Expansion tank

Hi All, just moved into a new condo with a gas fired water heater,there is an expansion tank installed on it,what is the purpose for it, never seen one on a water heater before.
Geo

 

When water is heated it expands and if the piping system is "closed" the pressure will rise very fast since water cannot be compressed. Having an expansion tank in the system gives the water a place to expand in that the expansion tank is mostly filled with air.

Years ago most water systems in houses were "open" in that the expanding water could flow backwards all the way to the city water main, relieving the pressure of heated water expansion. The noticeable exception was when a "pressure regulating valve" (PRV) was installed to lower the city main pressure to a more usable (and safer) pressure at the house, generally no more than 75 psi. The PRV changed the open system to a closed system because it would not allow the water to flow backwards through the PRV. Often times an additional pressure relief valve would be installed with a set point only a few psi higher than the regulated water pressure. This had the downside of wasting water and over time the valve seat/disc would erode and the valve would leak constantly. The expansion tank solves this problem.

Recently, maybe the last ten years or so, water utilities have been installing meters that also do not allow a backward flow thereby creating closed systems from what were originally open systems.

 

Curd,good info I have only seen PTV which would relieve if the temp or pressure was to hi, haven't been around to much new construction for awhile.