Condensing vs Non-Condensing Tankless Water Heater

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Quick answer from a licensed plumber: For most indoor installs, buy a condensing tankless such as the Rheem RTGH-84DVLN-3: it vents in plastic pipe with longer runs and Rheem lists up to 0.93 UEF, against 0.82 for its non-condensing RTG. The trade is an acidic condensate drain. Choose non-condensing for an outdoor unit in a mild climate, such as the RTG-84XLN-3, or for a swap onto an existing approved stainless vent.

Our pick for an indoor install: the Rheem RTGH-84DVLN-3 condensing tankless, with plastic vent up to 150 feet in 3-inch pipe. Check the Rheem RTGH-84DVLN-3 on Amazon

Every gas tankless water heater is either condensing or non-condensing, and the choice changes the install more than the hot water. A condensing unit recovers extra heat from its exhaust; Navien says that leaves exhaust temperatures low enough for PVC vent. The same process makes acidic condensate that needs a drain. A non-condensing unit indoors needs stainless or the maker’s listed vent, and an outdoor one needs no vent at all. This page explains the difference from Rheem, Rinnai and Navien manuals and spec sheets. To size and pick a unit, use our best gas tankless water heater guide.

Condensing vs Non-Condensing at a Glance

From the Rheem, Rinnai and Navien documents linked below. Vent rules differ by maker and model, so follow the manual for the exact unit you buy. Every linked listing was sold by Amazon.com and in stock on October 6, 2026. The RTGH-95DVLN-3 and RTG-70XLN-3 are named but not linked.

How a Condensing Tankless Works

Both types burn gas and pass the hot combustion gas over a heat exchanger full of water. A condensing unit adds a second heat exchanger. Navien’s brochure says its condensing stainless steel heat exchangers recover extra heat that would be lost in a non-condensing water heater, and that the low exhaust temperatures allow PVC venting. Navien’s manual says the unit creates condensation when it operates, with an acidic pH of 3 to 5. Rheem’s condensing RTGH lists a copper primary and a stainless steel secondary heat exchanger; its non-condensing RTG lists copper only.

You can see the gain in Rheem’s own numbers. The condensing RTGH-84 and the non-condensing RTG-84 both list 8.4 GPM at a 35°F rise, but the RTGH-84 does it on a 157,000 BTU/h maximum input and the RTG-84 on 180,000 BTU/h. That is 23,000 BTU/h less gas at full fire for the same flow (our arithmetic). Rheem lists the RTGH line at up to 0.93 UEF and the RTG at 0.82; Rinnai lists its condensing SENSEI RX at 0.97 to 0.98 and its non-condensing RE at 0.81 to 0.82, and Navien lists its condensing NPE-A2 at 0.95.

Venting: Plastic or Stainless

Venting is the biggest install difference. Rheem’s RTGH spec sheet lists Schedule 40 PVC, CPVC or polypropylene, in 2- or 3-inch pipe; its RTGH manual lists a longer set that also includes Schedule 80 PVC, ABS and UL 1738 Category III stainless, and says foam core PVC is never allowed. Rinnai’s SENSEI RX manual lists Schedule 40 PVC or CPVC and says not to use cellular core pipe. Navien lists PVC, CPVC, approved polypropylene and stainless for its NPE.

The non-condensing indoor units are stricter. Rheem’s RTG manual says to use only approved Category III stainless steel vent material, and its spec sheet adds Metal Fab, ProTech and UBBINK concentric vent. Rinnai’s RE manual says not to use PVC, CPVC, ABS or galvanized material, not to mix vent parts from different makers, and to use only vent components listed for the model. On Rheem’s sheets the condensing RTGH runs up to 150 feet of 3-inch pipe; the non-condensing RTG runs up to 43 feet.

Neither type can reuse an old tank water heater flue. Rheem’s RTG manual says not to vent through a chimney and to vent the heater separately from all other appliances, and both Rinnai manuals say not to connect the venting to an existing vent or chimney.

Condensate: Drain and Neutralizer

A condensing unit makes condensate whenever it runs, and Navien’s manual gives its pH as 3 to 5. Rheem’s RTGH manual says it is acidic and can corrode metal, recommends an external neutralizer, and says to run the drain in PVC, CPVC or flexible tube with a downward slope, open to the air at the end and never connected directly to the sewer. Rinnai sells a neutralizer kit for the SENSEI RX. Navien’s manual is looser: it lets you drain straight to an external drain, through a neutralizer or through a condensate pump, and recommends its own neutralizer kit if you use one. Follow your local code; it decides where condensate may go.

Non-condensing units can make condensate too, in the vent rather than the heat exchanger. Rheem’s RTG manual says to use the condensate collector on vertical vents and on horizontal vents that slope up toward the outside, and to plumb its hose to a sanitary drain. Rinnai’s RE manual says cold climates make more condensate in the vent and that the collector should be used there.

When Non-Condensing Still Makes Sense

• Outdoor installs in mild climates. An outdoor non-condensing unit has no vent pipe and no indoor drain. Rheem rates the outdoor RTG’s freeze protection to about -4°F outdoors.
• Swapping one tankless for another. If an older non-condensing unit already has listed stainless or concentric vent, Rheem markets the RTG for easier tankless-to-tankless replacements on Metal Fab, ProTech and UBBINK concentric vent. The installer still has to confirm the existing vent is approved for the new model.
• No place for a condensate drain. A condensing unit needs somewhere for its condensate to go, or a condensate pump.

