Tankless Water Heaters
Best Energy Efficient Tankless Water Heater (2026): 5 Models Side by Side
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For energy efficient tankless heating, the Rheem RTG Series leads this list: on-demand heating with a 0.82 UEF covers 2-3 simultaneous showers at 7 GPM, with digital LED temperature control. The Rinnai gives a UEF of .98 with 10 GPM, and the ThermoMate pairs propane heating with a 0.81 UEF.
Starting out? See the main guide: Best Tankless Water Heater
Top picks at a glance
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Best OverallRheem RTG Series 7.0 GPM Natural Gas9.8 View on Amazon -
9.7 View on Amazon -
9.6 View on Amazon -
Highest Flow RateRinnai RX180iN Condensing Smart Sense Tankless Water9.5 View on Amazon -
9.2 View on Amazon
Compare the specs
Ranked on flow rate, temperature control, phone app control from each listing, plus how many buyers have rated it. A dash (—) means we could not confirm that figure from the listing.
| Pick | Flow rate | Fuel | Temperature control | Phone app control | Weight | Warranty |
|---|---|---|---|---|---|---|
| 1. Rheem RTG Series 7.0 GPM Natural Gas | 7 GPM | Natural Gas | Yes | — | — | — |
| 2. Ranein Natural Gas Tankless Water Heater | 4.3 GPM | — | Yes | — | — | — |
| 3. ThermoMate CZ422LP Propane Tankless Water Heater Indoor | 5 GPM | Propane | Yes | — | — | — |
| 4. Rinnai RX180iN Condensing Smart Sense Tankless Water | 10 GPM | Natural Gas or Propane | — | — | — | 1-year |
| 5. Westinghouse Tankless Water Heater Propane Indoor | 7.9 GPM | Propane | Yes | Yes | — | — |
Rheem RTG Series 7.0 GPM Natural Gas
It pairs on-demand heating with a 0.82 UEF, digital LED temperature control and leak detection for whole-house use.
- Flow rate: 7 GPM
- Fuel: Natural Gas
- Temperature control
Good for
- Owners of multi-bathroom homes with 2-3 water points
- Buyers who want digital LED temperature control with ±1°F adjustments
Skip it if
- You need more than 7 GPM of flow
- You run more than 3 water points at once
It delivers 7 GPM at a 35°F temperature rise with 160,000 BTU/hr output for 2-3 simultaneous showers. It has a 0.82 UEF, a direct vent system for indoor installation, a digital LED temperature control panel with ±1°F adjustments, freeze protection and flame failure protection. LeakGuard leak detection and prevention is included, and the wall-mount design frees floor space.
Ranein Natural Gas Tankless Water Heater
It pairs 100,000 BTU on-demand heating with an oxygen-free copper heat exchanger and 4 preset modes.
- Flow rate: 4.3 GPM
- Temperature control
Good for
- Buyers who want 4 preset modes for seasons and rooms
- Owners who want 95°F to 155°F temperature adjustment
Skip it if
- You need more than 4.3 GPM of flow
- You serve more than 4 water points at once
It gives 4.3 GPM with 100,000 BTU for 3-4 water points at once. Four preset modes cover 4-season, Kitchen, Bathtub and Shower use, with a 95°F to 155°F adjustable range and a high-temp lock. An oxygen-free copper heat exchanger supports heat transfer, and anti-freeze protection resists 14°F low temperature. A vent pipe is included, and installation by licensed professionals is stated.
ThermoMate CZ422LP Propane Tankless Water Heater Indoor
It pairs propane on-demand heating with up to 98% thermal efficiency, a 0.81 UEF and 8 protections.
- Flow rate: 5 GPM
- Fuel: Propane
- Temperature control
Good for
- Buyers who want propane on-demand heating at up to 5 GPM
- Owners who want a 3.5-inch LED display with flow and flame status
Skip it if
- You need more than 5 GPM of flow
- You run more than two showers at once
It delivers up to 5 GPM with 102,000 BTU for two showers at the same time, holding 105°F from a 65°F inlet temperature. It has a 0.81 UEF with up to 98% thermal efficiency, an oxygen-free copper heat exchanger and 8 protections, including freeze and flame-out protection. Temperature adjusts from 90 to 140°F on a 3.5-inch LED display, and it measures 21 inches high, 13 inches wide and 7 inches deep. It carries a 3-year warranty.
