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What is a Water Heater Expansion Tank

What is an expansion tank?


An expansion tank is a metal tank connected to a building's water heater designed to accommodate fluctuations in the volume of the building's hot water supply system. These fluctuations occur because water increases in volume when heated and loses volume when cooled.

Expansion of water volume in a closed system can create dangerously high water pressure. As the water is forced into the tank by expansion, it compresses the air contained inside the rubber bag. Air is used as a cushion because it exerts less force on the container than water, which cannot be compressed.

The function of this bag is to prevent air from being absorbed into the water, a process that could cause the expansion tank to lose its ability to function as a shock absorber of sorts. If the bladder begins to leak air over time, a Schrader valve, identical to the inflation valve on bicycle and car tires, can be used to add more air.

What does it look like, inside and out?

Expansion tanks are considerably smaller than water heaters, typically holding about two gallons in residential systems, although tank sizes vary depending on the volume of water in the hot water supply system they serve. The design pressure for which the tank is rated is indicated on the label on the tank, normally 150 pounds per square inch (PSI) for a residential tank.

InterNACHI inspectors should check that the tanks are placed high enough above the water heater to allow water to flow easily back into the water heater tank. It is best located near the water heater and can be installed vertically, either above or below a horizontal supply pipe, but can also be placed horizontally. Horizontally suspended tanks will need additional support to reduce stress on the copper piping.

The expansion tank should be checked for proper placement and support, corrosion and leaks. Although many jurisdictions now require an expansion tank to be installed at the same time a water heater is installed, an expansion tank was not always required in the past and may not still be required in some areas.


It does not replace the temperature pressure relief valve (TPR).

If the water heater controls fail and the system pressure exceeds 150 PSI or the temperature exceeds 210°F, the temperature and pressure relief valve (TPR valve) installed on the tank side of the water heater will open and safely drain the water from the system. TPR valves are able to reduce water pressure at a rate greater than the water heater's capacity to increase it, eliminating the possibility of the water overheating (over 212 degrees) and becoming a serious threat. The importance of this valve cannot be overstated and it is very important that it be checked regularly for rust or corrosion and the release lever should be used monthly by the home owner, not the inspector.

If the TPR valve doesn't work when it needs to, the result can be disastrous. The pressure in the water heater tank will continue to build as the water temperature exceeds the boiling point and the water overheats. Eventually the pressure in the tank will exceed the ability of the weakest part of the water heater to hold it and the tank will burst, exposing the superheated water to air and causing an immediate and explosive expansion of steam. This situation can propel the water heater like a rocket or cause it to explode like a bomb, causing extensive property damage, injury, or death.

In short, the expansion tank should be checked for proper placement and support, corrosion and leaks. Although not required in every jurisdiction, they are absolutely critical to the successful and safe operation of domestic drinking water systems.


Inspectors at LiteHouse Inspect will take a look at any expansion tank that is part of your home's plumbing system, or lack thereof; and report on any noted deficiencies in the tank.

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