Condensing vs Non-Condensing Tankless

1. The Short Answer: Condensing Indoors, Non-Condensing Outdoors in the Sun Belt

Every gas tankless water heater burns gas across a heat exchanger and sends the hot combustion gases outside. A non-condensing unit stops there. Its exhaust leaves the cabinet hot, which wastes some heat and means the vent pipe has to be metal. A condensing unit adds a second heat exchanger in the exhaust path, pulls the exhaust temperature down far enough that water vapor in it turns back into liquid, and captures the heat released when that happens. That is where the extra efficiency comes from, and it is also why it needs a drain.

For an indoor installation, which is the norm anywhere winters get cold, condensing usually wins on both running cost and total project cost. The efficiency gap is real but modest in dollars. The venting gap is larger. A condensing unit vents through PVC, CPVC or polypropylene, typically a few hundred dollars for a simple sidewall run, while most indoor non-condensing units need Category III stainless steel that commonly runs well into four figures on a longer route. Price both complete jobs and the expensive heater is regularly the cheaper project.

Non-condensing still makes sense in specific places. An outdoor model mounted on an exterior wall in Florida, Texas, Arizona or Southern California needs no vent pipe at all, because the exhaust leaves straight out of the cabinet. There is no stainless to buy and no condensate to drain, and the unit price saving is kept. It also makes sense when you are swapping like for like and a correctly sized stainless vent is already in the wall, or when you will sell the house within a few years and lifetime gas savings will never reach you.

If you already know you want tankless and are choosing a model, our best tankless water heaters guide ranks condensing and non-condensing units side by side, and the reasons behind those picks come down to the same three questions below: how much gas each one wastes, what pipe it needs, and where its condensate goes.

2. How a Condensing Tankless Heater Gets Its Extra Heat

Burning natural gas or propane produces carbon dioxide and a surprising amount of water vapor. That vapor carries heat with it. As long as it stays a gas, the heat leaves the house through the vent. If the exhaust is cooled below its dew point, which for natural gas combustion is roughly 130 degrees, the vapor condenses into liquid and gives up its latent heat. A condensing water heater is designed to make that happen on purpose, inside a heat exchanger, where the released heat goes into your water instead of the sky.

Physically, that means two heat exchangers. The primary exchanger sits above the burner and does most of the work, exactly as in a non-condensing heater. The secondary exchanger sits in the exhaust stream and is fed with incoming cold water first. Because tap water arrives far below 130 degrees in every climate in North America, the secondary exchanger is cold enough to condense the exhaust nearly whenever the burner fires. The water is pre-warmed there and then finished in the primary.

The condensate that forms is mildly acidic, because combustion gases dissolve into it, so the secondary exchanger is built from stainless steel rather than the copper used in many primary exchangers. That acid is also why the condensate cannot simply be dumped anywhere, and why manufacturers exclude condensate damage from warranty coverage when the drain is installed badly. Rinnai's residential warranty, for example, lists condensate damage and improper venting among the causes it does not cover.

A non-condensing heater deliberately keeps its exhaust hot enough that vapor never condenses inside the unit or the vent. That is simpler, with one fewer heat exchanger, no drain and no neutralizer, but every unit of heat that leaves as hot exhaust is heat you paid for and did not use. It also means that if the vent cools too much, for example on a long run through a cold attic, condensation can form where it was never meant to, which is one reason those vents are specified so strictly.

3. Efficiency in Real Numbers: UEF 0.81 vs 0.93 to 0.98

Water heaters are rated by Uniform Energy Factor, a federal test number that approximates how much of the fuel energy ends up as delivered hot water over a typical day of use. Higher is better. Current non-condensing gas tankless units cluster around 0.80 to 0.82. Rinnai rates its non-condensing V65iN at about 0.81, and Navien rates its non-condensing NPN-U series at up to 0.81. Condensing units sit in a much higher band: Navien lists 0.95 for the NPE-240A2 and Rinnai lists 0.98 for the RX199iN.

ENERGY STAR raised the bar for gas tankless heaters to a UEF of 0.95 or higher for units made from April 18, 2023, which in practice means only condensing models qualify. That matters mostly for utility rebates, which are commonly tied to ENERGY STAR certification. The federal 25C tax credit for these heaters ended for equipment placed in service after December 31, 2025, so do not count on it, but check your gas utility, which may still pay a rebate on a certified unit.

