Expansion Tank: Preparation, Pressure and Installation

What is the proper way to install an expansion tank?  Many plumbers do not even do it right.  

First of all, an expansion tank has an air chamber and a water chamber separated by a diaphragm.  The air chamber comes pressurized from the factory, but the pressure is often far below the working pressure of the water system.  If an expansion tank has less pressure in the air chamber than the water system, and the air pressure in the tank is not adjusted, the air will compress as soon as the water is turned on, leaving no place for expansion.  So the pressure should always be adjusted to the system pressure.  

It is also important to consider the manufacturer's installation instructions.  How often to people actually read the instructions before installing something?  There are some specifics to installing an expansion tank.  The tank should be installed on the cold water inlet side of the water heater, down stream of the cold water shut off to the water heater.  The tank should be in a vertical position with the water chamber on top, and should be supported by a strap of some kind so that the weight of the tank is not on the pipes.  If the system pressure is greater than 80 psi, the water system should have a pressure reducing valve.  

Knowing how to install an expansion tank, and installing it correctly is the best way to receive the full benefits that the tank can provide for thermal expansion.