Condensing vs Non-Condensing Tankless Water Heaters

Condensing and non-condensing gas tankless water heaters both heat water on demand, but they differ in how much exhaust heat they recover. That single design choice drives the efficiency rating, the venting material, and whether a drain is needed. The comparison below keeps each of those points tied to the manufacturer statement it came from.

Efficiency Is Measured in UEF

Navien rates its condensing tankless units at up to 0.96 UEF and describes non-condensing units as high efficiency at greater than 0.80 UEF, listing a non-condensing example at 0.81 UEF. Navien states that a UEF rating of 0.96 means 96 percent energy efficiency. A. O. Smith frames the same gap through its product comparison, citing non-condensing models at 0.82 UEF versus condensing models at 0.95 UEF.

The useful takeaway is that UEF is the number that separates the two categories. A condensing unit recovers more of the heat it produces, which is why its rating sits higher. A non-condensing unit still runs in the low 0.80s, which A. O. Smith describes as using 82 cents of every dollar to heat water.

Because the ratings come from the manufacturers themselves, the honest comparison is a range rather than a single promised figure. Condensing sits near the mid 0.90s and non-condensing sits near the low 0.80s in these sources.

The Heat Exchanger Count Is the Core Difference

Navien states that condensing tankless water heaters have two heat exchangers while non-condensing models only have one. The second heat exchanger transfers heat from the exhaust gas to the incoming cold water, which improves energy efficiency. A. O. Smith describes the same mechanism as a second heat exchanger that captures and reuses exhaust heat, and says its condensing design captures 95 percent of produced heat.

On the non-condensing side, Navien states the model has no secondary heat exchanger to capture and recycle heat from the exhaust gas, and instead uses a single heat exchanger that heats water as it flows through the unit.

This is the design fact that explains everything downstream. Recovering exhaust heat is what raises efficiency, and it is also what cools the exhaust enough to change the venting and drainage requirements described in the next sections.

Venting Material Follows Exhaust Temperature

Because a condensing unit pulls more heat out of the exhaust, the exhaust leaves cooler, and that allows plastic venting. Navien states that condensing tankless water heaters can use a wider range of venting materials including PVC and polypropylene, while non-condensing models require metal venting due to much hotter exhaust gas. Navien notes condensing venting can reach up to 75 feet with a 2 inch PVC or polypropylene vent pipe, and lists 3 inch by 5 inch metal concentric for the non-condensing example.

A. O. Smith describes the same split, saying condensing tankless water heaters can use plastic venting such as PVC while non-condensing tankless water heaters require thick metal venting. The Navien comparison blog adds that condensing units can use PVC, CPVC, and polypropylene, while non-condensing units use metal Category III venting.

The practical point is that the venting choice is not a preference. It is set by the exhaust temperature of the unit type.

Condensing Units Produce Condensate That Needs a Drain

Recovering exhaust heat cools the flue gas enough that water vapor condenses inside the unit, so a condensing tankless water heater needs a drain. Navien states the condensate comes out through a condensate drain at the bottom of the unit, and that this condensate has an acidic pH of 3 to 5. A. O. Smith states that condensing tankless water heaters produce condensation that requires proper drainage through a condensate drain line connected to an appropriate drainage point during installation.

Non-condensing units do not have this requirement. Navien states that because the exhaust gas does not cool or condense inside the unit, non-condensing tankless water heaters do not need a drain. The Navien comparison blog puts it the same way, saying a condensing unit will require a dedicated drain line.

The acidic pH is worth flagging for the installer, since it affects how the condensate should be routed and whether neutralizing is appropriate for the drainage point.

Cost and Maintenance Tradeoffs

The two types trade efficiency against simplicity. Navien lists non-condensing units as lower cost than condensing models, with minimal ongoing maintenance compared to the annual service Navien recommends for condensing models. Navien also notes non-condensing units are smaller and lighter because they only have one heat exchanger.

A condensing unit earns its higher efficiency through more parts and more upkeep. A. O. Smith positions PVC venting as reducing installation complexity and operating costs, which offsets some of the added unit cost, but the condensing side still carries the drain requirement and the recommended annual service.

The balanced read across these sources is that condensing favors long-run efficiency and flexible plastic venting, while non-condensing favors lower upfront cost, lighter weight, and less scheduled maintenance. Both are legitimate choices depending on the install and the venting path available.