Now the honest part: the dollar gap is smaller than the percentage gap suggests. Navien's own label estimate for the NPE-240A2 is about 179 therms a year, roughly $195 at the national average price used for the label. A 0.81 heater doing the same job would burn something near 210 therms. That difference of about 30 therms is worth roughly $35 to $50 a year at typical prices, more in a big household or an expensive gas market, less in a small one. Over 20 years it adds up to a meaningful sum, but it will not pay for an expensive vent on its own.

That is why efficiency should be the tiebreaker, not the decision. If the installed prices come out close, condensing wins on running cost every year after. If a non-condensing unit is far cheaper installed because it can go outdoors with no vent, the gas savings of the condensing model may take a decade or more to catch up. For the fuller picture of what tankless saves against a storage tank, our tankless water heater cost breakdown covers operating cost alongside installation.

4. Venting: PVC vs Category III Stainless Steel

Vent material is the line item that decides most of these comparisons. Gas appliances are grouped into venting categories by exhaust temperature and pressure. Tankless water heaters are fan-driven, so they push exhaust under positive pressure, and that rules out the old Type B metal chimney vent used by many atmospheric tank heaters. A non-condensing tankless unit is typically a Category III appliance: positive pressure with hot exhaust that must not condense in the vent. A condensing unit is Category IV: positive pressure with cool exhaust that will condense, so the pipe has to tolerate water and mild acid.

Hot, positive-pressure exhaust needs sealed stainless steel vent, usually from a listed system specified in the manual, and sometimes only available through the manufacturer's approved suppliers. It is expensive per foot, the fittings are expensive, and every elbow adds up. Our cost research puts a Category III stainless vent at roughly $600 to $1,500 installed, and more for a long or vertical route. A condensing unit's cool exhaust goes through plastic: a simple sidewall PVC or polypropylene run is commonly $200 to $600 installed.

There are exceptions worth knowing. Navien's non-condensing NPN-U series is approved for PVC Schedule 40, CPVC Schedule 80, approved polypropylene and stainless steel, which narrows the venting gap considerably for that line. Some Rinnai non-condensing indoor models use a proprietary concentric vent system. And outdoor non-condensing models, such as Rinnai's V65eN or Rheem's outdoor mid-efficiency units, need no vent at all. Read the venting section of the installation manual for the exact model being quoted, because the rule of thumb does not hold for every unit.

Two more venting traps catch people replacing a gas tank. First, neither type of tankless heater can use the existing chimney or B-vent as it is, so plan on a new vent route. Second, if the old water heater shared a masonry chimney with a furnace, removing it can leave the furnace alone in a flue now too large for it, and codes often require a liner, which can add $1,000 or more to the job. Ask the installer about that on the first visit, not the last.

Vent length limits also differ. Condensing units typically allow long runs in plastic, which gives the installer freedom to choose a cheap, short sidewall exit or a longer route to a better location. Navien, for example, lists maximum runs of 75 feet in 2-inch pipe and 150 feet in 3-inch for the NPE-240A2. Whatever you buy, the termination needs the clearances from windows, doors, grade and other vents set out in the manual and your local code.

5. The Condensate Drain and Neutralizer

The price of condensing efficiency is a small, steady stream of acidic water. A residential unit makes a modest amount, typically well under a gallon an hour even when firing hard, but it makes it whenever the burner runs, and it has to go somewhere. The unit has a condensate outlet, often a 1/2-inch connection, which must be piped to a floor drain, a laundry standpipe, a sink tailpiece with a proper air gap, or a condensate pump if no drain is close by.

Because the liquid is acidic, many jurisdictions and most manufacturers call for a neutralizer between the heater and the drain. It is a simple cartridge or tube filled with limestone or marble chips, which react with the acid and raise the pH toward neutral before the water reaches cast iron drains, septic systems or concrete. A neutralizer kit generally costs $50 to $150, and the media gets used up, so expect to refill or replace it every few years for roughly $20 to $60.

