AO Smith Tankless Water Heaters
Review & Buying Tips

AO Smith ATO 540AO Smith ATO 540 tankless water heater

Explore small and compact AO Smith tankless water heaters designed for on-demand water heating in residential and commercial applications without occupying the floor space.

What to look for and review of the best of AO Smith condensing water heaters, engineered to provide hot water in endless supply, with the high flow rates and ultimate efficiency. Check out the best models to buy.

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Type and model review

AO Smith tankless water heaters are divided into three groups for residential and light commercial use, all mostly set to use natural gas and convert to liquid propane with the correct conversion kit.

Several types and sizes are available, from the economy, such as non-condensing, to the advanced condensing models with the built-in recirculation pump, ultimate efficiency, and ultra-low NOx - all coming from the ProLine XE series.

ATI (ATO) - 110 (U/C), 310 (U/C), 510 (U/C)

Tankless water heaters from these groups are the simplest of all tankless from AO Smith. As they are equipped with only one heat exchanger, these non-condensing units are available as indoor (ATI) with the power or power direct vent, or outdoor (ATO) - ventless, also with the ultra-low NOx (U), and concentric vent (C) option.

The ATI-510 model is the best, also the largest non-condensing tankless model to choose as it comes with a durable commercial-grade copper heat exchanger, high energy factor of 0.82, a maximum flow rate of 10 GPM, and capability to connect up to 4 units into one system. The users mostly like its compact size, nice appearance, reliable operation, and endless hot water, while the main "issue" is the longer wait time than with the tank-type.

An even better model to buy is ATI-510U, due to the ultra-low NOx emission - great for environment-conscious users and integrated control. In addition to an Easy-Link system, the Multi-Link can connect up to 20 units to operate as one system - delivering more power and hot water.

ATI (ATO) - 140H, 240H, 340H, 540H and 540P

This series consists of ultra-efficient and ultra-low NOx condensing tankless water heaters, where the energy factor is over 0.90. Indoor models have the temperature controller built-in, while the outdoor model comes with the wall mount controller.

A model 540H is the largest and most powerful - one of the top models to buy, and, as the 510 series, is built for heavier hot water usage - 10 GPM water flow and capability to use an Easy-Link and Multi-Unit system.

Basic features

On-demand technology

AO Smith tankless water heaters are designed for on-demand water heating at the point of service and for the whole house use, supplying hot water to one or several applications simultaneously. Also, they are used for high demanding applications such as to fill up the whirlpool tubs or simply supplementing primary storage tank-type water heaters.

High-quality components

The best AO Smith tankless water heaters have high efficiency and condensing technology, using one or two heat exchangers.

The primary heat exchanger is made of thick, commercial-grade copper - hard and durable, while the secondary is made of corrosion-resistant, stainless steel for utilizing "already lost" heat from the flue gases. This way, the total efficiency is significantly increased where the energy factor is over 0.90.

For example, one of the best tankless models, the 540 series, utilizes a recirculation pump for fast, hot water delivery and has an energy factor of high 0.93, meaning that only 7% of energy is wasted. This is the largest tankless unit, and due to the flow rate of 10 GPM is recommended for homes with up to four showers.


AO Smith tankless water heaters built for indoor use come with a direct venting system and 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 use the category III stainless steel venting, while condensing models use cheaper PVC pipes as the exhaust gas temperature is lower. The tankless installation allows flexible venting, including a long vertical or horizontal run through the roof or walls.

The outdoor units are ventless and have great advantages as the outdoor installation saves valuable indoor space, and any leak of the products of combustion or gas stays outside. A recommendation is to install the outdoor unit inside the recess box for protection from the elements, which is available as an optional accessory.

Water flow

AO Smith gas tankless water heaters are designed to provide hot water in continuous supply only when it is needed - on-demand. These can provide enough hot water for several fixtures simultaneously, where the water flow of 10 GPM, as found on the largest unit ATI-540H, is the maximum available.

Up to four units can be connected into one system without purchasing any additional parts or accessories, or up to 20 units connected into one system with some modifications.

In the situation with the high volume conditions (4 and more showers) found on heavy-duty water heaters, such as the series 510 and 540, AO Smith includes durable bypass and flow adjustment valves to handle units with less stress.

If you are looking for a tankless water heater for one to three showers, small and medium-size homes, series 110, 140, 310, and 340 are recommended.


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 one unit to have high performance, accurate temperature control, simple diagnostics, and troubleshooting. The built-in display shows the temperature and error codes if the problem occurs.

Remote control

The remote controller is included as the standard feature on some models. It shows the current inlet and outlet temperature, flow rate, and error codes when the fault is detected and allows you to increase or decrease the outgoing hot water temperature, a very convenient feature.

Safety features

AO Smith gas units are equipped with several safety features for greater protection:

  • An Air-Fuel Ration Sensor or AFR ensures proper air/gas mixture for the clean gas combustion inside the combustion chamber. This way, gas emission is reduced, the performance is optimized, and parts can last longer.
  • An Overheat protection will shut down the unit if the pre-set temperature is exceeded.
  • Power failure protection will cut off the gas supply to the unit if the unit loses electrical power.
  • Flame failure protection allows the tankless heater to shut down if, for any reason, the burner flame is extinguished.
  • A fusible link would break and shut down the tankless water heater if overheat protection didn't prevent the temperature from rising.
  • A power surge fuse is a glass fuse that blows in the case of an over-current.
  • Internal freeze protection, an essential feature when the unit is exposed to cold weather.


AO Smith tankless water heaters come with a 15-year limited warranty on the heat exchanger and for residential water heating, 6 years in the commercial application (or 10 years on 510 and 540 models), and 5 years on all other parts. The extended warranty and advanced features are what make tankless models the best type to buy.

How to contact AO Smith:

Official website: hotwater.com

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