower). More important, this cushion effect prevents short-cycling of the pump on and off, which in turn prevents pump burnout or water pressure control switch damage which could occur if the system switches on and off too rapidly.
Why is a pressure tank with a buffer or spring or cushion of air needed? Water itself is not very compressible. Since a pump can usually pump faster than the flow of water out of a single faucet, as soon as the pump switched on, water pressure would build way up and the pump would immediately switch off. Water would not flow nicely from the tap. Instead it would come in a series of squirts. Worse, you'd burn up the water pump or pump control switch by this rapid cycling.
The air cushion in the tank acts like a big spring which is compressed by water pushed into the tank by the pump at the same time that water is also flowing out of the pump, tank, and piping system into the building to whatever faucet has been opened. So while water is running in a building served by a private well, part of the time the water flow is being maintained only by air pressure in the pressure tank, and part of the time the water flow is being boosted by the pump during its "on" cycle. And during the pump's "on" cycle, it is both pushing water into the house and re-pressurizing (by compressing) air in the pressure tank.
When do you need a new pressure tank?
When an older type (non-captive-air) tank needs replacement (for example because it has rusted through and has perforations leaking water or air, or if adding air frequently becomes a hassle, you'll want the newer type of "captive air" or "bladder" tank. If your tank looks like it has leprosy, if you see water streaming down its side from a "pinhole" leak, it may be possible to make a temporary emergency repair using a rubber washer and screw. Your hardware store sells special screws for this purpose. This is only a stopgap measure. You need a new pressure tank.
The newer type "captive air" tanks, one which use an internal bladder to contain the water separately from the air charge, can also fail. The bladder can rupture - you need a new tank. The tank itself can develop an air leak - you need a new tank. But these failures occur less often than with the older tank, largely because the tank bladder holding the water supply protects the tank interior from corrosion.
If the building water supply pump is "short cycling" - coming on and off rapidly, you may need to add air to the water pressure tank. This step is important to protect your private pump and well from damage due to short-cycling due to loss of air in the pressure tank. Short cycling of the pump motor can burn up the pump relay control.