Water Storage & Pressure Tanks - how they work
InspectAPedia^®  -    

• The purpose of water storage or water pressure tanks on private well systems
• How to diagnose & correct short cycling "on-off" of the building water pump
• What is the difference between a bladder type water tank and a glass lined no-bladder water tank?
• How to adjust the water pump pressure control switch
• Well pump & water tank diagnosis & repair procedures

Private well and pump systems include a well (the water source), piping from the well to the building, a water pump, and a water 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.

How water pressure tanks work

When water is turned on at a fixture in the building, compressed air in the water tank acts like a spring: it pushes water out of the water tank and into the building water supply piping and thus water is sent on to the building plumbing fixtures.

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.

As water leaves the water tank, water pressure in the water tank drops. Since the water tank also contains air, the air pressure drops too. In the tank water pressure and air pressure will be at the same psi. Sketch courtesy of Carson Dunlop.

A pressure control switch, usually mounted on or near the water tank, senses the pressure drop, and at a pre-set "pump cut-in pressure" (typically 20 or 30 psi) the pressure switch turns on the water pump. See WATER PUMP PRESSURE CONTROL ADJUSTMENT for details of this control.

The water 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. See WATER PUMP TYPES & LIFE EXPECTANCY for types of water well pumps, how they work, how they are diagnosed and repaired.

Because the water pressure tank is connected to the water pump (water in from the well) and also to the building water supply piping (water out to the building) the water tank is said to be "floated on the water line" and when the water pump is running water is pushed simultaneously into the water pressure tank and into the building supply piping.

The pressure control switch turns off the water pump when water pressure in the pressure tank reaches the "pump cut-out pressure" (typically 40 or 50 psi) - pressure switch turns off the well pump.

Bladder type or "captive air" water tanks (shown in our photo at left and in the sketch above) store the water tank's air charge in the upper portion of the steel water tank. Water in the tank moves in and out of a rubber bladder in the tank bottom. Because the air charge is kept separate from the water in the tank, air is not absorbed into the water and bladder type water tanks do not normally need to have makeup air added.

Water pressure or water pump short cycling problems with bladder-type water tanks are usually traced to a problem with the pump controls, with well and water piping leaks, or less often, to a failure of the internal tank bladder itself - a component that may be replaceable.

If the water-containing rubber bladder in a "captive air" water tank is defective (it can become stuck to itself and remain collapsed), the result can be a rapid on-off short cycling of the water pump. We test water pressure tanks to see if they're empty or nearly empty of water by seeing if we can gently rock or move the tank.

If the water tank is heavy with water it does not move easily. Be careful not to jiggle and break a pipe! Bladder type or captive-air water pressure tanks and their repairs are described just above and in more detail at WATER TANK TYPES.

Bladderless Steel Water Pressure Tanks (photo at left and sketch just below) use a single steel tank interior to hold both the air charge and the water supply.

Modern steel bladderless type water tanks may be coated internally to increase the water tank life by resisting corrosion. That's what "glass lined" refers to on some water tanks. (A "glass lined" or "epoxy coated" water tank will not be a bladder type water tank which we discussed above.)

Bladderless water pressure tanks, because the air charge and water are in the same container, can lose their air charge over time (air is absorbed into the water) and may need air added. See WATER TANK AIR, HOW TO ADD for details.

Air in the water 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 we call this the water pressure tank rather than a water storage tank - though for most people these are the same device. Sketch courtesy of Carson Dunlop.

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 water 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 water tank.

Contact Daniel Friedman for website content suggestions or for fee-paid consulting