A blocked or frozen condensate line is the classic way a condensing heater fails on the coldest night of the year. When condensate cannot drain, it backs up into the unit, and a float switch or sensor shuts the burner down. Rinnai, for instance, uses a dedicated diagnostic code for condensate backing up into the trap. Keep the line short, pitched downhill, out of unheated spaces where possible, and insulated or heat-traced where it is not. If a condensate pump is involved, its failure becomes your water heater's failure, so check it whenever you service the heater.

If there is no practical drain near the planned location, price the condensate pump and its outlet into the condensing quote. It is a small cost, usually, but it is one more part to maintain. This is also the single most common reason a non-condensing unit gets chosen for a garage or utility closet: no drain, no neutralizer, nothing to freeze. If you are diagnosing a unit that keeps shutting down, our guide to a tankless water heater not heating covers the condensate and venting checks in order.

6. Unit Cost vs Installed Cost

On the shelf, non-condensing gas units generally run about $700 to $1,500 and condensing units about $1,200 to $3,000, depending on capacity and features such as a built-in recirculation pump. That gap, often $500 to $800 for comparable flow rates, is what the cheaper heater has going for it. Everything else in the installed price either erases that saving or keeps it.

Work through a typical indoor replacement and the pattern is plain. Say the non-condensing unit is $700 cheaper. Its stainless vent costs perhaps $900 more than a PVC run, and the condensing unit needs a $100 neutralizer and a drain connection. The non-condensing project ends up a little more expensive on day one and then costs more to run every year. That is the result we see far more often than not, and it is why our cost guide warns that a non-condensing heater can be cheaper on the shelf and dearer installed.

Gas supply is a cost both share. Both types fire at up to roughly 199,000 BTU an hour in whole-house sizes, which is far more than a typical tank heater, and many homes need a 3/4-inch line run to the heater. Some condensing units, including Navien's NPE-A2 series, are rated to run on an existing 1/2-inch line for a limited length, up to 24 feet in Navien's case, which can save a retrofit, but that depends on the rest of the gas system. Our tankless sizing guide explains why the BTU rating, not the box GPM, drives this.

Long-term cost tips the same way. The condensing unit saves perhaps $35 to $50 a year in gas at average prices, and it is ENERGY STAR eligible for rebates. The non-condensing unit has one fewer heat exchanger to fail and no neutralizer to maintain, which is a small advantage in parts but not one that changes the arithmetic for most homes.

7. Which Climates and Homes Suit Each Type

Cold-climate homes with an indoor install, basement, utility room or closet, should start with condensing. Cold groundwater means the secondary exchanger condenses hard all year, the unit is already indoors so there is no freeze risk to the drain, and the vent has to be built anyway. Size for winter groundwater, which in the northern states can drop into the 40s, and the larger condensing units with 199,000 BTU burners are where you will end up.

Warm-climate homes that can mount the heater outdoors should seriously price non-condensing. In much of the South and Southwest, outdoor installation is normal, groundwater is warm enough that a smaller unit covers the house, and an outdoor non-condensing model with built-in freeze protection needs no vent and no drain. The installed price can be the lowest of any gas tankless option. Condensing outdoor units exist too, such as Rinnai's RX series with its outdoor vent accessory, but in a place that can freeze even occasionally their condensate line needs protecting.

Homes with an existing, correctly sized stainless vent from a previous non-condensing tankless are the other good case for non-condensing. Replacing like for like on the same vent, if the manual for the new unit approves it, avoids most of the condensing advantage. Conversely, if the old vent is a brand-specific system that the new heater does not list, you are pricing a new vent either way, and condensing returns to the front.

Households planning to stay a long time should lean condensing, because the efficiency gap compounds and Rinnai and Navien back their condensing heat exchangers for 15 years in residential use; Rinnai's non-condensing V65iN carries a 10-year heat exchanger warranty, 12 with an isolation valve kit. How long either type really lasts depends far more on water hardness and descaling than on the design, which our guide to how long tankless water heaters last covers in detail.

Finally, if you are still deciding whether gas tankless is right at all, compare honestly against the alternatives. A high-efficiency tank may be the better fit for a short ownership horizon, and in a mild climate with good electrical service a heat pump unit can beat both on running cost. Our best gas water heaters and best heat pump water heaters guides cover those options.