Types of Tanks Found in Buildings
How to Find & Identify Expansion Tanks, Oil Tanks, Range Boilers, Water Softener Tanks, Water Pressure Tanks, Water Heater Tanks

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Guide to identifying the types of tanks & water tanks found in or at buildings: this article describes water tanks and other kinds of storage tanks found in buildings, captive air and traditional water storage tanks or water pressure tanks, and we provide advice about what to do when things go wrong, such as finding air and water leaks or deciding to replace a water tank.

Water storage tanks, cisterns, rooftop tanks, open tanks, water pressure tanks, steel tanks, range boilers, indirect-fired water heatersExpansion tanks in attics & basements, oil storage tanks, other tanks in attics, basements, etc.; water heater tanks, cisterns, oil tanks, others.

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Types & Uses of Residential & Light Commercial Water Tanks & Other Liquid Storage Tanks

Photograph of a heating boiler expansion tankVarious types of tanks uses in and around buildings are identified and explained below, including water pressure tanks, water storage tanks, range boilers for hot water, indirect fired hot water tanks, expansion tanks in attics, basements, and on heating boilers, oil storage tanks, rooftop tanks, cisterns, water pressure booster systems. We also discuss when to replace water tanks.

Article Series Contents

CAPTIVE AIR TANKS bladder-type water tanks for Building Water Pressure Regulation

Newer "captive air" tanks use a rubber bladder to keep water and air separate. This prevents the air charge from being absorbed into the water, so you should never need to add air to those systems.

At WATER PRESSURE TANKS, BLADDER TYPE we discuss this type of water tank in detail.

At WATER TANKS HOW THEY WORK we provide details of how water pressure tanks work.

Loss of air in "captive air" bladder type water pressure or water storage tanks

If a "captive air" or internal "bladder" (usually rubber) type tank is installed, and if the pump is short-cycling on and off, you should turn off the pump and call a plumber.

We would suspect that the bladder has ruptured, or that the tank itself has developed a leak. We explain waterlogged water tanks and water pump short cycling in detail at WATER TANK REPAIRS where we also explain in detail what well pump short cycling is - how to diagnose captive-air type water tank problems and how to correct these conditions by repairing or replacing the water tank bladder or the tank itself.

Schematic of a bladder type captive air water pressure tank (C) Carson Dunlop AssociatesRegardless, short cycling of the well pump would damage the pump or pump controls.

These tanks are made of steel or fiberglass, and typically have an air schrader valve at the top of the tank. The air valve is used for adjustment of the air pressure in the tank at the time of initial installation. Normally you never need to add air to these tanks after initial installation. Sketch courtesy of Carson Dunlop Associates.

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 as we discussed above - 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 single chamber steel water pressure tank, largely because the tank bladder holding the water supply protects the tank interior from corrosion.

On some captive air water tanks this design is reversed. For example on the WellMate™ water tank the water is in the tank and air is in the tank bladder. This difference can confuse the burst water tank bladder diagnosis procedure which we describe below.

At WellMate Diagnosis we provide separate water tank diagnosis and repair advice for captive-air water tanks in which the air is in the bladder and the water is outside the bladder in the water tank.

If the captive air water pressure tank bladder is ruptured, the air charge in the tank becomes lost over time and the tank acts like a water-logged steel tank discussed below.

If the captive air water pressure tank bladder is collapsed, defective, jammed, and stuck on itself it may not accept much volume of water, also leading to a short draw-down cycle before the pump has to turn on again. This is an unusual case but has been reported to me on occasion.

A burst water tank bladder can collapse at the water tank bottom, preventing water from leaving the tank. The result is no water pressure in the building and perhaps an inability to drain water from the water tank itself. You'll need to replace the water tank bladder or the entire water tank assembly.

See these detailed articles on bladder-type "captive air" water tank diagnosis and repair:

WATER TANK BLADDERS & CAPTIVE AIR - how water pressure tanks work, the difference between internal bladder type and bladderless wat4er tanks, causes of bladder type water tank failures.

WATER TANK BLADDER PRESSURE ADJUSTMENT how to adjust bladder type water tank pressures or air pre-charge

WATER TANK BLADDER REPLACEMENT for leaky water tank bladders.

Bladderless Fiberglass Water Tanks

Bladderless Fiberglass Water Pressure Tanks, such as the WellMate traditional hydro-pneumatic water tank operate similar to the steel water pressure tank, that is, no internal bladder is used to maintain and separate the tank's air charge and water charge pressure. These tanks incorporate a tank-top mounted air volume control and offer the advantage (over steel water tanks) of no risk of rust perforation and leak at the water tank.

