Older water heaters, electric and gas, are notorious for allowing hot water to seep back into the cold water lines that feed them.  Water is amazing in its versatility, and even more amazing as it changes physical states.  As water freezes, it expands in volume.  Try filling a bottle to its maximum capacity and then sticking that bottle in a freezer.  As the water becomes ice, it expands and damages the bottle it is confined in. The same thing is true for water when it is heated.  A 40 gallon water heater that holds 40 gallons of cold water can not hold the same 40 gallons when heated to 120 degrees.  So where does the extra water go?   How do you prevent damage to your water heater?   In the past, the water released back into the cold water line, pushing backward towards the source.  This greatly decreased the efficiency of the water heater, and caused damage to other appliances such as dishwashers and washing machines.

A dripping Temperature and Pressure Relief (T&P) Valve on the water heater is an indicator of a potential thermal expansion problem. This is the valve with the small handle or lever that automatically opens when either the temperature or pressure inside the heater tank exceed a set limit (usually 150psi/210°F). These are emergency valves, and are not meant to operate regularly. When one of them leaks, the first step is usually to replace the valve. 

 The solution to this thermal expansion problem is to install a Thermal Expansion Tank above the supply water inlet on your water heater (JON R15202).  These tanks feature a rubber bladder that separates an air chamber from the rest of the tank.  Air is pumped into the tank to match the pressure of the water supply.  When water expands while being heated in the water heater, it enters the expansion tank, compressing the air.  Once expansion has stopped and there is room in the heater, the compressed air pushes the water back into the heater.  

It is required by codes throughout our country to have one of these tanks installed with each new water heater installation.  All manufacturers, including American Water Heater, recommend that you install an expansion tank with all new heaters.  A Thermal Expansion tank is an economical and practical way to protect your new water heater from damage.  It will increase the efficiency of your heater, and keep it running safely for many years.