AO Smith Tankless
Water Heaters
Review and Buying Tips

AO Smith tankless water heaters are built with the condensing and non-condensing technology for on-demand water heating in residential and light commercial applications. Find out which one is the best for you, and what are the main advantages and top features.

Advantages and benefits

AO Smith tankless (sold on are on-demand water heaters that come with many great benefits, such as endless and continuous supply of fresh hot water, anytime and almost anywhere, and advanced safety features such as the modulating gas valve, diagnostics system, AFR sensor to ensure proper combustion, internal anti-freeze system to protect the system from freezing temperatures and other.

AO Smith tankless water heaters are designed small, compact, and wall-hanging, which saves on space, and with its power-venting and sealed gas combustion allows more flexibility and installation.

All the appliances are using natural gas or propane for high efficient water heating, which makes them Energy Star approved, and eligible for the federal tax credit and utility rebates.

Great design allows them to supply enough hot water from small (apartments, condos, cottages) to larger homes with several bathrooms or applications working simultaneously. Which one you will choose, depends on your budget, house and family size, installation options, and your preference. Check out our article on how to select a tankless water heater for more info.

An overview of AO Smith tankless water heaters

AO Smith tankless water heater

All tankless water heaters from AO Smith are designed as indoor and outdoor units, where indoor units are using the power or power direct (optional) vent system to move the byproducts of combustion out. The outdoor models are ventless; the fan and air passages are enough to discharge the gases and get the fresh air in for combustion.

Strong heat exchangers

Some residential tankless heaters, such as models from 510 and 540 series, utilize one commercial-grade heat exchanger made of the thick HRS 35 copper alloy, which provides more strength, resistance, and longer life than the standard type. The secondary heat exchanger, which is found only on condensing models, is made of stainless steel providing better resistance to corrosion and longer life.

Modulating gas valve

Both condensing and non-condensing water heaters are fully-modulating, which means that the burner works in a range from its minimum to its maximum power, so the temperature is based on the existing water flow. This is what provides more comfort during the shower.


The remote control is a standard feature and is used to control the temperature of the hot water, and it shows the error codes when the problem occurs. Some models have it built-in while for others are provided. For additional convenience, the power cord is also included, which saves time during the installation.

The units have advanced electronics that provide a better performance, accuracy, and easier troubleshooting. If the remote control is installed and if there is a problem with the unit, the numerical error code will display. Use the troubleshooting guide to fix the problem.


All the indoor models use the vent system, which consists of the power vent and 4" stainless steel vent pipe, Category III, which brings excellent flexibility in moving the flue gases out safely. The installation can be horizontal or vertical and with a maximum length of 50 ft. This applies to non-condensing models, while condensing use PVC.

The outdoor models do not require such a vent system; they utilize only a fan.

Capable of delivering hot water to high demanding applications

For larger applications with the high hot water demand, an Easy-Link system is used, which connects up to four identical units into one system, while with the Multi-Unit system, up to 20 units. From residential models, this is possible only on 510 and 540 series.


AO Smith tankless appliances are very safe. They safely transfer the byproducts of combustion through the venting system, use the sealed combustion chamber for gas combustion, including the AFR or Air-Fuel Ratio sensor for the proper combustion and reduced gas emission.

The freeze protection is there to protect the unit during the cold freezing periods while the elements such as the hi-limit switch and overheat cutoff fuse, protect the units and its components from overheating. Other safety features include the flame sensor, which shuts the unit off when the flame is extinguished and flue backdraft pressure switch, which shuts the unit off if something is wrong with the flue vent.

Related: Rinnai RUR98

Non-condensing water heaters

AO Smith offers several models with the non-condensing tankless technology, which are still high-efficient but with the lower energy factor than the condensing units, reaching energy factors of 0.81-0.82.

The models are as follows:

  • Series 110/U/C* - with the maximum water flow of 6.6 gallons per minute, the smallest tankless water heaters from AO Smith are used for heating water in homes with one to two baths (depends on the climate or groundwater temperature). These models are recommended for smaller houses, apartments, condos, and cottages. They come with the maximum power of 140,000 BTU and an energy factor of up to 0.81.
  • Series 310/U/C* is used for supplying hot water to two to three bathrooms, due to its maximum power of 190,000 BTU, maximum water flow rates of 8 GPM and energy factor of 0.82.
  • Series 510/U/C* is powered by an eco-friendly gas burner that can provide up to 199,900 BTU of power and maximum water flow rate of 10 GPM, which makes it ideal for homes with two to four bathrooms. These models can combine more units into one system if the demand for hot water is higher.

*ATI models are indoor while ATO models are outdoor, using power or power direct venting system.

Models such as ATI/ATO 110, 310, and 510 are non-condensing and low NOx water heaters that come with the remote controller.

Models such as ATI/ATO 110U, 310U, and 510U are non-condensing and ultra-low NOx water heaters that utilize an integrated temperature controller and two-pipe vent system for indoor models, while the controller for outdoor ventless models is provided.

AO Smith ATI 110C, 310C, and 510C are indoor and non-condensing models that utilize a built-in temperature controller and concentric or one-pipe venting system for easier installation. They are also ultra-low NOx.

Note: All indoor, non-condensing tankless water heaters from AO Smith must use category III stainless steel venting.

Condensing type

Condensing water heaters from AO Smith are also designed for indoor (ATI models) and outdoor (ATO models) installation.

  • 140H - used for 1.5 to 2.5 bathrooms (maximum water flow rates are up to 6.6 GPM).
  • 240H - used for 2.5 to 3.5 baths (maximum water flow rates are up to 6.6 GPM).
  • 340H - used for 2.5 to 3.5 baths (maximum water flow rates are up to 8 GPM).
  • 540 - used for 2.5 to 4.5 baths (maximum water flow rates are up to 10 GPM). It uses an Easy-Link system to connect up to 4 units, or 20 units with the multi-unit controller if the demand is higher.

All the models that are built for indoor installation (ATI models) come with integrated controls and diagnostics system, also a two-pipe vent system, while outdoor models (ATO) come with the remote controller and ventless configuration.

The main difference between condensing and non-condensing heaters is in the technology used to heat water. Condensing water heaters are much more efficient as the water is preheated in the heat exchanger from flue gases, which increases the energy factor to high 0.95.

Condensing units are more expensive, but are using stronger and more durable materials also, such as the thicker copper for the primary exchanger and stainless steel for secondary HE or the advanced burner for low NOx emission. Use this article for the detailed review of condensing water heaters.

Best of AO Smith

The best model from Proline XE tankless and non-condensing series is ATI-510C that features concentric venting and ultra-low NOx gas burner. This model comes with the highest BTU rating and water flow rate. If the demand for hot water is higher, it can use Easy-Link cable to connect up to 4 units, while when using the Multi-Link, it can connect up to 20 units and operate as an integrated system.

From the condensing series, choose a model ATI-540P-N, which comes with an integrated recirculation pump so hot water can be delivered fast through the plumbing with the pipe length of up to 500 feet. For larger applications, it can also use an Easy-Link cable and Multi-Unit System and increase the flow rate, substantially.


AO Smith tankless water heaters are an ideal solution for those who are looking for on-demand fresh hot water delivery with an endless supply, without occupying too much of living space. It is excellent for homeowners who are looking to heat potable water directly or indirectly, use it in radiant floor heating, recirculation, or combined with the space heating. Either one of the AO Smith tankless models you select, you will save; energy, water, space, and money. Like most of the tankless water heaters, they last long and are backed by the manufacturer for 15 years.


Review, Ask or Comment

> > AO Smith tankless review