Best Water Heater Ultra Low NOx Options for Efficient on-Demand Hot Water
Homeowners seeking ultra-low NOx gas tankless water heaters can reduce emissions while enjoying on-demand hot water. This guide spotlight five leading picks that balance efficiency, installation flexibility, and reliability. Each product is evaluated for NOx performance, flow capability, and compatibility with common venting and fuel types.
| Product | Brand | Type | Approximate Flow (GPM) | NOx Feature |
|---|---|---|---|---|
| A. O. Smith ProLine XE ULN Outdoor | AO Smith | Outdoor Tankless | 6.6 | Ultra-Low NOx |
| Takagi TK-510U Series | Takagi | Indoor Non-Condensing | Variable | Ultra-Low NOx |
| A. O. Smith Propane Outdoor ULN | AO Smith | Outdoor Propane | 6.6 | Ultra-Low NOx |
| Rheem ULN Chamber Sensor Kit SP21030 | Rheem | Sensor Kit | — | ULN Compatible |
| Gulfmew Device for AP19299 ULN | Gulfmew | Sensor Replacement | — | ULN Compatible |
The products below were chosen for relevance to ultra-low NOx operation, availability of replacement parts, and compatibility with common ULN-rated setups. Each section provides a concise overview and practical considerations to help with decision-making.
AO Smith ProLine XE ULN Outdoor 6.6 GPM (ATO-140H)
The ProLine XE Ultra-Low NOx Outdoor unit delivers up to 6.6 GPM of continuous hot water, making it suitable for households with multiple fixtures. Its wall-mount design saves space and enhances flexibility for outdoor mounting. A primary commercial-grade copper heat exchanger offers superior heat transfer, helping stabilize outlet temperatures quickly. This model operates at 120,000 BTU input and is designed for condensing natural gas usage, with safety features that guard against overheating and temperature deviations.
Takagi TK-510U Series ULN Indoor (NG/Propane)
Takagi’s TK-510U Series provides precise combustion control through a sealed combustion system and power-vented flue. It supports both natural gas and propane, enabling flexible installation without multiple model variants. As a non-condensing ULN unit, it emphasizes efficiency and reliable on-demand hot water. Its compact design and fuel flexibility make it a practical choice for indoor installations where venting considerations are critical.
AO Smith Propane ULN Outdoor 6.6 GPM (ATO-140H)
This propane outdoor model supports continuous hot water with up to 6.6 GPM and features venting options using PVC up to significant lengths. Its safety suite includes exhaust and water temperature safeguards, an overheat cut-off, and an air-fuel ratio sensor. A 15-year limited warranty on the heat exchanger applies in residential applications, emphasizing long-term reliability in ULN setups where propane is the primary fuel.
Rheem ULN Chamber Sensor Kit SP21045
This chamber sensor replacement kit is designed to repair ULN Rheem units by restoring accurate combustion chamber temperature monitoring. Included are a thermistor, temperature probe assembly, a burner door gasket, installation instructions, and replacement hardware. OEM quality ensures compatibility with ULN self-powered models that use 14-inch burners, aiding safe shutdown and protection against overheating.
Gulfmew Chamber Sensor for AP19299 ULN
This thermistor sensor is compatible with AP19299 Ultra Low NOx tankless models, providing precise temperature measurement and reliable control board data. Engineered for a plug-and-play installation, it features robust brass components and heat-resistant housing to withstand harsh conditions inside water heaters, ensuring stable performance and durability over time.
Buying Guide: Key Considerations For Ultra-Low NOx Water Heaters
- NOx performance: Look for ULN or Ultra-Low NOx ratings and certify that the unit meets your local emission standards.
- Fuel type and flexibility: Decide between natural gas and propane, and check for models that are field-convertible where applicable.
- Flow capacity: Confirm GPM requirements for simultaneous fixtures (showers, sinks, appliances) to avoid cold water sandwiches.
- Vent and installation: Check venting needs (PVC options, distance limits) and whether outdoor or indoor installation better fits your space.
- Durability and warranty: Assess heat exchangers, materials, and warranty length to gauge long-term value.
- Replaceable parts availability: Sensor, thermistor, and burner kits should be readily available for ongoing maintenance.
- Space and mounting: Wall-mount or compact footprints can optimize storage and garage or utility room layouts.
- Maintenance requirements: Consider ease of service, access to parts, and compatibility with common service tools.