Ultra low NOx tankless water heaters are a smart choice if you want efficient, on-demand hot water with lower emissions and less wasted energy. The best models balance flow rate, fuel type, and installation style for your home.
In this roundup, we focus on 10 options that cover a wide range of household sizes, venting needs, and performance levels so you can find the right fit faster.
Best 10 Ultra Low Nox Tankless Water Heaters Picks for 2026
Ultra-Efficient Whole-Home Outdoor Pick
Rheem 9.5 GPM Outdoor NG Tankless
- 9.5 GPM flow rate
- Outdoor mount saves space
- EcoNet smart monitoring
Best For: Outdoor whole-house natural gas installs
Space-Saving Family Choice
Rinnai V53DeN Natural Gas Tankless
- 5.3 GPM supports up to 4 appliances
- Compact outdoor footprint
- 10-year heat exchanger warranty
Best For: Smaller to mid-size households needing compact outdoor hot water
Scale-Protection Indoor Performer
A. O. Smith ATI-240HX3-N 160K NG
- Up to .94 UEF efficiency
- X3 scale prevention technology
- Indoor wall-mount design
Best For: Indoor 2-3 bath homes needing scale protection
High-Capacity Recirculation Pick
Rheem Prestige 11 GPM Indoor NG
- 11.0 GPM high-capacity output
- Built-in recirculation
- Leak detection and auto-learn programming
Best For: Larger indoor homes wanting recirculation and high flow
Outdoor Performance Pick
FOGATTI 120,000 BTU Propane Tankless
- 5.1 GPM and 120,000 BTU output
- Precision control holds temperature within ±1°F
- Compact outdoor unit with CSA certification
Best For: Medium households needing outdoor propane hot water
Smart Efficiency Choice
- 10.1 GPM and 180,000 BTUs for whole-home demand
- Condensing ultra-low NOx, super high efficiency
- EcoNet Wi-Fi for remote control and alerts
Best For: Large homes wanting indoor smart gas hot water
High-Flow Legacy Pick
- Up to 9.8 GPM for high-demand households
- Commercial-grade copper heat exchanger
- Indoor/outdoor install flexibility
Best For: High-flow natural gas setups needing flexible installation
High-Efficiency Whole-House Pick
- 6.8 GPM for whole-house use
- 0.93 UEF condensing efficiency
- Ultra-low NOx, indoor direct vent
Best For: Large propane homes needing steady hot water
Most Flexible Install
- Runs on natural gas or propane
- Indoor/outdoor installation flexibility
- 11.1 GPM with UEF .98
Best For: Homes needing flexible fuel and install options
Smart High-Output Upgrade
- 11.2 GPM for whole-house demand
- EcoNet Wi-Fi remote control
- Ultra-low NOx with condensing design
Best For: Propane homes wanting smart controls and high flow
Ultra-Efficient Whole-Home Outdoor Pick – Rheem 9.5 GPM Outdoor NG Tankless
If you are comparing ultra low nox tankless water heaters for a full household setup, this Rheem outdoor model is built for high-demand natural gas use with a 9.5 GPM flow rate and 199,900 BTU output. Its 120-volt electronic ignition, copper and stainless steel construction, and EcoNet monitoring add convenience and durability for homeowners who want a capable whole-house unit.
Best For: Outdoor whole-house natural gas installs that need strong flow and smart monitoring.
Pros:
- Up to 9.5 GPM for whole-home hot water needs
- Outdoor mount design saves indoor space
- EcoNet smart monitoring system adds protection and energy-awareness
- Copper and stainless steel construction for rugged durability
Cons:
- Outdoor installation only
- Requires 120-volt power for electronic ignition
- No built-in recirculation feature listed
For buyers focused on ultra low nox tankless water heaters with solid output and a space-saving exterior install, this Rheem model stands out as a practical whole-house option. It balances high flow, durable materials, and smart monitoring without adding extra indoor footprint.
Space-Saving Family Choice – Rinnai V53DeN Natural Gas Tankless
Among ultra low nox tankless water heaters, the Rinnai V53DeN is a straightforward outdoor natural gas option for households that want dependable on-demand hot water without taking up much room. It delivers 5.3 GPM, is rated for up to four appliances, and uses water only when needed to help keep operating costs down.
Best For: Smaller to mid-size households wanting a compact outdoor unit with dependable hot water delivery.
Pros:
- 5.3 GPM flow rate serves up to 4 appliances
- Compact outdoor design fits tight spaces
- Energy-efficient on-demand heating helps reduce waste
- Long heat exchanger warranty for added peace of mind
Cons:
- Lower flow than larger whole-house units
- Professional installation is recommended
- Outdoor installation limits placement options
This Rinnai model is a good fit if you want a practical, space-saving tankless setup with a smaller footprint. It is not the highest-capacity option here, but it offers an efficient and easy-to-place solution for many homes.
Scale-Protection Indoor Performer – A. O. Smith ATI-240HX3-N 160K NG
If you are shopping ultra low nox tankless water heaters for an indoor natural gas installation, this A. O. Smith ProLine XE model is designed around efficiency and scale control. It delivers up to .94 UEF, uses X3 scale prevention technology, and combines copper and stainless steel heat exchangers to support stable, long-term performance.
