Typical Expansion Fitted Water Heater 


High pressure in the residential water system causes the most common problems facing the plumbing trade. State, Counties and Municipalities have been working to find a solution to these problems especially the one that is caused by "Thermal Expansion".

Thermal Expansion is what happens when water is heated in a closed system and expands in volume as its pressure rises. As water is not compressible the water heater tank and piping system react by expanding to tank and piping containment limits.  The higher pressurized water can not be physically compressed, so the expanded volume, even in small quantity, has no space to go and results in a rapid and dangerous pressure increase in the system. Just for illustration, a 40-gallon capacity water heater during the recovery cycle of hot water usage will expand to approximately  40.53 gallons by the time the set desired temperature (usually 120-degrees Fahrenheit) is reached. The 0.53 gallons has to go somewhere.  Initially, excess pressure and water is expelled through the pressure release valve resulting in a loss of water.  Over time as the system fatigues, a rupture of one or more of the component of the system may occur.  

The current available market solution is to install a Thermal Expansion Tank on the cold water feed line prior to the water heater along with a pressure control valve or check valve.  The Thermal Expansion Tank solution works, by allowing water to expand against an air-filled diaphragm inside. 

 U.S. Patent 5,690,061, Juan A. Lopez, Inventor

Thermal Expansion Topic