Note: This article contains affiliate links. If you click on these links and make a purchase, we may earn a small commission at no extra cost to you. This helps support our research and content. Read our full affiliate disclosure here.
If you are upgrading to a condensing tankless water heater, your main goals are higher efficiency and reliable on‑demand hot water while minimizing fuel and maintenance costs. The right unit can reduce fuel consumption and recover heat from exhaust gases, but installation and venting choices have a big impact on real-world performance. This guide shortlists the top condensing tankless water heaters available and explains the practical tradeoffs buyers face. We evaluated units for efficiency, venting requirements, serviceability, and suitability for typical household loads. Read on for the factors that matter, expert selection criteria, and clear guidance to help you pick the best condensing tankless water heater for your home.
Top Picks at a Glance
| Best Overall The best all-around choice for most situations | Rheem RTGH-90DVLN-3 Tankless Water Heater |  |
| Best Quality Built to a higher standard if you want something more solid | A. O. Smith ATI-540P-N Tankless Water Heater |  |
| Best Value The sweet spot between price and what you get | Ranein Condensing Tankless RH12KG-C-NG |  |
| Best Budget The cheapest option that still gets the job done reliably | ThermoMate CZ480NG Tankless Water Heater |  |
| Top Rated A top-rated pick with lots of positive feedback | Rinnai RX180iN 10 GPM Tankless Water Heater |  |
| Most Popular A crowd favorite that many people choose with confidence | Noritz NRC66DVNG Tankless Water Heater |  |
| Best Seller A high-volume bestseller chosen again and again | Rinnai RX199iN Tankless Water Heater | |
Rheem RTGH-90DVLN-3 Tankless Water Heater
| Power Source | Natural Gas |
|---|---|
| Flow Rate | 9 gpm |
| Maximum Temperature | 140 °F |
| Heat Output | 180 british thermal units |
| Wattage | 132 watts |
The Rheem RTGH-90DVLN-3 is a high-efficiency indoor condensing natural gas tankless water heater designed to deliver continuous hot water for whole-house use. It produces up to 9.0 gallons per minute at a 35°F rise and 180,000 BTU input, which is typically enough to support three to four simultaneous showers in a family home. The unit uses condensing technology with a 0.93 UEF to reduce fuel consumption and lower operating costs, and it includes practical features such as Hot Start Programming to avoid cold-water bursts, an LED temperature control with ±1°F precision, built-in freeze protection, and overheat protection for the heat exchanger. Its wall-mounted, compact design frees floor space and allows installation in tighter mechanical closets with minimal side clearance. Expect typical professional-install requirements for direct venting and gas hookups; the condensing design also means you will need to plan for condensate routing. For buyers seeking a balance of strong flow capacity, high efficiency, and family-oriented reliability, this model is a solid choice, though installation complexity and the need for proper venting and condensate handling are important considerations.
Best for: Homeowners who want a balanced whole-house tankless that prioritizes fuel efficiency and steady hot-water capacity for medium to large families.
Less Ideal for: People who need a plug-and-play or DIY water heater replacement or those who prefer the simplest, lowest-cost installation option.
- Super high efficiency condensing design with 0.93 UEF reduces gas use compared with standard tanks
- 9.0 GPM flow supports multiple simultaneous showers for large families
- Hot Start Programming and precise LED temperature control reduce cold-water bursts and improve comfort
- Compact wall-mount form factor saves floor space and allows tighter installation clearances
- Built-in safety features including overheat protection and freeze protection
- Requires professional installation with direct venting and condensate management
- Condensing operation adds complexity for drain routing and potential maintenance
- Higher initial equipment and installation complexity compared with simple tank heaters
Verdict: Buy this model if you want a compact, high-efficiency condensing tankless that reliably supplies hot water for medium to large households while cutting operating costs.
