A. O. SMITH TK-510C-NI 1**

The registry lists a UEF, the ratio of hot water delivered to energy used, of 0.81 for this unit. This site did not take the annual figure from the registry. It computed 209 therms of natural gas from that UEF and the DOE test's daily hot-water draw for the high draw pattern, using DOE CCD data retrieved 2026-09-19. The method is set out on the methodology page. Among the 397 gas tankless models at this draw pattern, the unit ranks 323rd lowest for energy use, which places it in the highest quarter of its class. Its use is 27 therms above the class median of 182 therms.

The high draw pattern describes a larger household, or one where showers and laundry overlap. The figure of 209 therms is stated for that pattern only. The registry gives a maximum flow rate in gallons per minute of 4.7 for this model. A household that matches this draw pattern can read 209 therms as the use under test conditions. A household with lighter use does not match the figure, and its actual use is not available from the registry. Because the unit sits 27 therms above the class median of 182 therms, a reader who compares by energy use alone may want to look at other models in the same class. The water heater selector filters by fuel, space and size, and the same-class replacement finder works from a model number.

The running cost is not stated in the text because it depends on the state average residential gas price, which differs by reader. Multiplying 209 therms by the price per therm gives the yearly fuel cost at the federal test's draw. The cost calculator does this from your own ZIP. Actual household use may differ from the test's daily hot-water draw, so the result is a test-condition figure and not a measured bill. The 27 therms gap to the class median of 182 therms is the extra gas this model is computed to use each year at the same draw pattern. The installed price range appears in its own block on the page, with its source and date, and is not repeated here.