A. O. SMITH TK-510U-I 2**

The annual figure of 209 therms is not a registry entry. This site computed it from the registry's UEF, the ratio of hot water delivered to energy used, which is 0.81 for this model, and the DOE test's daily hot-water draw for the high draw pattern. The source is DOE CCD, retrieved 2026-09-19. Among the 397 models registered as a gas tankless unit at the same draw pattern, this one ranks 323rd lowest in energy use, which places it in the highest quarter of its class. The class median is 182 therms, so this unit sits 27 therms above it. The steps of the calculation are set out in the methodology for water heater costs.

The high draw pattern describes a larger household, or one where showers and laundry overlap. The figure of 209 therms applies to that use, and the unit's maximum flow rate in gallons per minute is 5. For a household that matches this pattern, the gap of 27 therms from the class median of 182 therms is the difference in gas use at the test draw. A household that draws less hot water than the test assumes would use a different amount. The registry does not give that amount, so it is not available here. Readers who compare units by energy use alone should note the position of 323rd lowest of 397. The water heater selector filters by fuel, space and size.

Running cost is not written in this text because it depends on your state's average residential gas price. It is that price applied to 209 therms at the high draw pattern, so the same model gives a different result in each state. The cost calculator applies the price for your own ZIP. The modeled installed-cost range appears in its own block on this page with its source and date, and this text does not repeat it. If you are checking a swap for TK-510U-I 2**, you can find a same-class replacement by model number, then compare each candidate's annual energy with the class median of 182 therms.