The average lifetime of gas water heater is 9 years. The range is 5-14 years.
The first hour rating (FHR) tells you the number of gallons per hour of hot water. Therms relate to the gas consumption of water heater. When you are shopping for hot water heater, compare the energy factor on the energy guide label. FHR and Therms are not sufficient to calculate the energy factor. Also remember to compare water heaters of equal capacity and FHR.
For a dual unit, it must meet the efficiency standard requirements of both applications. Your local building department may have additional requirements for such units.
A “hot water re-circulating” system usually refers to a domestic hot water system that circulates the heated water through the house continuously so as to eliminate or shorten the delay in hot water reaching the faucet after the user turns it on. These systems are most common in multifamily or large building applications where otherwise the delays would be truly unacceptable. They can be real energy hogs if the pipes aren’t well insulated.
Sizing of hot water heaters should be based on the number of occupants and their peak hour water demand and not on the size of the house.
Tankless water heater efficiencies are not addressed by the Federal standards. Check the EREC fact sheet at http://www.eren.doe.gov/consumerinfo/refbriefs/bc1.html for additional resources.