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 ultra-low 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 using tankless technology and is made for 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 promotes this category as the "BEST".

AO Smith condensing and tankless models bring significant benefits to the homeowners not only because they are small, lightweight, and have a compact design, but because of state-of-the-art engineering. They heat water on demand, continually supplying bathrooms and kitchens with minimal temperature fluctuation, high accuracy, and energy savings using ultra-high efficient and 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, making them Energy Star compliant and eligible for rebates and other incentives.

Indoor models from the ATI series include a built-in temperature 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 ultra-low NOx emission.

The following four sizes are available for both indoor and outdoor installation:

  • 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 following three sizes are available only as indoor:

  • AT-160X3P – with the maximum water flow of 6.6 GPM, it is used for homes with 1.5-2.5 showers
  • AT-180X3P – with the maximum water flow of 8 GPM, it is used for homes with 2.5-4 showers
  • AT-199X3P - 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 high power of 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 families, water heaters from the 540 series can link up to 4 units 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 integrated recirculation pump as it can deliver hot water through the ¾" pipe with a 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 is made of commercial-grade copper, which provides better resistance to thermal and mechanical stress for longer and reliable life. The secondary heat exchanger is made of high-quality stainless steel, preventing corrosion due to acidic condensation. The secondary HE is used to increase the incoming water temperature by "stealing" the heat from the flue gases.


Venting pipes are mandatory for indoor models. They utilize a 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 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.

As a convenient feature, a temperature controller 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 the "increase" and "decrease" buttons found on the control panel, you can change settings.

Similar: Rinnai (sold on


  • Heating elements for freeze protection are activated when the surrounding temperature is below 36.5 F.
  • Air-Fuel Ration (AFR) sensor ensures proper gas combustion.
  • A 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
  • A flame sensor ensures the proper functioning of the heater.
  • Tankless models that are equipped with the X3 scale prevention technology are recommended for homeowners who live in areas with hard water.


Model/Feature Type BTU (min-max) Max water flow (GPM) UEF Weight (lbs)
ATI-140H-N ATO-140H-N Indoor
15,000-120,000 6.6 0.90/0.91 44
ATI-240H-N ATO-240H-N Indoor
15,000-160,000 6.6 0.94/0.95 58
ATI-340H-N ATO-340H-N Indoor
15,000-180,000 8.0 0.95/0.94 58
ATI-540H-N ATO-540H-N Indoor
15,000-199,000 10 0.93/0.95 59

Model/Feature Type BTU (min-max) Max water flow (GPM) UEF Weight (lbs)
AT-160X3P Indoor 9,000-160,000 6.6 0.93 80
AT-180X3P Indoor 9,000-180,000 8.0 0.93 80
AT-199X3P Indoor 9,000-199,000 10 0.93 80

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%, reducing energy loss and making 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 an excellent solution for the brand-new house or retrofits. All models come with a 15-year warranty on the heat exchangers and five on parts.


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