At WellMate Diagnosis we provide separate water tank diagnosis and repair advice for this water tank type.

OLDER STEEL TANKS - Bladder-less Traditional Steel Water Pressure & Water Storage Tanks

There may be lots of kinds of tanks found in buildings, storing water, fuel, hot water, or serving other purposes. We review quite a few of them here and include photographs to help you figure out what's what.

Steel Water Storage/Pressure Tanks

Photograph of a steel water tank Photograph of a steel water tank

The water storage tank in the photographs above is leaking, having rusted through from inside the tank. (This tank is less needed for storage than to smooth or regulate the water pressure in the building as the pump cycles on and off) Leaks like the ones in these photos can also leak the air charge out of the upper portion of the tank when the in-tank water level is below the leak point. So you might trace a water pump short cycling problem to an air loss in the tank to a leak in the tank itself.

Waterlogged water pressure tank schematic (C) Carson Dunlop AssociatesHow to detect a waterlogged water tank

If a steel water tank has lost its air charge, or most of it, the condensate line on the tank will be high, near the top few inches of the tank side (unless you live in an arid climate where there is never condensation on the water tank anyway.). Sketch courtesy of Carson Dunlop Associates.

In this condition the steel tank is called a waterlogged water pressure tank. Such tanks will also be heavy and won't be so easily rocked or moved - a simple test we make to see if the tank is empty or full of water. (Don't move the tank so much that you cause a water piping leak!

When to add air to a water tank

The discussion on how to add air to a water pressure tank at this website beginning
at HOW TO ADD AIR applies principally to older steel water tanks that don't use an internal bladder to keep water and air separate.

In these simpler single-internal chamber steel tanks the air charge is lost either by absorption into the water as it passes through the tank during use, or it may be lost by leaks at the tank or nearby plumbing fittings.

Automatic ways to add air to a water tank

Some old-style bladderless steel water pressure tanks are equipped with an air volume control device which is intended to put makeup air into the tank as air is lost or absorbed. I discuss these further
at AIR VOLUME CONTROLS but suffice it to say these often stop working. But if you see one on your water tank you know this is a non-bladder type older style water pressure tank.

Attic Expansion Tanks for Heating Boilers & Other Storage Tanks Found in Attics

Photograph of an attic expansion tank for a heating boiler

Attic expansion tanks and pressure relief systems for boilers: Don't confuse an old heating system attic-mounted expansion tank like the one shown here for a water tank storage tank or a range boiler.

The heating system expansion tank will be connected to the heating system radiators or basement boiler and may have a simple overflow pipe to permit excessive water (or system pressure) to spill outside.

Systems which rely on a remote attic-mounted expansion tank are less safe since than a boiler that has a pressure and temperature relief valve mounted right on or at the boiler.

See ATTIC EXPANSION TANKS, HEATING for details about this device.



Basement Expansion Tanks for Heating Boilers & Other Tanks found in Basements

Our client is pointing to a do-it-yourself insulating job on a water heater. But that reddish-brown horizontal tank over his head is an expansion tank for the hydronic heating boiler in this building.

Photograph of a heating boiler expansion tank

This is not a water storage tank, it's not a range boiler, it's an expansion tank.

This basement expansion tank is expected to be on a heating boiler that also has a pressure and temperature relief valve.

See EXPANSION TANKS for details about expansion tanks and pressure tanks used on heating systems.

Also see RANGE BOILERS and CISTERNS and of course OIL TANKS

Indirect-Fired Domestic Hot Water Tanks

Photograph of an indirect fired water heater

Indirect fired water heaters: In some other buildings domestic hot water is produced by cycling hot water from a hydronic or steam boiler through a loop inside of a steel tank. The water in the tank is heated by the water in the coil.

Modern systems using this approach use the term indirect-fired water heater and such systems are sold by various companies such as the SuperStorTM unit shown here as the white tank to the right of the heating boiler.

Indirect fired water heater tanks for domestic hot water, such as the SuperStorTM are usually located close to the heating boiler and will have both cold and hot water lines leaving the tank to supply the building with domestic hot water and a loop of piping that runs between the [usually the] bottom of the tank and a nearby heating boiler.

Follow the pipes to see which pipes are performing which function.

The range boiler is an old concept in use for about 100 years. Indirect fired water heaters are a modern system and are in current sales and use.

See HOT WATER TANKS, INDIRECT FIRED for details about this type of water heater.

See WATER HEATERS for details about residential hot water systems.

