A.Martin Holladay responds: Before you consider installing an instantaneous gas water heater for energy efficiency, consider whether your customers will be satisfied with the flow rate of the model chosen. The most common models of instantaneous water heaters have maximum flow rates in the range of 2 to 3 gallons per minute. Three gpm is the bare minimum to supply two simultaneous showers, and most American families expect their water heater to provide up to 6 gpm of hot water when necessary.

Assuming you’ve decided to install an instantaneous gas water heater with at least 3 gpm of flow, a typical choice would be the AquaStar 240FX (800/642-3199, www.controlledenergy.com), which is available for about $900 to $1,030, depending on whether you need the outside vent hood. The AquaStar 240FX has an efficiency factor (EF) of 0.84. An...

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