A. O. Smith ATI-540P-N Tankless Water Heater
| Power Source | Natural Gas |
|---|---|
| Flow Rate | 10 gpm |
| Heat Output | 199000 british thermal units |
| Efficiency | 0.93 UEF |
| Mount Type | Wall |
This condensing tankless unit is built for steady, high-demand hot water delivery in mid-to-large homes. It uses natural gas with a 199,000 BTU heat output and a 0.93 UEF efficiency rating to deliver up to 10 gallons per minute, which the manufacturer says can support three or more bathrooms depending on climate. The ATI-540P-N includes an integrated return pump for near-instant hot water when paired with an appropriate recirc setup and offers multiple safety controls such as exhaust and water temperature monitoring, an overheat cut-off fuse, and an air-fuel ratio sensor. Construction notes and specs favor durability, with alloy steel and a commercial-grade heat exchanger covered by a long 15-year limited warranty. Expect professional installation and gas-line sizing; the unit is wall-mounted and compact for a high-output condensing heater but still requires proper venting and possible bypass plumbing for recirculation. The result is a reliable, energy-efficient option when continuous hot water and performance are the priorities.
Best for: Buyers who prioritize steady high-output hot water and long-term efficiency for homes with multiple bathrooms and who are prepared to arrange professional installation and plumbing adjustments.
Less Ideal for: If you need a simple drop-in replacement or have a very small household with minimal hot-water demand, a lower-capacity or non-condensing unit may be a better fit.
- High continuous flow up to 10 GPM suitable for multiple bathrooms
- High heat output at 199,000 BTU for strong temperature rise under load
- Condensing design with 0.93 UEF for better energy efficiency
- Integrated recirculation pump for faster hot-water delivery
- Multiple safety controls including overheat cutoff and AFR sensor
- Requires professional installation, proper venting, and possible plumbing changes for recirculation
- Recirculation may need a separate bypass valve which is not included
Verdict: Choose the ATI-540P-N if you need a high-capacity, energy-efficient condensing tankless heater that delivers reliable continuous hot water for multi-bath homes.
Ranein Condensing Tankless RH12KG-C-NG
| Flow Rate | 5.1 gpm |
|---|---|
| Heat Output | 120000 british thermal units |
| Maximum Temperature | 149 °F |
| Maximum Pressure | 150 psi |
This Ranein condensing tankless water heater is built to provide continuous hot water for about two to three bathrooms by producing 5.1 gallons per minute from a 120,000 BTU modulating burner. It uses condensing technology to recover exhaust heat and improve efficiency, and the LED touch control lets you set temperatures from 95 to 149°F with four preset modes for Eco, kitchen, bathtub, and an auto seasonal setting. The unit includes an integrated anti-freeze heater, multiple safety cutoffs, a stainless steel Category III vent, condensate drain, and a simple wall-mount footprint that replaces a bulky tank. In practice this means steadier shower temperatures when two fixtures run at once, lower gas consumption than many non-condensing alternatives, and clear fault and flow readouts for easier troubleshooting. Important practical limits are that it needs professional installation with proper venting and condensate handling, requires 120V power, and is not recommended above 2,000 ft elevation which affects some mountain installations.
Best for: Buyers who want a space-saving, fuel-efficient on-demand water heater to cut gas use and supply reliable hot water for 2–3 bathrooms while prioritizing measurable efficiency and straightforward controls.
Less Ideal for: Homes at high elevation or DIY installers who cannot accommodate professional venting and condensate plumbing should consider other options.
- Condensing design recovers exhaust heat to improve fuel efficiency versus non-condensing units
- 120,000 BTU modulating burner supplies up to 5.1 GPM for simultaneous use of two showers and a faucet
- Smart LED touch control with four preset modes and precise temperature adjustment
- Integrated anti-freeze heater and multiple safety protections for cold-weather reliability
- Includes vent pipe, condensate drain line, and mounting hardware for a more complete kit
- Requires professional installation and proper Category III venting and condensate management
- Not recommended for installations above 2,000 ft elevation
Verdict: Choose this unit if you want a compact, condensing natural gas tankless heater that lowers fuel use while reliably supplying hot water for two to three bathrooms.