Best For: Indoor 2-3 bath homes that want high efficiency and scale protection.
Pros:
- Up to .94 UEF for efficient operation
- X3 scale prevention technology helps maintain performance
- Primary copper heat exchanger supports quick heat transfer
- Wall-mount design saves floor space
Cons:
- Indoor installation only
- 4.7 GPM flow rate is below larger whole-house models
- Best suited to 2-3 bath homes rather than large-demand setups
This A. O. Smith unit is especially appealing if scale protection matters as much as efficiency. It is built for a controlled indoor install, and its warranty coverage adds confidence for buyers who want a long-term tankless solution.
High-Capacity Recirculation Pick – Rheem Prestige 11 GPM Indoor NG
For buyers comparing ultra low nox tankless water heaters with premium convenience features, this Rheem Prestige indoor model stands out with built-in recirculation and an 11.0 GPM flow rate. It is designed to reduce wait time for hot water, improve space efficiency during replacement, and add leak detection plus auto-learn programming for everyday use.
Best For: Larger homes that want recirculation and strong hot water output in an indoor unit.
Pros:
- 11.0 GPM flow rate supports higher demand
- Built-in recirculation helps reduce hot-water wait time
- Water and gas leak detection adds protection
- Auto-learn programming adapts to household usage
Cons:
- Indoor floor-mount format may not suit every layout
- Higher-capacity unit may be more than smaller homes need
- No outdoor installation option listed
This Rheem model is a strong match if you want fast hot water delivery and convenience features in one package. It is one of the more feature-rich options here, making it a practical upgrade for households that value comfort and efficiency.
Outdoor Performance Pick – FOGATTI 120,000 BTU Propane Tankless
If you want ultra low nox tankless water heaters for an outdoor propane setup, this FOGATTI model is built around efficient whole-home delivery without taking up much space. It delivers up to 5.1 GPM and supports 2-4 outlets, with precision temperature control designed to keep hot water consistent for everyday use.
Best For: Medium households wanting an outdoor propane tankless unit with strong output and compact installation.
Pros:
- 120,000 BTU output and up to 5.1 GPM for whole-house demand
- Water servo technology and sensors hold temperature within ±1°F
- Compact outdoor design with remote control panel and 120V AC power
- CSA certified with multiple safety features and 2-year full unit coverage
Cons:
- Outdoor install only, so it is not a fit for indoor placement
- Best use is below 2,000 feet of altitude
- Requires draining in very cold conditions or long periods of non-use
Overall, this is a practical choice if you want propane tankless performance, space savings, and stable temperature control in an outdoor installation. It emphasizes value and efficiency more than premium smart-home features.
Smart Efficiency Choice – Rheem IKONIC 10.1 GPM Indoor
For shoppers comparing ultra low nox tankless water heaters, this Rheem IKONIC stands out for high-capacity indoor performance and connected controls. The condensing design delivers up to 10.1 GPM and 180,000 BTUs, making it suitable for whole-house hot water while helping you manage settings and usage from your phone.
Best For: Large homes that want an indoor high-efficiency gas unit with Wi-Fi control.
Pros:
- 10.1 GPM flow rate supports multiple simultaneous hot-water uses
- Condensing ultra-low NOx design with super high efficiency
- EcoNet Wi-Fi lets you adjust settings and monitor usage remotely
- Compact footprint with quick hang bracket and outdoor conversion capability
Cons:
- Indoor model may require venting and proper installation planning
- Outdoor use needs an outdoor conversion kit
- Higher-capacity unit may be more than needed for smaller households
This is a strong fit if you want a modern tankless heater that combines high hot-water output with app-based control. Its feature set is especially useful for homes that value convenience and efficiency together.
High-Flow Legacy Pick – Noritz NR98SVNG 9.8 GPM
In a roundup of ultra low nox tankless water heaters, the Noritz NR98SVNG is worth considering if you want a proven natural gas model with strong flow and flexible installation. It offers up to 9.8 GPM, a commercial-grade copper heat exchanger, and indoor/outdoor placement support for setups that need versatility.
Best For: Homes needing a high-flow natural gas unit with flexible install options.
Pros:
- Up to 9.8 GPM flow rate for demanding hot-water use
- Commercial-grade copper heat exchanger with thicker piping
- Indoor/outdoor installation flexibility
- Remote thermostat and 6-foot power cord included
Cons:
- Does not list Wi-Fi or app-based controls
- Parts warranty is shorter than some newer competitors
- Natural gas only, so it will not suit propane setups
This model makes sense if you value high flow, durable construction, and installation flexibility over smart features. It is a straightforward option for buyers who want a dependable gas tankless heater with a strong track record.
High-Efficiency Whole-House Pick – Rheem RTGH 6.8 GPM LP Indoor
If you’re comparing ultra low nox tankless water heaters for a propane-powered home, this Rheem RTGH model is built for steady whole-house demand. It delivers up to 6.8 GPM at a 35°F rise, uses condensing technology with a 0.93 UEF, and is designed for indoor direct vent installation.
