Here in California we have adopted the California 2010 plumbing code as of January 1 2011. One of the many changes is that you can now put your water heater back on the floor as long as it is a Flammable vapor ignition resistant approved water heater, We are now required to protect the water heater from damage by a car in the garage by installing pipe Ballard or other similar devices
Another standing requirement is that water pressure over 80 pounds per square inch must be reduced to 80 psi however the new code requires when a pressure regulator is installed to reduce pressure a properly sized expansion tank to be included in the system.
The new code has addressed many concerns of plumbers and water heater manufactures you see the new water heater that are flamable vapor ignition resistant prevent ignition of flammable vapors better than raising the water heater. and keeping cars from crashing into water heaters just makes sense.
One of the major causes of premature water heater failure is excessive pressure and metal fatigue. the more pressure in a water heater the more the tank expands creating more stress on the tank, by limiting the pressure to 80 psi that reduces the pressure to approximately 1/2 of the maximum operating pressure the tank was designed for, When you have a pressure regulator installed the pressure goes in but usually cannot go back so when the water heats up it expands creating more pressure this causes the thank to expand, when your turn on the water the pressure drops back to your 80 psi. A expansion tank absorbs the pressure increase when the water heats up so the tank expands and contracts less, helping reduce metal fatigue.
Excessive pressure and expansion tanks are so important that most water heater companies will not warrant leaking water heaters when the pressure exceeds 80 psi