LARGE WATER TANKS - Large Water Storage Tanks, Why They Are Used, What They Imply about Well Recovery Rate or Well Yield

Details about the use of large water storage tanks to handle low-flow wells or limited municipal water supply are provided at WATER STORAGE TANKS, LARGE. Excerpts are below.

Photograph of a storage type steel water tank

As we explain at WATER STORAGE TANKS, LARGE, large water storage tanks, such as the one whose end is visible in our photo (left) usually mean that the well flow rate is very slow, even inadequate by contemporary standards.

The life expectancy or future usability of the well must be questioned.

We explain how people determine the necessary water tank size and volume at WATER TANK SIZE & VOLUME .

Oil Storage Tanks in Buildings

Photograph of a heating boiler expansion tank

This is a typical indoor oil storage tank in a residential building. We have a lot to say about oil tanks in buildings, oil tank leaks, environmental risks, potential costly cleanups, and effects of oil tank problems on the heating system and its operation.

For information about oil tank issues and solutions see OIL TANKS - Heating Oil Underground & Above ground Oil Storage Tank Leaks, Testing, Problems & Solutions, Home Buyer's / Home Owner's Guide. These online articles answer most questions about above ground or buried oil storage tanks.

Watch out: What's really important in this indoor oil storage tank photo is the black wall-mounted gauge our client is pointing to. He's found an indication that there is or was a buried underground oil tank at this property - potentially a costly environmental problem if that tank leaked.

Given the leakage all over the old oil tank that we can see indoors in this photo, we weren't too optimistic about what might have happened with an old outdoor buried tank. Some testing was ordered.

OPEN WATER TANKS - gravity fed water systems or pump-up rooftop systems with and without pressure-boosting pumps and water tanks

Rooftop Water Tanks or Free-standing Water Tanks at Ground Level

Sketch of open top water storage tank and well design used both on rooftops and in rural areas. Rooftop water tanks in urban areas such as New York City may be used to supply water at high pressure to the building below.

Water is pumped to the rooftop tank from its municipal source, then redistributed at good pressure to the points of use in the building below.

When passing through New York City, look at rooftops and you'll often see these tanks still in use.

This sketch shows how a rooftop tank might be constructed, though this particular sketch has the tank next to a well.

See details at CISTERNS.

Plastic & Fiberglass Water Storage Tanks - HDPE Tanks

Plastic cistern water tank (C) InspectApediaReader Herman Voegel has pointed out that an up-and-coming area of storage containers includes spun-plastic tanks.

Specifically, new storage tank types include High Density Polyethylene (HDPE), currently available as tank storage for water, chemicals, waste oils, etc.. General HDPE containers come in all shapes and sizes and are quite rugged and relatively cheap compared to using typical 12-gauge steel home heating oil tanks.

The ruggedness of HDPE plastic containers comes from their material density which is typically at a minimum specific-gravity of 1.7, and for heavy-duty at 1.9.

Home heating oil has not yet been approved for storage in HDPE tanks, basically for two reasons:

  1. UV (ultra-violet) light degrading HDPE plastics over time, making them weak, and
  2. Problems with slight permeation seepage through their plastic walls.

However, fixes have been put in place to properly address these problems. UV-light is checked by using special color additives that prevent their light from penetrating and degrading the plastic walls. Permeation or seepage of oil through container walls is checked by coating them with fiberglass.

Unfortunately, even with these fixes, HDPE plastics for heating oil storage have yet to be universally approved and accepted.

Readers should also see PLASTIC CONTAINERS, TANKS, TYPES where we describe health and other concerns involving plastic tanks and other containers used for water storage.

Range Boilers for Domestic Hot Water Systems

Photograph of an attic expansion tank for a heating boiler Photograph of a coal fired range boiler water heater

Range boilers are vertical or horizontal hot water systems whose water is heated by circulating the water from within a water storage tank (the range boiler) through a heat exchanger which is inside or connected to the exterior of a heating boiler. The water in the hot water tank range boiler is heated by circulating its water through the heat exchanger which itself is heated by the water inside or from the heating boiler.

The sketch illustrates how a very early type of coal-fired water heater range boiler worked. As homeowners shifted fuels from coal to oil or gas and installed central heating boilers, often the range boiler water heater was adapted to work with these systems as well, as you can see in the photograph.

As with the indirect-fired boiler described next, range boiler water heating tanks are usually located close to the heating boiler and will have both cold and hot water lines leaving the tank to supply the building with domestic hot water and a loop of piping that runs between the bottom of the hot water tank and a nearby heating boiler.