Rinnai RX180iN Condensing Smart Sense Tankless Water
It pairs condensing design with a UEF of .98, 10 GPM flow and a Smart Sense gas control system for natural gas or propane.
- Flow rate: 10 GPM, the highest here
- Fuel: Natural Gas or Propane
- 1-year warranty stated
Good for
- Buyers who want indoor or outdoor installation
- Owners who run 6 fixtures at once
Skip it if
- You need more than 10 GPM of flow
- You run more than 6 fixtures at once
It gives up to 10 GPM for 6 fixtures at once through a Smart Sense gas control system for natural gas or propane. It installs indoors or outdoors with a Versa Vent, and an RX-OVC vent cap is required for outdoor install. It has a UEF of .98 and a Quick Flush feature. Professional installation is recommended.
Westinghouse Tankless Water Heater Propane Indoor
It pairs propane on-demand heating with app and voice control, ±1°F stability and 7.9 GPM flow.
- Flow rate: 7.9 GPM
- Fuel: Propane
- Temperature control
- Phone app control
Good for
- Buyers who want app and voice control
- Owners who serve 3-5 fixtures at once
Skip it if
- You need more than 7.9 GPM of flow
- You serve more than 5 fixtures at once
It delivers up to 7.9 GPM with 180,000 BTU for 3-5 fixtures at once. App and voice control adjust temperature, monitor operating status and send fault alerts, with no extra accessories required. Self-modulating technology holds temperature within ±1°F, and freeze protection, a variable-speed combustion fan and a copper heat exchanger are included. It is CSA-certified.
How to choose
Read UEF as the efficiency figure
UEF is the efficiency figure each maker gives for a tankless heater. The Rheem gives 0.82, the ThermoMate gives 0.81 and the Rinnai gives .98. On-demand heating also matters for efficiency: the Westinghouse heats water only when needed to reduce standby energy loss, and the ThermoMate design helps reduce wasted energy. Compare UEF with flow rate and fixtures, since a high UEF on a small-flow unit still limits simultaneous use.
Match flow to simultaneous fixtures
Flow rate sets how many fixtures run at once. The Rheem gives 7 GPM for 2-3 simultaneous showers, the Ranein gives 4.3 GPM for 3-4 water points, the ThermoMate gives 5 GPM for two showers at the same time, the Rinnai gives 10 GPM for 6 fixtures at once, and the Westinghouse gives 7.9 GPM for 3-5 fixtures. Count the showers and fixtures you run together, then pick a flow figure that covers them.
Check fuel, venting and installation
Fuel differs across this list: the Rheem uses natural gas, the ThermoMate and the Westinghouse use propane, and the Rinnai runs on natural gas or propane through Smart Sense. Venting differs too: the Rheem uses a direct vent system for indoor installation, the Rinnai uses a Versa Vent with an RX-OVC vent cap required for outdoor install, and the ThermoMate uses a 3-inch vent connection. The Ranein calls for installation by licensed professionals, and professional installation is recommended for the Rinnai.
FAQ
What does UEF mean on a tankless water heater?
UEF is the efficiency figure the maker gives. The Rheem lists 0.82, the ThermoMate lists 0.81 and the Rinnai lists .98. Compare those figures with flow rate and fixtures before choosing.
Which of these heaters use propane?
The ThermoMate and the Westinghouse use propane, and the Rinnai runs on natural gas or propane through Smart Sense. The Rheem uses natural gas.
Do these heaters need professional installation?
The Ranein requires installation by licensed professionals, and professional installation is recommended for the Rinnai. Check the installation note on each product before ordering.
Which heater here has app control?
The Westinghouse has app and voice control for temperature, operating status and fault alerts, with no extra accessories required. The Rinnai WiFi control module is not included.