ThermoMate CZ480NG Tankless Water Heater
| Power Source | Natural Gas |
|---|---|
| Flow Rate | 5.28 gpm |
| Maximum Temperature | 140 °F |
| Heat Output | 114000 BTU |
| Maximum Pressure | 150 psi |
The ThermoMate CZ480NG is a wall-mounted condensing tankless natural gas heater designed to supply continuous hot water for residential use. It heats water on demand using a dual heat-exchanger system (stainless steel condensing exchanger plus oxygen-free copper exchanger) to reclaim waste heat, which produces a high 0.86 UEF efficiency and claims up to 15% energy savings over non-condensing units. The unit supports a peak flow of 5.28 GPM and 114,000 BTU output, enough to run two showers concurrently under typical conditions, and it keeps outlet temperature steady within about ±1°F thanks to a fast-flow sensor and rapid control. Built-in protections cover freeze, flame failure, dry-fire, overheating and more, and a 3.5-inch LED touchscreen gives live readouts and basic diagnostics. As a condensing model it produces acidic condensate that must be drained and generally requires a neutralizer kit for safe disposal. Installation needs standard NPT water and gas connections, a 3-inch vent, and 120V power for controls, so expect some plumbing and venting work during setup.
Best for: Buyers who prioritize lower operating costs on a budget and want a compact, energy-efficient condensing tankless heater for everyday household hot water needs.
Less Ideal for: Homes that cannot accommodate a 3-inch vent, lack a suitable condensate drain or where a plug-and-play electric solution is preferred over a gas-fired unit.
- High 0.86 UEF condensing efficiency that reduces energy use compared with non-condensing units
- Dual heat exchangers (stainless steel + oxygen-free copper) to improve heat recovery
- 5.28 GPM and 114,000 BTU output suitable for two simultaneous showers in many homes
- Comprehensive safety protections including freeze and flame-failure safeguards
- 3.5-inch LED touchscreen with real-time diagnostics and precise temperature control
- Produces mildly acidic condensate that requires proper neutralizing and drainage
- Indoor installation requires appropriate venting and professional plumbing/gas hookup
Verdict: If you want a compact, budget-friendly condensing tankless heater that cuts fuel use while delivering enough flow for typical two-shower households, the ThermoMate CZ480NG is a strong choice.
Rinnai RX180iN 10 GPM Tankless Water Heater
| Power Source | Dual-Fuel (Natural Gas or Propane) |
|---|---|
| Flow Rate | 10 gpm |
| Maximum Temperature | 140 °F |
| Heat Output | 180000 british thermal units |
| Efficiency | 0.98 UEF |
The Rinnai RX180iN is a compact condensing tankless water heater designed to supply continuous hot water for a whole house. It converts between natural gas and propane and, with the Versa Vent option, can be installed indoors or outdoors, which adds flexibility during retrofit or replacement jobs. The unit emphasizes efficiency with a UEF of 0.98 and a 180,000 BTU output that supports multiple fixtures simultaneously. Practical features include an easy-to-use interface, Quick Flush for simpler maintenance, and optional Wi-Fi connectivity via a sold-separately module for remote diagnostics. Because it is condensing and compact, it reclaims space compared with tank systems and reduces standby losses. Expect professional installation and periodic flushing for long life, and note that some advanced connectivity requires an extra module. For buyers prioritizing dependable, long-lasting hot water and strong efficiency, this model balances performance and serviceability well.
Best for: Buyers who want a reliable, energy-efficient whole-house hot water source that minimizes space and operating cost and who value proven, serviceable equipment.
Less Ideal for: Renters or buyers seeking a plug-and-play, DIY water heater solution without professional installation or those who want built-in Wi‑Fi without add-on modules.
- Very high energy efficiency (UEF 0.98) lowers operating costs
- Convertible to natural gas or propane for installation flexibility
- Supports up to about 10 GPM to run multiple fixtures at once
- Compact, wall-mount design saves space compared with tank systems
- Quick Flush feature and simple interface simplify maintenance and servicing
- Wi‑Fi control requires purchasing the Control-R module separately
- Professional installation is recommended, adding installation cost
Verdict: Choose the Rinnai RX180iN if you want a compact, ultra-efficient condensing tankless heater that reliably supplies whole-house hot water with flexible fuel and installation options.