Follow the pipes to see which pipes are performing which function. Our photo shows a silver steel range boiler hiding back in the corner behind the newer (though pretty old) gas fired water heater. (Notice also the efflorescence on the masonry block foundation, where the downspout has been spilling by the house foundation?)

See HOT WATER TANKS, RANGE BOILERS for details about range boiler water heaters.

See WATER HEATERS for details about residential hot water systems

What's the difference between a range boiler, an indirect fired water heater, and an expansion tank?

The range boiler (HOT WATER TANKS, RANGE BOILERS) is an old concept in use for about 100 years.

A similar concept is found at indirect fired water heaters (HOT WATER TANKS, INDIRECT FIRED), a modern system and are in current sales and use. But indirect water heaters use a circulator pump to cycle heating boiler water between the boiler and a separate hot water tank. Inside the hot water tank a coil containing boiler water heats the physically separate domestic hot water in the tank.

Also see EXPANSION TANKS for a guide to smaller tanks used to absorb pressure increases on hot water heating systems.

Reverse Osmosis (RO) water treatment tanks

Reverse osmosis tanks are generally small in size and located as centralized treatment equipment or more oftren as a point of use under-sink water treatment unit.

Details about RO systems are at

Rooftop Water Storage Systems

Details about rooftop water systems, tanks, and booster pumps are at ROOFTOP WATER TANKS. Excerpts are below:

Rooftop water storage tanks In some areas, Mexico, for example in our photo (above-left), rooftop water reservoirs are supplied intermittently with water from a water main in the street.

Photograph of a rooftop storage tank in Manhattan


and also see CISTERNS.

We discuss water pressure booster pump and tank systems in detail at PUMP, WATER PRESSURE BOOSTING

Open Water Tanks Indoors

Photograph of an open top gravity type storage type steel water tank Photograph of a storage open top gravity type steel water tank

At some locations there is an up-hill or rooftop water source which is fed into the building entirely by gravity. The open top water tank in these photos used a simple float valve to let water into this storage tank.

See details at CISTERNS.

Cisterns for Water Storage

Photograph of an open cistern at an old property. Photograph of an open cistern at an old property.

Basement & Outdoor Cisterns, are often located in the basement or courtyard of buildings where they collect rainwater for future use. In the U.S. cisterns were often located in the basement of a (pre-1900) home. This cistern was originally filled by downspouts directing roof runoff into the basement.

Details about cisterns are found at CISTERNS.

Rainwater Storage Tanks & Cisterns

In arid areas such as the U.S. Southwest and parts of Mexico, very large cisterns are often placed in a courtyard where they collect rainwater for use during the dry season.

The above-ground water cistern storage tank shown in our photo is located in Mexico and is discussed

Rainwater holding tank for a passive solar home (C) Daniel Friedman

Rainwater for this cistern is collected from a near-flat rooftop [photo] and channeled to a large fiberglass holding tank - the blue tank in our photograph, (above left).

Piping also permits directing water into this tank from a well-fed cistern located atop the concrete block tower [photo]).

See rainwater collection and storage cistern details at CISTERNS.

Water Pressure Boosting Systems

Photograph of a water pressure booster pump and tank system

Water pressure booster pumps and tanks may be installed in buildings where municipal water is supplied, at a cistern water supply, and to boost the water pressure in buildings where a water tank is located on rooftops or anywhere in a building.

Just because you see a pump and pressure tank, don't assume that the building is served by a private well.

Water pressure boosting systems using a water pump and water tank are installed in homes where the municipal water supply or cistern or water storage tank supply pressure is low.

We discuss water pressure booster pump and tank systems in detail at PUMP, WATER PRESSURE BOOSTING

See WATER PRESSURE LOSS DIAGNOSIS & REPAIR for details on how to correct low water pressure in a building.

Water Softener Tanks

Water softeners use one or a pair of tanks to treat water for hardness and to contain salt or a similar compound.


Water Treatment Equipment Tanks

We have listed reverse osmosis treatment and water softeners as examples of uses of tanks in buildings. But there is a wider variety of water treatment equipment, some of which also uses tanks, such as treatment of water for odors, water chlorinators, etc. Details are at WATER TREATMENT EQUIPMENT CHOICES

When do you need a new water tank?

Details about when to replace a damaged water pressure tank or water storage tank and also about where to locate the water tank are at WATER TANK REPLACEMENT. Excerpts are below.

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 water storage tank looks like this one, or if you see a "pinhole" leak, it may be possible to make a temporary emergency repair using a rubber washer and screw, as we describe at


But you need a new water tank.



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TANK TYPES: EXPANSION, OIL, SEPTIC WATER, ALL at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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