Water Storage & Pressure Tanks - Home
Water tank purpose, uses, diagnosis, repair, replacement

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Water tanks:

This article describes the purpose of water storage and water pressure tanks, how water tanks work, what goes wrong with the water tank (such as water tank leaks, loss of air charge, corrosion, failure to admit water) and how to fix water tank troubles.

We explain the purpose of water storage or water pressure tanks on private well systems, or in pressure boosting systems on municipal water supplies. We discuss 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 are well water tanks recycled?

We explain the different types of water tanks such as tanks using an internal bladder or diaphragm, bladderless steel, fiberglass, or plastic water tanks, and water tank pressure and air controls and valves.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What is the purpose of the water tank on building water supply systems

Photograph of a sketch of a water pressure tank in cross sectionWater tanks in or at building water supplies serve one or both of these basic functions:

1. Prevent short cycling pump: An air charge in the tank delivers water pressure out of the tank and into the building to avoid water pump rapid on-off cycling or "short cycling" that is annoying and that ultimately damages the equipment.

2. Provide water storage: At some properties where the supply capacity of the water well is limited, a large water tank may also be used to store water for use by building occupants. That storage capacity assures that occupants always have water and permits the tank to be re-filled slowly during off-use periods.

Here we provide information on the purpose and function of water tanks and on water tank selection, installation, troubleshooting, and repair. Our complete list of water tank installation, diagnosis, & repair articles is at the end of this page.

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.

[Click to enlarge any image]

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.

Schematic of a bladder type captive air water pressure tank (C) Carson Dunlop Associates

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 Associates.

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 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.

What's the Difference Between a Bladder Type Captive Air Water Tank and a Conventional Steel Bladderless Water Tank

Bladder type water tank (C) Daniel Friedman

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.

Watch out: Be careful not to jiggle the tank so hard that you cause a leak or 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 where we describe all of the types of storage tanks found in, on, or around buildings.

Steel water tank (C) Daniel Friedman

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.

Why Air is Needed in a Water Tank

Waterlogged water pressure tank schematic (C) Carson Dunlop Associates

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 Associates, a Toronto home inspection, report writing, & education company.

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.

Water Pressure Requirements for Flushometer Type Toilet & Urinal Valves

Urinal with flushometer valve © D Friedman at

Reader question: Installing a pressurized system & Flushometer Valve Toilets & Urinals on a Well & Pump Water Supply System

I have a well system at a marina and would like to install commercial flushometer toilets and urinals. It is old and is currently served by a standard residential system.

I have space above the rest rooms and would like to install a stand alone pressurized system that is fed by the existing residential system, but would provide the gpm and pressure to function the commercial units. Likely to be a total of 3 toilets and two urinals.

Any ideas or plans available? Thanks, Bill

Reply: Be sure that your well water supply system, pressure tank, pressure, and water supply piping diameter & flow rate can support flushometer valves; consider waterless urinal fixtures.

Bill, we provide a detailed answer to this question

While flushometer valves by various manufacturers can operate at a range of water pressures (10-100 psi) the typical operating pressure requirement is 25 psi. But the fixtures also need a high water flow rate at that pressure to operate the valve and flush the fixture properly.

That's why the standard water supply pipe diameter to the fixture is 1 1/4" rather than the common 1/2" or 3/4" water supply piping found in residential and light commercial buildings using conventional tank-type toilets.

When choosing fixtures and deciding on water storage volume, pressure, and plumbing specifications you'll need to be sure to check the specific requirements of the flush valves you are buying for the urinals and toilets.

Are well water tanks recyclable?

Well water tank headed for recycling © D Friedman at

Reader Question: I'm replacing my well tank. The plumber says they recycle these tanks. Is that possible? - Anon

Reply: Yes well water tanks are often recycled

Well water tanks are often collected and recycled, especially by larger plumbing and well drilling companies. To recycle an internal-bladder well water pressure tank the tank is cut open and the bladder is removed.

The remaining steel is delivered to a recycler. This is a useful detail to know about not just because we recommend recycling whenever possible.

Our photo (left) of a bladderless steel water pressure tank illustrates a unit that was replaced by a newer internal bladder tank

. The steel tank will be taken away by the installer (avoiding leaving something difficult for the homeowner to dispose-of), and stored in the installer's junkyard until that company has accumulated enough tanks to have them hauled en-masse to the recycler.

The plumbing company that opens water pressure tanks has an inside view of problems that may have developed with the bladder or with the tank itself - useful water tank failure cause and effect data.


Continue reading at CISTERNS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see WATER TANK FAQs - questions & answers posted originally at this article.

Or see WATER PRESSURE BOOSTER PUMP - using a pump and tank to improve building water pressure

Or see WATER PUMP PRESSURE SENSITIVE - pressure boosting without a pressure tank



Or see WATER TANK TYPES - what are the different kinds of water pressure tanks & water storage tanks & how are they used?

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WATER TANK: USES, TROUBLESHOOTING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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