A. O. SMITH TK-310U-E 2**

The registry lists a UEF, the ratio of hot water delivered to energy used, of 0.81 for this model. This site combined that value with the DOE test's daily hot-water draw for the high draw pattern. The result is 209 therms of natural gas a year. The registry does not publish that annual figure itself. The underlying data comes from the DOE CCD, retrieved 2026-09-19. The method is described in how the annual cost is calculated. Among the 397 models registered as a gas tankless unit at the high draw pattern, this one ranks 323rd lowest for energy use.

The maximum flow rate in gallons per minute for this unit is 4.8. The registry data available here does not say how many fixtures that supports at once, so this page makes no claim on it. The high draw pattern describes a larger household, or one where showers and laundry overlap. The 209 therms figure applies to that pattern only. A household with a lighter draw would need the figure for its own pattern, and that value is not available here. Because this model sits above the class median of 182 therms, a reader who wants lower gas use at the same draw pattern can find a same-class replacement by model number.

The 209 therms figure is an amount of natural gas, not a running cost. Cost depends on the average residential gas price in your state, and this page does not state one. The cost calculator for your own ZIP applies your rate to the figure. The same rate applies to the 27 therms gap above the 182 therms class median. A higher local gas rate widens that yearly gap, and a lower rate narrows it. Actual use in a home depends on real hot-water demand, and the federal test draw is only a standard. A modeled installed-cost range appears in its own block on this page, with its source and date. It is a modeled range, not a quote.