Best High Efficiency Gas Heaters and Tankless Water Heaters

Quick Summary: This guide compares five high-efficiency gas heaters — primarily condensing and non-condensing tankless water heaters plus a garage/space unit heater — suited for whole-house hot water or targeted space heating. The table below highlights brand, model type, and key performance metrics to help you compare at a glance.

Product	Type	Key Metric	Notable Feature
Rinnai RXP199iN	Condensing Tankless Water Heater	Up To 11.1 GPM, 199,000 BTU	UEF .98, Indoor/Outdoor Install
Rheem RTGH-95DVELN-3	Super High Efficiency Condensing Tankless	9.5 GPM	Built-In Wi‑Fi (EcoNet)
Rheem IKONIC RECTGH-SR10iN	Super High Efficiency Condensing Tankless	10.1 GPM	Integrated Recirculation
Rheem RTG-95XLN-3	Non-Condensing Outdoor Tankless	9.5 GPM	Water Saving Setting
ThermoMate 102,000 BTU	High Efficiency Tankless Water Heater	4.7 GPM, 102,000 BTU	High Thermal Efficiency 98%

Rinnai RXP199iN Condensing Tankless

Check Price on Amazon

This Rinnai RXP199iN is a condensing tankless water heater designed for both indoor and outdoor installation with the proper vent cap. The model supports natural gas or propane via Smart Sense gas control and lists up to 11.1 GPM output and 199,000 BTU—suitable for multiple simultaneous fixtures.

Key features include a compact footprint, UEF .98 energy performance, a Quick Flush service feature, and an interface designed for simplified operation. Outdoor installation requires an RX-OVC vent cap. Wi‑Fi control is possible with an optional Control-R module (sold separately).

Best For: Homes needing very high flow rates and flexible installation options that prioritize maximum efficiency and modern controls.

Rheem RTGH-95DVELN-3 Super High Efficiency

Check Price on Amazon

The Rheem RTGH-95DVELN-3 is a condensing tankless water heater offering 9.5 GPM and built-in EcoNet Wi‑Fi technology to monitor and control temperature, recirculation settings, and usage data. It operates only when hot water is needed, which supports energy savings.

Functionality highlights include remote control and diagnostics via EcoNet, continuous hot water capacity for multiple applications, and a focus on energy and water savings through efficient operation patterns. As a super high-efficiency condensing unit, it targets lower operating costs compared to non-condensing models.

Best For: Households wanting integrated smart controls and a condensing design to maximize energy savings on hot water demand.

Rheem IKONIC RECTGH-SR10iN With Recirculation

Check Price on Amazon

The Rheem IKONIC RECTGH-SR10iN is a professional-grade condensing tankless heater with 10.1 GPM capacity and an integrated recirculation pump option. Recirculation minimizes the wait for hot water, saving water usage over time.

It features EcoNet Wi‑Fi for customization and alerts, a condensing heat exchanger designed for high thermal efficiency, and customizable recirculation schedules to balance comfort and energy use. The recirculation system is marketed as saving significant water by eliminating prolonged cold-water draws.

Best For: Homes that prioritize immediate hot water delivery at fixtures and want the water-saving benefits of built-in recirculation controls.

Rheem RTG-95XLN-3 Outdoor Non-Condensing

Check Price on Amazon

The Rheem RTG-95XLN-3 is a high-efficiency, non-condensing outdoor tankless water heater rated at 9.5 GPM. As an outdoor model, it removes the need for indoor venting infrastructure and is designed to withstand exterior installations.

Features include continuous hot water for multiple applications, a water-saving setting that can reduce household water use, and a design focused on simplified installation outdoors. Non-condensing units typically have simpler venting and may fit certain installation budgets or site constraints.

Best For: Properties where outdoor installation is preferred or necessary and where a reliable, high-flow non-condensing option is acceptable.

