We do not have any specific product information or recommendation. However you can access the several manufacturers found in the contacts and links page in our web site and obtain the necessary information. Here are some links for additional information. The average lifetime of gas water heater is 9 years. The range is 5-14 years.
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. 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. These systems are most common in multifamily or large building applications where otherwise the delays would be truly unacceptable.