Best For: Larger households that want continuous hot water for multiple fixtures with ultra-low NOx compliance.
Pros:
- 6.8 GPM output supports up to 2-3 simultaneous showers
- 0.93 UEF and condensing design help reduce gas use
- Ultra-low NOx emissions and CSA-certified safety
- Built-in freeze protection and multi-point safety system
Cons:
- Indoor direct vent setup limits placement flexibility
- Professional installation is recommended
- Best suited to liquid propane homes only
Overall, this is a practical choice if you want efficient, continuous hot water with strong output and a cleaner-burning burner profile. Its compact wall-mounted design and long heat exchanger warranty make it appealing for busy homes with high daily hot water use.
Most Flexible Install – Rinnai RX199iN Smart Sense
Among ultra low nox tankless water heaters, this Rinnai stands out for flexibility because it can run on natural gas or propane and be installed indoors or outdoors. It offers up to 11.1 GPM, a 199k BTU output, and a condensing design with UEF .98 for high-demand homes.
Best For: Homeowners who want one tankless unit that can adapt to gas type and installation location.
Pros:
- Supports natural gas or propane with Smart Sense control
- Indoor or outdoor installation options with Versa Vent
- High 11.1 GPM flow rate for up to 7 fixtures at once
- UEF .98 and Quick Flush feature for efficient operation
Cons:
- Outdoor installation requires the RX-OVC vent cap
- Wi-Fi control needs a separate Controlr module
- Professional installation is recommended
This is a strong fit for shoppers who value installation flexibility and high output over a simple one-setup solution. Its compact footprint, efficient condensing design, and broad fuel compatibility make it especially useful for larger or changing home setups.
Smart High-Output Upgrade – Rheem IKONIC 11.2 GPM LP
If your shortlist for ultra low nox tankless water heaters includes smart controls and high flow, this Rheem IKONIC is designed for whole-house propane use. It delivers up to 11.2 GPM, uses a 199,900 BTU condensing system, and includes built-in EcoNet Wi-Fi for remote monitoring and temperature control.
Best For: Propane households that want high hot-water capacity plus app-based control.
Pros:
- 11.2 GPM output supports multiple simultaneous uses
- EcoNet Wi-Fi lets you track usage and adjust settings remotely
- Ultra-low NOx burner helps reduce greenhouse gas NOx emissions
- Compact wall-mounted design with outdoor conversion capability
Cons:
- Liquid propane only
- Outdoor use requires an outdoor conversion kit
- Professional installation may be necessary for best results
This model makes sense if you want a cleaner-burning propane unit with strong output and modern connectivity. Its compact size and installation flexibility are useful, but the real appeal is the combination of capacity, smart features, and high-efficiency condensing performance.
How We Picked the Best Ultra Low Nox Tankless Water Heaters
We selected Ultra Low Nox Tankless Water Heaters based on practical buyer priorities: hot water output, fuel compatibility, efficiency features, installation flexibility, and whole-home suitability. We also considered whether each unit is better suited to indoor or outdoor use, single- or multi-bath homes, and natural gas versus propane setups.
Quick Comparison
Start by matching the heater’s maximum flow rate and BTU output to your home’s peak demand. Larger homes or homes with simultaneous showers, laundry, and dishwashing usually need higher-capacity models. If your utility setup is fixed, the fuel type matters just as much as the performance rating. Indoor models may offer higher efficiency features, while outdoor models can simplify venting and free up interior space.
Key Buying Factors for Ultra Low Nox Tankless Water Heaters
Flow Rate and Demand
Look at gallons per minute, not just BTUs. A higher GPM helps maintain hot water when multiple fixtures run at once. For smaller homes, 5 to 7 GPM may be enough. Larger households often benefit from 9 GPM or more.
Fuel Type and Site Conditions
Choose natural gas or propane based on what is already available at your property. Converting fuel types is rarely worth the added cost unless the model is designed for it. Also check whether the unit is indoor, outdoor, or dual-location compatible before you buy.
Venting, Recirculation, and Installation
Some Ultra Low Nox Tankless Water Heaters require more involved venting, while outdoor units can reduce that complexity. Built-in recirculation can improve hot water convenience, especially in larger homes, but may add to install cost. Make sure the heater’s electrical requirements and gas line sizing are compatible with your existing setup.
Efficiency and Temperature Stability
Condensing models generally deliver stronger efficiency and may be a better long-term value if you want lower operating costs. Temperature control features also matter, especially if your household wants consistent showers without swings when another faucet opens.
Who Should Buy Which Ultra Low Nox Tankless Water Heaters?
Choose a higher-capacity condensing model if you have a larger household, multiple bathrooms, or want the best long-term efficiency. Pick a mid-range unit if you need reliable on-demand hot water for a typical family home. Choose an outdoor or simpler non-recirculating model if you want easier installation and less venting complexity. If you are shopping for a smaller home, an apartment, or a lighter hot-water load, a lower-GPM model can be the most cost-effective option.
For most buyers, the best Ultra Low Nox Tankless Water Heaters are the ones that match real household demand first, then optimize efficiency, convenience, and installation fit.
