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AO Smith ATO-310U-N Outdoor Nat.Gas Ultra-Low NOx Tankless Water Heater

Original price $899.44 - Original price $919.39
Original price
$899.44 - $919.39
Current price $899.44


190,000 BTU tankless gas water heater for homes with up to three bathrooms.

The ATO-310U is A. O. Smith's most popular and versatile tankless model. It features a maximum flow rate of 8 GPM, providing enough hot water for three showers in warm climates, or up to two showers in colder climates at the same time.

New, Improved Features

• Heat-resistant copper heat exchanger tubing
• A remote control is included (item number 100209924). Install up to 400 feet from the heater using minimum 20 gauge control wire (not included).
• Thicker heat exchanger drum
• Includes a pump control port
• Easy-Link System for up to 4 units

Specification highlights

• Set up for natural gas from factory, but field convertible from NG to LP with included gas conversion kit
• 190,000 BTUs
• Maximum flow rate of 8 GPM
• Non-condensing technology
• Ultra-Low NOx emissions (natural gas only). Complies with SCAQMD Rule 1146.2 for NOx emissions requirement of 14 ng/J or 20 ppm
• Complies with lead-free standards
• Built-in freeze protection
• Indoor models include factory-installed 120V power cord with plug
• Manual reset high limit (Set at 194°F)
• Overheat cutoff fuse
• Inlet and outlet thermistors for constant temperature monitoring
• Can be used for radiant/hydronic/space heating as well as the usual running hot water (domestic hot water/DHW). See page 36 of the installation manual (download below).

Optional Accessories

• Recess Box (for outdoor models)
• Pipe Cover
Isolation Valve Kit



Emission Ultra-Low NOx
Maximum BTUs 190,000
Minimum BTUs
Limited Heat Exchanger Warranty 15 Years for residential installation
5 Years for commercial installation
Limited Parts Warranty 5 Years
Water and Gas Connection Location Bottom
Water and Gas Connection Size 3/4" male NPT
Heat Exchanger HRS35 Copper Alloy
Secondary Heat Exchanger Stainless Steel
Electrical power 120V 60 Hz
Uniform Energy Factor (UEF) 0.82
ENERGY STAR qualified Yes
NSF Certification NSF/ANSI Standard 5
CSA Certification Yes
CEC Compliant Yes
Technology Non-Condensing
Minimum water heating flow rate

0.5 GPM initially, 0.4 GPM thereafter

Type Space Saving
Height 20.5"
Width 13.75"
Depth 9.5" for outdoor model
10.0" for indoor model
Product Weight 39lbs/17.7kg
Shipping Weight 44lbs


Ultra-Low NOx Manual
Spec Sheet