Noritz NRC66DVNG Tankless Water Heater
| Power Source | Natural Gas |
|---|---|
| Flow Rate | 6.5 gpm |
| Maximum Temperature | 140 °F |
| Mount Type | Wall |
| Capacity | 6.6 gallons |
The Noritz NRC66DVNG is a high-efficiency indoor condensing tankless water heater designed to supply continuous hot water at flow rates up to about 6.5 gallons per minute. It uses an advanced heat exchanger and a built-in controller to prioritize efficiency and steady temperature, and it accepts 3" Schedule 40/80 PVC, PP, or CPVC venting which simplifies installation in many homes. The unit is vented and runs on natural gas, ships with a remote thermostat and a 6 foot power cord, and produces low NOx emissions. In practice it suits households that need reliable hot water for one or two simultaneous fixtures such as a shower and a sink; the higher flow capability within its class helps reduce temperature drop under load. Important installation items to note are the required IK-WV-200-1-XX-LF valve kit (sold separately) and possible additional venting accessories for terminations or atmospheric conversion. The heater offers strong manufacturer support with an extended heat exchanger warranty, but buyers should plan professional gas and venting installation to get the listed performance and safety benefits.
Best for: Buyers who prioritize energy-efficient continuous hot water and want a condensing indoor unit with flexible PVC venting and solid manufacturer support.
Less Ideal for: Large households that need very high simultaneous hot water demand or anyone seeking a plug-and-play replacement without professional gas and venting work.
- High-efficiency condensing design for better fuel use
- Delivers up to 6.5 gpm, supporting multiple fixtures simultaneously
- Compatible with 3" Schedule 40/80 PVC, PP, or CPVC venting for flexible installation
- Includes a remote thermostat and a 6' power cord for convenient control
- Required installation valve kit is sold separately which adds to install cost
- Flow rate may still be marginal for larger households requiring multiple high-demand fixtures at once
Verdict: Choose the NRC66DVNG if you want a condensing, low-NOx indoor gas tankless heater that maximizes efficiency and supports moderate simultaneous hot-water use.
Rinnai RX199iN Tankless Water Heater
| Flow Rate | 11 gpm |
|---|---|
| Maximum Temperature | 140 °F |
| Heat Output | 199000 british thermal units |
| Efficiency | UEF 0.98 |
| Power Source | Gas Powered |
The Rinnai RX199iN is a compact condensing tankless water heater designed to supply continuous hot water for larger homes and several simultaneous fixtures. It supports both natural gas and propane via a Smart Sense gas control, and the Versa Vent option lets you install it indoors or outdoors with a simple adaptation. The unit advertises up to 11.1 gallons per minute and a high UEF of 0.98, which translates to strong flow performance and excellent energy efficiency compared with traditional tank heaters. Practical features include an easy interface, a Quick Flush routine to simplify maintenance, and optional Wi-Fi control through a separate module for remote diagnostics. Expect pro installation to be recommended, and while the compact footprint saves space, the system requires correct venting and plumbing work that a qualified technician should handle. For buyers prioritizing reliable continuous hot water and industry-leading efficiency, this model balances power and energy savings, though initial installation complexity and the need for professional setup are important considerations.
Best for: Homeowners who want a reliable, energy-efficient tankless heater that can serve several fixtures simultaneously and prefer a factory-quality product backed by established warranty support.
Less Ideal for: Buyers seeking a plug-and-play DIY replacement or those unwilling to arrange professional venting and gas conversion should consider simpler non-condensing or tank-style options instead.