For most indoor installs, especially a first tankless replacing a tank, the condensing unit is usually the easier job: plastic vent, longer runs and the higher UEF. For the gas line, which matters more than either type, see our gas tankless guide, and for the costs see what a tankless install costs.

I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.

Before You Buy

1. Decide indoor or outdoor first. That decides most of the vent question. Rheem’s RTGH is indoor (outdoor with the RTGH-X kit); its RTG comes in separate indoor and outdoor models.
2. Plan the vent route. Count the elbows: Rheem takes 6 feet off the RTGH maximum for each 90-degree elbow and lists a shorter table for the RTG.
3. Find the condensate path. A floor drain, laundry tub or condensate pump within reach of a condensing unit, and a neutralizer if your maker or code calls for one.
4. Check the gas line. The units on this page burn 157,000 to 199,900 BTU/h at full fire. Rheem says the gas pipe and meter must be sized correctly, and that a qualified installer, service agency or the gas supplier must do the installation.
5. Size for winter. Flow drops as the temperature rise goes up. Our tankless size calculator does the arithmetic.

Install Notes

Rheem’s manuals say installation and service must be performed by a qualified installer, service agency or the gas supplier. The job covers a gas line sized for the burner, a new vent through the wall or roof, a 120-volt outlet, and for a condensing unit a condensate drain. Rheem’s RTGH manual says not to convert the heater to a different gas than the one on its rating plate, so buy the natural gas or propane model you need. Rheem says to glue the RTGH’s plastic vent to the heater’s vent connection and not to use screws.

Plumber’s note: a tankless needs descaling on hard water whichever type you buy; see how to descale a tankless water heater. If you are still deciding between gas and electric, read electric vs gas tankless first, and for Rinnai and Navien condensing models see Navien vs Rinnai.

FAQ

What is the difference between condensing and non-condensing tankless water heaters?

A condensing unit has a second heat exchanger that takes more heat out of the exhaust. Rheem lists a copper primary and stainless secondary exchanger on its condensing RTGH and a single copper exchanger on its non-condensing RTG. The condensing unit vents in plastic pipe and needs a condensate drain; the non-condensing indoor unit vents in Category III stainless or approved concentric vent.

Is a condensing tankless water heater worth it?

For an indoor install, usually. Rheem lists up to 0.93 UEF for its condensing RTGH line against 0.82 for its non-condensing RTG, and the RTGH vents in plastic pipe with longer runs. The trade is an acidic condensate drain, which Rheem says should run through a neutralizer. Outdoor non-condensing units skip the vent entirely.

Can a condensing tankless use PVC vent?

Yes, per its maker. Rheem lists Schedule 40 PVC, CPVC and polypropylene on the RTGH spec sheet, Rinnai lists Schedule 40 PVC or CPVC for the SENSEI RX, and Navien lists PVC, CPVC, approved polypropylene and stainless for its NPE. Rinnai’s RE manual says not to use PVC, CPVC, ABS or galvanized material on its non-condensing RE.

Does a non-condensing tankless need a condensate drain?

Sometimes. Rheem’s RTG manual says acidic condensate can form in the vent and requires a condensate collector on vertical vents and on horizontal vents that slope up toward the outside. Rinnai’s RE manual says to use the condensate collector on vertical terminations and in cold climates.

Can a tankless water heater use my old water heater flue?

Not on these models. Rheem’s RTG manual says not to vent through a chimney and to vent the heater separately from all other appliances, and Rinnai’s RE and SENSEI RX manuals say not to connect the venting to an existing vent or chimney. Plan a new vent run, or an outdoor unit.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia journeyman plumber. Every spec here comes from the maker documents linked below, or from the Amazon listings we link; nothing comes from hands-on testing. On October 6, 2026, every linked listing was in stock and sold by Amazon.com. If you buy through a link we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.

Sources

• Rheem: RTGH Series condensing tankless gas water heaters, specification sheet TK-RTGH Rev. 2, RTGH Use & Care manual
• Rheem: RTG/RTG-R Series non-condensing tankless gas water heaters, specification sheet RH-RTG-RTGR-SS Rev. 1, RTG Use & Care manual
• Rinnai: RE series non-condensing tankless (product page with spec sheet and Installation and Operation Manual)
• Rinnai: SENSEI RX series condensing tankless (product page with spec sheet and Installation and Owner’s Manual)
• Navien: NPE condensing water heaters installation manual, NPE-2 specification sheet, NPE-2 consumer brochure
• Amazon listings for each linked water heater (linked above)

Bottom Line

Condensing tankless heaters get more heat out of the gas, vent in plastic and need a condensate drain. Non-condensing units vent in stainless or listed concentric pipe indoors, or skip the vent entirely outdoors. For most indoor installs the condensing Rheem RTGH-84DVLN-3 is the one to buy; for a mild-climate outside wall, the RTG-84XLN-3. Plan the vent, the drain and the gas line before you order.