AO Smith Condensing
Water Heaters
Review and Buying Tips

Models, features, and advantages of the AO Smith condensing water heaters. Built as the most advanced products, condensing units provide ultra-efficient, high-performing water heating with significant energy savings and a reduction in gas emission. 

Are you having problems with condensing? Check out the troubleshooting guide.

This article covers only condensing, gas-powered water heaters from AO Smith that are using tankless technology and are made for the residential and light-commercial water and space heating, with and without recirculation. Only models from the Proline XE incorporate condensing technology. The AO Smith manufacturer advertises this category as the "Best".

AO Smith condensing models can bring significant benefits to the homeowners not only because they are small and lightweight with the compact design, but because they heat water on demand, continually supplying bathrooms and kitchens with minimal temperature fluctuation, high accuracy and energy savings using the ultra-high efficient, eco-friendly gas burners.

You might also want to check: Best condensing tankless water heaters

AO Smith condensing water heater

Models, Features, Benefits

Available models

AO Smith condensing water heaters with tankless technology are designed as indoor models with the power direct venting (ATI) and outdoor models (ATO), which are ventless. Both types come with a high energy factor of over 0.9, which makes them Energy Star compliant and eligible for rebates and other incentives.

Indoor models from ATI series, include a built-in remote controller and diagnostics system for convenient monitoring, control, and easier troubleshooting. Outdoor models from the ATO series also contain these features, but the temperature controller is not built-in but supplied, and it mounts on the wall.

All models use either natural or propane gas for water heating, and thanks to the improved burner designed, gas combustion is very efficient with the ultra-low NOx emission.

There are four sizes available:

  • ATI/ATO-140H – with the maximum water flow of 6.6 GPM, it is used for homes with 1.5-2.5 showers
  • ATI/ATO-240H – with the maximum water flow of 6.6 GPM, it is used for homes with 2.5-3.5 showers
  • ATI/ATO-340H – with the maximum water flow of 8 GPM, it is used for homes with 2.5-4 showers
  • ATI/ATO-540H – with the maximum water flow of 10 GPM, it is used for homes with 3-4.5 showers

The best model from AO Smith condensing water heaters is coming from the 540 series. Models ATI-540 and ATO-540 use an eco-friendly gas burner that can provide a high 199,000 BTU and water flow of up to 10 gallons per minute, which quickly covers homes with 3 to 4.5 showers. For high demanding homes, water heaters from 540 series can link up to 4 units by using the Easy-Link cable, and up to 20 units when using the Multi-Unit controller.

For faster hot water delivery and even better savings than other condensing models, buy ATI-540P with an integrate recirculation pump as it can deliver hot water through the ¾" pipe with the maximum length of 500 feet. What makes it even greater is the high-efficiency rate of up to 0.95 and no standby heat loss, as found in tank-type models.


Two heat exchangers

AO Smith condensing water heaters are equipped with two heat exchangers; the main or primary one, made of the commercial-grade copper, which provides better resistance to thermal and mechanical stress for longer and reliable life, while the secondary heat exchanger is made of the high-quality stainless steel, which prevents corrosion due to the acidic condensation. The secondary HE is used to increase the temperature of the incoming water by "stealing" the heat from the flue gases.


Venting pipes are mandatory for indoor models. They utilize power vent or power direct vent design where the maximum length of the vents is up to 50 feet and are made of PVC, CPVC, ABS, or Stainless Steel. Outdoor models do not need a vent pipe since they are venting directly to the outside atmosphere by using the electric blower. Using the eco-friendly gas burner, heaters would emit ultra-low NOx gases.


AO Smith condensing water heaters are computerized, and that includes the electronic circuit board, which initiates and monitors the electronic ignition, the temperature of hot water, water flow, heat power, and other parameters, including error code diagnostics and troubleshooting.

A remote controller, as the convenient feature, is either built-in or included in your purchase, and it allows you to change and monitor the temperature and check for potential problems. AO Smith also offers an option to change the water temperature directly on the heater. There are 16 temperature settings to choose from, and by using "increase" and "decrease" buttons found on the computer board, you can change the settings.


  • Heating elements for freeze protection activated when the surrounding temperature is below 36.5 F.
  • Air-Fuel Ration (AFR) sensor ensures proper gas combustion.
  • Hi-limit switch is a standard feature that shuts the unit off when the temperature of the exhaust gases is too high.
  • Temperature sensors for the outgoing hot water
  • Flame sensor ensures the proper functioning of the heater.

AO Smith condensing water heaters are expensive but provide considerable energy savings for the users in the long run. Energy efficiency on all condensing models is over 90%, which reduces the energy loss and makes them eligible for Energy Star approval.

They are perfect for larger homes and families where the high hot water demand is needed. With the vent pipe run of over 50 feet, outdoor installation options, and recirculation systems, they are a great solution either for the brand-new house or retrofits. All models come with the 15-year warranty on the heat exchangers and five on parts.


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