- Very high energy efficiency (UEF 0.98) lowers operating costs compared with standard tank heaters
- Convertible to natural gas or propane for flexible fuel options
- Supports up to 11.1 GPM, able to run multiple fixtures at once
- Versa Vent allows indoor or outdoor installation with simple adaptation
- Quick Flush and app support simplify maintenance and troubleshooting
- Requires professional installation and correct venting for indoor or outdoor setups
- Wi-Fi control requires an additional module sold separately
Verdict: Choose the RX199iN if you need a compact, pro-installed condensing tankless unit that delivers strong flow and industry-leading efficiency for larger households.
Choosing the Right Condensing Tankless Water Heater: Key Factors to Consider
Efficiency and Condensing Performance
Condensing tankless water heaters recover heat from exhaust gases that would otherwise be lost, which increases thermal efficiency and lowers fuel consumption. Look for documented seasonal efficiency ratings or AFUE-equivalent figures to compare expected energy use across models.
Higher efficiencies reduce operating costs over time but depend on correct installation, adequate return temperatures, and condensate management. Consider whether your household hot-water profile supports the higher initial investment for improved efficiency when calculating payback.
Venting Requirements and Materials
Condensing units exhaust cooler, acidic condensate, so they require corrosion-resistant venting and, often, a dedicated condensate drain. Venting may use stainless steel or specialized plastic vent piping rated for low-temperature exhaust and acidic condensate.
Venting runs, terminations, and clearances affect installation cost and placement options. Verify whether the unit supports direct-vent, power-vent, or atmospheric venting and whether it can be installed horizontally or vertically in your available space.
Hot Water Flow Rate and Temperature Rise
A tankless heater must match your peak simultaneous demand at the expected incoming ground temperature to deliver desired outlet temperatures. Flow capacity is expressed as gallons per minute at a given temperature rise; choose a unit whose published charts align with your household needs.
If you live in a cold climate with low inlet water temperature, you will need a higher-flow-capacity unit or supplemental tanks to maintain multiple fixtures running at once. Always account for peak uses like showers plus appliances when sizing.
Serviceability and Access
Condensing tankless units require periodic service such as cleaning heat exchangers, flushing mineral scale, and inspecting condensate traps. Units designed with easy access panels and replaceable components reduce labor time and long-term service costs.
Consider how easily the unit can be isolated, drained, and flushed. Clear documentation and accessible parts make routine maintenance faster and less expensive, which matters more in hard-water areas.
Condensate Management and Drainage
Condensing boilers and heaters produce acidic condensate that must be neutralized or drained properly to avoid damaging drains or materials. Some installations require a condensate neutralizer or an indirect drain to a suitable waste connection.
Plan the condensate route during installation and confirm local code requirements for disposal. Neglecting condensate management can lead to corrosion issues and code violations.
Installation Constraints and Permitting
Installation complexity affects overall project cost and determines where the unit can be located. Condensing units may need electrical supplies for fans and controls, gas line sizing for high-input units, and appropriate venting runs.
Local building codes may require licensed installation, specific vent material, and condensate disposal methods. Factor in access for future service when choosing mounting location and orientation.
Sizing a Condensing Tankless Heater for Your Home
Calculate your required flow rate by listing likely simultaneous fixtures and their typical flow rates. Use conservative estimates for showers and appliances to avoid undersizing during peak demand.
Next, determine the expected inlet water temperature in winter to calculate the needed temperature rise. Use manufacturer flow-temperature charts to find units that deliver the required gallons per minute at that rise.
- Estimate peak simultaneous flow from showers and appliances
- Measure or estimate winter inlet water temperature
- Match required temperature rise to published flow charts
Venting Options and Best Practices
Because condensing units produce cooler, acidic exhaust, choose vent materials designed for low-temperature condensate, such as stainless steel or special PVC/CPVC rated by the manufacturer. Avoid standard galvanized materials that can corrode.
Keep vent runs as short and straight as feasible. Each elbow and additional length reduces efficiency and can increase condensate accumulation in the duct, so plan the shortest acceptable route and maintain proper slope for condensate drainage.