ThermoMate 102,000 BTU Tankless Water Heater

Check Price on Amazon

The ThermoMate 102,000 BTU tankless water heater lists 4.7 GPM and a high reported thermal efficiency of 98% based on its oxygen-free copper heat exchanger. The unit claims precise temperature control via an advanced flow sensor and a suite of safety protections.

Built-in protections include freeze, flame-out, overheat, and pipe-blockage safeguards. The manufacturer states a UEF of 0.81 and highlights corrosion resistance from copper construction. The lower GPM rating positions this model for smaller homes or point-of-use applications where simultaneous high-flow use is limited.

Best For: Smaller households or multi-point setups requiring precise temperature control and a compact high-efficiency heat exchanger design.

Buying Guide: Choosing A High Efficiency Gas Heater

This guide covers the most important factors when comparing high-efficiency gas heaters, with perspectives on performance, installation, and lifecycle costs.

1. Efficiency Type: Condensing vs Non-Condensing

Condensing units reclaim latent heat from exhaust gases via a secondary heat exchanger, delivering higher UEFs and lower operating costs but usually require corrosion-resistant venting and condensate management. Non-condensing units have simpler venting and sometimes lower upfront cost but typically lower efficiency.

2. Flow Rate And Sizing (GPM)

Match the unit’s GPM rating to peak household demand. Higher GPM (8–11+) supports simultaneous showers and appliances. For point-of-use or smaller homes, 3–5 GPM may suffice. Oversizing can increase install complexity and cost; undersizing causes lukewarm water during concurrent use.

3. Fuel Type And Flexibility

Confirm natural gas or propane compatibility. Some models support both with conversions or multi-fuel control systems. Verify local gas supply capacity and required BTU input to ensure the line and meter can support the chosen unit.

4. Installation Considerations

Decide on indoor vs outdoor installations. Outdoor units remove the need for special venting but require freeze protection in colder climates. Indoor installations need appropriate venting, condensate drains for condensing models, and clearance requirements.

5. Controls, Connectivity, And Recirculation

Built-in Wi‑Fi and app controls (EcoNet or vendor-specific modules) enable remote monitoring, temperature scheduling, and diagnostics. Integrated or optional recirculation systems reduce wait times and water waste but add pump and control complexity.

6. Durability And Heat Exchanger Material

Heat exchangers are commonly stainless steel or copper. Copper offers strong thermal conductivity but may vary in corrosion characteristics; stainless steel is common for condensing applications due to corrosion resistance. Consider local water chemistry and scale management needs.

7. Safety Features And Protections

Look for blowout/ flame-out detection, freeze protection, overheat protection, and freeze-resistant components for outdoor installs. Condensing units should include safe condensate routing and materials resistant to acidic condensate.

8. Permits, Local Codes, And Professional Installation

Gas appliance installation often requires permits and licensed professionals. Venting, clearances, combustion air, and gas line sizing must meet code. Include professional estimates for installation complexity and necessary venting modifications.

9. Lifecycle Cost: Upfront Vs Operating Expenses

Compare UEF or efficiency ratings to estimate yearly energy use. Condensing units often cost more upfront but may deliver lower operating costs. Factor in maintenance needs, expected service life, and manufacturer warranty coverage.

10. Comparative Perspectives

• High Flow Need: Choose models with higher GPM (e.g., Rinnai RXP199iN) for large households.
• Smart Controls & Water Savings: Select units with Wi‑Fi and recirculation (Rheem RTGH-95DVELN-3 or IKONIC) to optimize efficiency and user convenience.
• Outdoor Install Preference: Use outdoor-rated units (Rheem RTG-95XLN-3) to avoid indoor venting but ensure freeze protection.
• Smaller Homes/Point Use: Lower GPM, high thermal-efficiency units (ThermoMate) may be more cost-effective and compact.

When comparing models, weigh installation constraints, peak demand, long-term energy savings, and whether integrated features such as recirculation or app connectivity address your household priorities.