How is FHR and Therms used to compare the efficiency of water heaters?
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.
What efficiency standard does a dual use residential water heater need to meet as a water heater or a boiler (used for both potable water and space heating purposes)?
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.
What is a hot water recirculating system?
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.
How do I choose the best electric or gas tankless water heater for my home (2300 sq. ft.)?
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.
Are tankless water heaters more efficient?
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.