GC: n

CT: An expansion tank is a metal tank connected to a building’s water heating appliance designed to accommodate fluctuations in the volume of a building’s hot water supply system. These fluctuations occur because water expands in volume as it gets hot and loses volume as it cools.
Expanding water volume in a closed system can create dangerously high water pressure. As water is forced into the tank by expansion, it compresses air contained inside a rubber bladder. Air is used as a cushion because it exerts less force on its container than water, which cannot be compressed.
The function of this bladder is to prevent air from becoming absorbed into the water, a process that could cause the expansion tank to lose its ability to act as a sort of shock absorber. If, over time, the bladder begins to leak some air, a Schrader valve, identical to the fill valve found on bicycle and car tires, can be used to add more air.

S: NACHI – http://www.nachi.org/water-heater-expansion-tanks.htm (last access: 9 December 2014)

N: 1. expansion (n): 1610s, “anything spread out;” 1640s, “act of expanding,” from French expansion, from Late Latin expansionem (nominative expansio) “a spreading out,” noun of action from past participle stem of Latin expandere “to spread out”.
tank (n): 1610s, “pool or lake for irrigation or drinking water,” a word originally brought by the Portuguese from India, from a Hindi source, such as Gujarati tankh “cistern, underground reservoir for water,” Marathi tanken, or tanka “reservoir of water, tank.” Perhaps ultimately from Sanskrit tadaga-m “pond, lake pool,” and reinforced in later sense of “large artificial container for liquid” (1680s) by Portuguese tanque “reservoir,” from estancar “hold back a current of water,” from Vulgar Latin stanticare. But other sources say the Portuguese word is the source of the Indian ones. Meaning “fuel container” is recorded from 1902. Slang meaning “detention cell” is from 1912. Railroad tank-car is from 1874.
2. In the field of Solar Energy: Takes up the overflow created when the heat-transfer fluid expands.
3. Solar Expansion Tanks are one of the most important components of any solar heating system. Most system installed in North America fail to use proper sized expansion tanks or proper high temperature rated tanks. Most installers tend to use tanks designed for hydronic heating which requires about 1/4 the size and operates on a much lower temperature. This represents a substantial liability and will ultimately cause the solar system to fail well before its useful life expires.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=expansion&searchmode=none (last access: 9 December 2014); http://www.etymonline.com/index.php?allowed_in_frame=0&search=tank&searchmode=none (last access: 9 December 2014). 2. GDT – http://www.granddictionnaire.com/ficheOqlf.aspx?Id_Fiche=9486916 (last access: 10 December 2014). 3. NL – http://www.solartubs.com/solar-expansion-tanks.html (last access: 10 December 2014).

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CR: solar energy, water hammer.