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

A. O. Smith TK-310U-E 2** is in the DOE listing as of Oct 26, 2026.

It has a certified UEF of 0.81 and a certified flow of 4.8 gal/min at a 67°F rise.

Its certified flow is 4.8 gal/min at a 67°F rise, while A. O. Smith advertises up to 8.0 gal/min, no rise stated (Read Oct 5, 2026), so a comparison at the same rise is not possible.

  • Gas tankless water heater
  • 2 listed models

Not listed as ENERGY STAR certified on Oct 26, 2026.

Flow at a 67°F rise
4.8gallons per minuteCertified
Advertised flow
8.0gallons per minuteA. O. Smith, up to, no rise stated
UEF
0.81Uniform energy factor
See it among every gas tankless model

Flow where you live

A bigger rise from colder incoming water leaves the heater with less flow.

Cold water
Cold water must be at least 10°F colder than the hot water setting.
Flow estimate
The estimate uses the lowest certified flow among the listed models. It is about 4.6 gal/min at a 70°F rise.

Flow limits

  • The estimate excludes the flow needed to turn the heater on, heat lost in pipes, and installation.
  • Cold water from a well or long pipe run can differ from the temperature you enter.
  • Without a stated flow limit, estimates for warmer water may exceed the heater’s actual flow.

Advertised and certified figures

A. O. Smith TK-310U-E 2**: advertised and certified
A. O. Smith TK-310U-E 2**: advertised and certified UEF: advertised 0.81, certified 0.81. UEF: advertised 0.81, certified 0.81 (advertised up to). Flow: advertised 8.0 gal/min, certified 4.8 gal/min (advertised up to, no rise stated, certified at a 67°F rise). Flow: advertised 4.8 gal/min, certified 4.8 gal/min (advertised at a 67°F rise, certified at a 67°F rise). UEF Advertised 0.81 Certified 0.81 UEF · advertised up to Advertised 0.81 Certified 0.81 Flow · advertised up to, no rise stated, certified at a 67°F rise Advertised 8.0 gal/min Certified 4.8 gal/min Flow · advertised at a 67°F rise, certified at a 67°F rise Advertised 4.8 gal/min Certified 4.8 gal/min
FigureAdvertisedCertifiedAdvertiserRead
UEF 0.81 0.81 A. O. Smith Read Oct 5, 2026
UEF up to 0.81 0.81 A. O. Smith Read Oct 5, 2026
Flow up to 8.0 gal/min, no rise stated 4.8 gal/min at a 67°F rise A. O. Smith Read Oct 5, 2026
Flow 4.8 gal/min at a 67°F rise 4.8 gal/min at a 67°F rise A. O. Smith Read Oct 5, 2026

See how advertised flow compares across tankless models

Certified figures by listed model

Model numberUEFFlow at a 67°F rise (gal/min)Recovery efficiencyDraw pattern
ATO-310U 2**0.814.886%High
GT15-310U-O 2**0.814.886%High

UEF is the uniform energy factor. A dash means the model has no such figure.

It warms 4.8 gallons per minute of water from 58°F to 125°F in the test. The gas tankless type has a median flow of 4.8 gallons per minute. Among 487 gas tankless models, 221 give more flow at a 67°F rise.

On Oct 26, 2026, this heater was not listed as ENERGY STAR certified. In tests, 81% of the energy it uses ends up as hot water (UEF 0.81). Its test ran at the high level of daily hot water use. Its UEF is lower than that of 327 of the 399 gas tankless models with its draw pattern. On its draw pattern, the median gas tankless UEF is 0.93. Of the energy it spends heating water, 86% ends up in it (its recovery efficiency).

It is sold as ATO-310U 2** and GT15-310U-O 2**. There are 5 more basic models with the same test figures. Each of the 5 other brands lists one of them.

Also listed as

They carry every figure in the table above.

Updated Oct 26, 2026 · Methodology

Not covered

  • Changes to an advertised figure after the date beside it.