AO Smith Gas Water Heater

Expert review of gas-fired A.O. Smith tankless water heaters

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Tankless AO Smith gas water heater is the whole house, on demand heating unit designed for both residential and commercial use, also applicable for residential hydronic and radiant floor heating applications. New condensing heaters feature not only an improved flow rates, but save you money because of the amazing energy efficiency.

Basic Features

On demand technology

AO Smith tankless water heaters are designed for the whole house water heating as they supply hot water to several applications simultaneously; also whirlpool tubs, point-of-use applications for single fixtures, or supplement your home's primary storage tank type water heater.

AO Smith gas water heater with a tankless technology is available in three sizes, for indoor and outdoor installation and comes with the different hot water flow and BTUs.

High efficient components

New models from AO Smith include high efficiency and condensing gas water heaters that are using not one but two heat exchangers. The primary heat exchanger is made of the thicker copper while the secondary is made of the durable stainless steel for utilizing the "already lost" heat from the flue gases. This way the total efficiency is significantly increased where the energy factor is 0.91. The built-in display shows the temperature and error codes if the problem occurs.


ao smith gas water heater tankless

Indoor on demand models are designed as direct vent units, with the sealed combustion, so the water heater can only take the fresh air and vent the gases outside, keeping the inside air protected and untouched. Non-condensing units are using Category III stainless steel venting while condensing cheaper PVC pipe. Tankless installation allows flexible venting, which includes long vertical or horizontal run through the roof or walls.

The outdoor unit has great advantages as the outside installation saves the valuable indoor space and any leak of the products of combustion or gas stays outside. Recommendation is to install them in the recess box, which come as the optional accessories.

Water flow

AO Smith gas water heater designed with the tankless technology is providing continuous hot water, on demand and when it is needed. AO Smith tankless units are able to provide enough hot water for several fixtures simultaneously, where the water flow of 9.8 gpm is the maximum available. Up to four units can be connected into one system without purchasing any additional part or accessories, or up to 20 units connected into one system with some modifications.


Electronic ignition, temperature and other heating processes inside the AO Smith tankless gas heaters, are monitored by the electronic control board. This is what helps to have great performance, an accurate temperature control and easier diagnostics and troubleshooting.

Remote control

Temperature controller is optional and included with 510 models, and it shows the current inlet and outlet temperature, flow rate, error codes when a fault is detected, and allows you to increase or decrease the temperature of the outgoing hot water.

emergency plumber

Note: According to AO Smith and in order to increase the value of its tankless water heaters, the manufacturer will include remote controllers and power cord in every residential on demand heater purchase.

Safety features

AO Smith gas units are equipped with several safety features which protect the unit and your property:

  • Overheat protection will shut down the unit if the pre-set temperature is exceeded.

  • Power failure protection will cut off the gas to the unit if the unit loses the electrical power.

  • Flame failure protection allows tankless heater to shut down if for any reason the burner flame is extinguished.

  • Fusible link will break to shut off the tankless water heater if overheat protection didn't prevent the temperature from rising.

  • Power surge fuse is the glass fuse which blows in a case of the over-current.

AO Smith gas water heater is Energy Star qualified as the energy factor of 0.82 for non-condensing and 0.91 for condensing, complies with the requirements. You will also find models that don't have the energy factor rating and that is because the maximum BTU gas input exceeds 199,000, which is used as the maximum power for EF consideration.

Types of Tankless Units

Tankless or on demand AO Smith gas water heaters are divided into three different groups for residential and light commercial use: 305, 505 and 705 models, or new 110, 310 and 510 models with the addition of condensing models from 520 series. All the models from below are factory set to use natural gas, but with the correct conversion kit they can use liquid propane.

ATI-110 and ATO-110

ATI-110 and ATO-110 are gas water heaters designed for indoor and outdoor installation and with the water flow rate of 6.6 GPM (which is the lowest) they are ideal to supply sufficient hot water for up to two and three showers. Fully modulating gas burner allows them to operate within the power range from 19,500 to 140,000 Btu.

ATI-310 and ATO-310

An indoor ATI-310 and outdoor ATO-310 water heaters are designed with lower minimum gas input of 11,000 and higher maximum gas input of 190,000 Btu where the maximum water flow rate of 8 GPM is higher than found in the previous models.

ATI-510 and ATO-510

ATI-510 and ATO-510 have the highest water flow rate of 10 GPM and maximum power of 199,000 Btu. They are used for up to four applications in warmer climates and up to two in colder. The other difference between 510 series and previous two is in the thicker and stronger heat exchangers which makes it more durable and resistant to stress.

ATI-520 and ATO-520

ATI-520 and ATO-520 are indoor and outdoor condensing water heaters that are, with the water flow rate of 9.0 GPM recommended for three and four fixtures.

AO Smith ATI-305N

AO Smith ATI-305N (replace N with P for propane gas) tankless models can be found as indoor and outdoor units, with an option to use natural or propane gas. Maximum gas input for tankless water heaters from this 305 series is 180,000 BTU/hr with the EF 0.82, which qualifies them as the high efficiency units.


AO Smith tankless heaters ATI-505N are also available as indoor and outdoor units, where the maximum power for propane gas can reach 190,000 BTU/hr while for natural gas 199,000.

ATI-705N and ATI-705AN

AO Smith ATI-705N and AO Smith ATI-705AN with 237,000 BTU/hr are designed as outdoor and indoor units but without EF rating. AO Smith gas water heater from ATI-705AN series is certified as ASME.


Every AO Smith gas water heater with the tankless technology comes with the 12-year limited warranty on the heat exchanger for residential water heating, 5 years in commercial application and 5 years on all component parts.

(Pictures are courtesy of AO Smith)