Direct answer: Ultra Low NOx (ULN) water heaters and burner assemblies reduce nitrogen oxide emissions while delivering reliable hot water. This article highlights five top ULN options ideal for households seeking cleaner combustion, better efficiency, and scalable hot water for different spaces. The best fits vary from compact outdoor units to indoor high-capacity systems, and from replacement kits to whole-unit solutions. The following buyer’s guide and product comparisons help you choose based on installation type, space, and hot-water needs.
| Product | Best For | Notes |
|---|---|---|
| Rheem SP21098 ULN Replacement Kit | Existing Rheem ULN systems | Self-powered burner assembly with ignitor control |
| Takagi T-H3S-OS-N Condensing Tankless | Small homes needing endless hot water | Ultra-low NOx, no pilot light |
| A. O. Smith ATI-240HX3-N ProLine XE | Indoor installations with high efficiency | X3 technology, high efficiency |
| A. O. Smith ATO-140H ProLine XE Outdoor | Outdoor spaces needing compact, space-saving unit | 6.6 GPM; outdoor rated |
| Rheem SP21099 ULN Replacement Kit | New burner assemblies for ULN Rheem heaters | OEM-style kit with burner and hardware |
Rheem Burner Assembly Replacement Kit ULN SP21098
The Rheem ULN replacement kit is designed for existing Ultra Low NOx Rheem water heaters. It includes a burner assembly and all necessary hardware. This kit is best for homeowners who want a straightforward, OEM-style replacement without replacing the entire heater. Best for: homeowners with a compatible Rheem ULN model seeking a self-contained burner upgrade. Caution: ensure compatibility with your exact model and serial before ordering to avoid mismatches.
Takagi T-H3S-OS-N Condensing Tankless Water Heater
This outdoor tankless model provides endless hot water with on-demand usage and no pilot light. It’s compact and space-saving, suitable for small-to-mid homes. It satisfies ultra-low NOx requirements and includes computerized safety features. Best for: homes needing space efficiency and reliable continuous hot water, with emphasis on ultra-low emissions. Caution: installation outdoors requires weatherproofing and proper venting that complies with local codes.
A. O. Smith ProLine XE Indoor 160,000 BTU Tankless
This indoor natural gas tankless unit uses X3 Technology for high efficiency and temperature stability. It features a primary copper heat exchanger that transfers heat efficiently, with a secondary 316L stainless steel exchanger for corrosion resistance. Best for: larger households or applications prioritizing energy efficiency and stable hot water, with a focus on long-term performance. Caution: higher upfront cost; ensure adequate venting and electrical support for installation.
A. O. Smith ProLine XE Outdoor 120,000 BTU
This outdoor model delivers up to 6.6 GPM with a space-saving wall-mount design. It uses a primary copper heat exchanger for efficient heat transfer and quicker temperature stabilization. Best for: homes needing outdoor installation with consistent hot water and a compact footprint. Caution: outdoor units require proper weatherproofing and seasonal maintenance to maximize lifespan.
Rheem ProTech ULN Burner Assembly SP21099
This replacement kit targets ULN Rheem models, including the OEM burner assembly, thermostat/thermistor components, and hardware. Best for: users upgrading or repairing ULN Rheem heaters with a complete OEM-style kit. Caution: verify exact model compatibility to ensure proper fit and function.
FIXIT PARTS Direct Replacement ULN Burner Assembly
This direct replacement assembly is positioned as a direct replacement for certain Ultra Low NOx water heater models, with a focus on natural gas burners. Best for: customers seeking a quick, compatible burner replacement with a robust build. Caution: model compatibility should be confirmed using the heater’s serial number and exact part matching.
Buying Guide: Key Considerations For ULN Water Heaters And Replacement Kits
What type of unit fits your home? Indoor versus outdoor units affect installation costs, venting, and space use. Ultra Low NOx options vary from compact tankless designs to larger residential boilers. If space is limited, consider wall-mounted or compact outdoor models with efficient heat exchangers.
How important is efficiency and emissions? ULN models aim to reduce NOx emissions while delivering reliable hot water. Look for features like X3 technology, high UEF/UEF ratings, and corrosion-resistant heat exchangers for long-term performance.
Is replacement or full unit preferred? Replacement kits are ideal for existing ULN Rheem or other compatible units, reducing downtime. Full units are better when the heater is older, failing, or when compatibility with non-integrated parts is a concern.
What about installation considerations? Check venting requirements, gas supply size, space clearance, and outdoor versus indoor suitability. Outdoor units must withstand weather and moisture, while indoor units may require dedicated venting and combustion air supply.
What is the best for high-demand households? For high flow rates, outdoor or indoor high-capacity tankless models with efficient heat exchangers deliver steady hot water. Consider units with larger heat exchangers and features that stabilize outlet temperatures.
Frequently Asked Questions
- What does Ultra Low NOx mean? Ultra Low NOx indicates reduced nitrogen oxide emissions compared to standard gas units, helping meet stricter air-quality standards.
- Are ULN replacement kits compatible with all models? Compatibility depends on the exact heater model and serial number. Verify part numbers before purchase.
- Is a tankless ULN heater better for small homes? Tankless ULN heaters can be a space-saving choice with endless hot water, but ensure the unit’s flow rate matches household demands.
- Do ULN units require different venting? Some ULN units have specific venting requirements; follow manufacturer instructions and local codes for installation.
- Can I install ULN products myself? Installation often requires a licensed professional to ensure safe gas connections, venting, and compliance with codes.
- What maintenance is needed for ULN systems? Regular inspection of burners, heat exchangers, and sensors is recommended to maintain efficiency and emissions control.