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, what are the main advantages and top features.

Advantages and benefits

AO Smith tankless (sold on are on-demand water heaters that come with some great benefits, such as an endless and continuous supply of fresh hot water, anytime and almost anywhere and advanced safety features such as the AFR sensor and anti-freeze system.

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.

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

Models and features

AO Smith tankless water heater

Indoor/outdoor types

All tankless water heaters from AO Smith are designed as indoor and outdoor units, where indoor units are using the power or direct (optional) vent system to move the byproducts of combustion out. The outdoor models are not using the venting pipes; the fan and air passages are enough to discharge the gases and get the fresh air in for combustion.

Heat exchanger

Residential tankless heaters from 510 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.

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. For additional convenience, the power cord is also included, which saves time during the installation.

The units have advanced electronics which provides better performance, accuracy and easier troubleshooting. If the remote control is installed and there is a problem with the unit, the numerical error code will display.


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 the maximum length of 50 ft.

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

Easy-link system

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

Freeze protection is there to protect the unit during the cold freezing periods while the elements such as 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

Several models with non-condensing technology are available from AO Smith tankless category, still high-efficient but with the lower energy factor than the condensing units (0.82-0.83). These are:

  • ATI / ATO 110 - 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.
  • ATI / ATO 310 - used for two to three bathrooms (maximum water flow rates are up to 8 GPM).
  • ATI / ATO 510 - it comes with the maximum flow rate of 10 GPM, which makes it ideal for homes with two to four bathrooms. It uses an Easy-Link system if demand is higher.

Condensing type

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 up to 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.

AO Smith tankless water heaters are ideal solution for those who are looking for on-demand fresh hot water delivery with an endless supply, packaged in the small and compact design, or looking to heat potable water directly or indirectly, use in radiant floor heating, recirculation or combined space and potable water heating.


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