A. O. SMITH ATO-140X3-N 1**

The registry lists a UEF, the ratio of hot water delivered to energy used, of 0.81 for this model. This site computed the annual figure of 137 therms from that UEF and the DOE test's daily hot-water draw. The registry does not publish the annual figure itself. The value was retrieved from the DOE CCD on 2026-09-19. It belongs only to the medium draw pattern and changes if a household uses more or less hot water than the test's daily draw. Among the 80 models registered as a gas tankless unit at this draw pattern, this one ranks 52nd lowest in energy use. That places it in the second-highest quarter of its class, with a median of 135 therms.

This unit uses 2 therms more than the class median of 135 therms. It fits a reader whose household resembles the medium draw pattern and who wants a figure that sits close to the median. It does not fit a reader who needs the lowest energy use in the class, because 52nd lowest of 80 leaves lower-use models registered. Those can be found with the same-class replacement search by model number. The registry's maximum flow rate in gallons per minute is 3.6. The energy figure does not depend on that value, and this page does not judge whether it meets a particular home's simultaneous demand. The water heater selector filters by fuel, space and size.

Running cost is not written here, because it depends on your state's average residential gas price. At any rate, the yearly gap to the class median is 2 therms multiplied by your price per therm. The cost calculator applies the price for your ZIP to the 137 therms shown above. The annual cost method explains how that calculation works. Measured gas use in an actual home is not available for this model. The 137 therms is a test-based computation, so real use may differ. The modeled installed-cost range appears in its own block with its source and date. It is not a quote and is not repeated in this text.