Mandatory Water Heater Expansion Tank

[delaney]

Does anyone know what purpose an expansion tank serves when hooked up to a water heater?

[LazyPup]

Most elements or compounds expand when they are heated and contract when they are cooled but water has a very unusual property. Water reaches maximum density at 39 degF and it expands if heated or cooled from that temperature. When cooled to freezing water expands approximately 10% by volume which explains why pipes rupture when they freeze. On the other hand , water expands about 15% by volume until it reaches the boiling point. When water converts to steam it then expands to 12 to the third power by volume. This means one cubic inch of water will produce one cubic foot of saturated steam.

The average geothermal temperature of the earths crust is 55degF however in northern climates subject to winter freezing the surface of the soil freezes. The depth of the freeze varies from one region to another and is norally indexed by the "Average Frost Depth" which is the depth where frost normally occurs during winter.

In order to prevent the water supply lines from freezing we are required to bury them at a minimum depth of 6" below the average frost level. While freezing may occur the the frost level cooling continues to a depth approximately twice the average frost depth. This means that while the water line is buried deep enough to prevent freezing none-the-less the ground is colder than normal at the water line depth. Quite often the actual temperature at the burial depth is only slightly above freezing. Understanding that freezing occurs at 32 degF at standard atmospheric pressure, it would then stand that the temperature at the water line depth would be at or very near 39 degF which is the maximum density of water.

At maximum density a gallon of water occupies 231 cubic inches of volume. As the water at maximum density enters the structure and begins to warm up it will begin expanding. The rate of expansion is rapidly increased as water is heated in a water heater.

Normally if a house is connected to a municipal water main as the water expands the increased volume is absorbed by a small portion of the water backflowing out the water line and into the municipal main.

In the case of a home well there is a backflow preventer on the line from the well pump to the structure and in the case of a municipal water supply where the pressure may exceed 80psi we are required to install a Pressure Reducing Valve which will function as a backflow preventer. This then makes the inhouse water distribution system a closed system so we must now make a provision for the expansion of the water as it is heated. For that reason the code requires that when we install a Pressure Reducing Valve or backflow preventer on the main supply line we must also install an expansion tank on the cold water supply line to the water heater in order to provide the necessary additional capacity as the water expands from heating.

In a closed water system (one which has a backflow preventer) without an expansion tank even the simple warming of the water from the underground temp to room temp could cause enough expansoin to rupture a pipe or cause a fitting to fail.