- Use corrosion-resistant venting materials
- Minimize elbows and long horizontal runs
- Ensure proper slope to condensate drain connections
Condensate Handling and Neutralization
Condensate is typically acidic and may require a neutralizer before being discharged to a sanitary drain. Neutralizer kits use media to raise pH and protect plumbing and municipal systems.
Position the neutralizer and drain to be accessible for routine replacement of media and inspection. Some jurisdictions have specific requirements for condensate disposal; check local codes before finalizing the install.
- Install a neutralizer when required or recommended
- Provide access for periodic media replacement
- Confirm local disposal codes before installation
Maintenance Checklist for Longevity
Plan a regular maintenance schedule that includes descaling or flushing when water hardness is elevated, inspecting condensate traps, and cleaning or replacing filters. A proactive approach prevents loss of efficiency and premature component failure.
Keep service valves, isolation valves, and documentation handy. If you have hard water, consider a water softening or conditioning strategy to reduce scale buildup and extend heat exchanger life.
- Flush system annually or per manufacturer guidance
- Inspect condensate drains and traps regularly
- Address hard-water scaling proactively
Electrical and Gas Supply Considerations
Condensing tankless units often require a dedicated electrical circuit for fans, controls, and pumps. Verify voltage and circuit needs before installation to avoid costly rework.
For gas-fired condensing units, ensure gas piping is sized to supply the maximum input rating plus allowances for other appliances. Proper gas pressure and fuel quality are critical for stable combustion and full-rated output.
- Confirm electrical circuit and control wiring requirements
- Ensure gas piping is correctly sized and pressure-tested
- Allow clearance for servicing near valves and connections
How We Chose the Best Condensing Tankless Water Heater
We selected condensing tankless water heaters based on measurable, buyer-relevant attributes that predict real-world performance and lifecycle cost. Primary evaluation criteria included thermal efficiency and condensing heat recovery, venting and combustion options, maximum flow rate and temperature rise, serviceability and access for routine maintenance, and warranty and parts availability. We prioritized units with proven methods for recovering latent heat from exhaust gases to raise AFUE-equivalent efficiency and reduce fuel usage. Installation complexity and venting material compatibility were treated as a separate filtering dimension because condensing units typically require corrosion-resistant venting and a drain for condensate. We excluded non-condensing tankless models and small point-of-use units that do not use condensing logic. We also filtered out units with undocumented performance data or lacking clear service access. The final list reflects products that balance high seasonal efficiency, practical venting and condensate handling, sufficient hot-water flow for common household demands, and reasonable maintenance needs.
Our full evaluation process is outlined in our review methodology.
FAQ
How often should a condensing tankless water heater be serviced?
Typically it should be inspected and flushed at least once a year, but frequency depends on water hardness and usage. In hard-water areas, more frequent descaling or maintenance is usually necessary.
Can a condensing tankless unit be installed outdoors?
Some models are rated for outdoor installation, but outdoor units still require appropriate venting and protection from freezing. Confirm the unit's outdoor rating and follow manufacturer guidance for freeze protection.
Will a condensing tankless heater save money on my utility bills?
They often reduce fuel consumption compared with non-condensing alternatives because they recover latent heat, but actual savings depend on usage patterns, correct sizing, and proper installation. Payback varies by local fuel costs and usage.
Do condensing units require special drain connections?
Yes. Condensate is mildly acidic and should be routed to a drain that can accept it, often through a neutralizer. Local codes may dictate acceptable disposal methods, so verify requirements before installation.
Are condensing tankless heaters louder than non-condensing models?
They may have similar noise profiles, but condensing units often include fans for exhaust that can add a steady noise. Placement, mounting, and acoustic isolation reduce perceived sound in living spaces.
Final Verdict
When choosing a condensing tankless water heater, prioritize accurate sizing, proper venting and condensate management, and serviceability. High seasonal efficiency matters, but only if the venting and condensate systems are installed and maintained correctly. Balance expected fuel savings against installation complexity and maintenance needs. Focus on units that match your peak flow and temperature requirements, offer clear maintenance access, and fit the venting constraints of your installation to ensure reliable long-term performance.