Tankless Water Heaters
Best Natural Gas Indoor Tankless Water Heater (2026): 5 Models Side by Side
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The ThermoMate CZ750NG leads this search for a natural gas indoor tankless water heater: it delivers up to 8 GPM at a 33°F temperature rise, with 199,000 BTU and temperature stability within ±1°F. It is built for whole house use with 3 to 5 fixtures at once.
Starting out? See the main guide: Best Tankless Water Heater
Top picks at a glance
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9.8 View on Amazon -
9.5 View on Amazon -
Highest Flow RateRinnai RE199iN Non-Condensing Natural Gas Tankless Water9.4 View on Amazon -
9.3 View on Amazon -
Also GreatRheem RTGH Series 9.0 GPM Natural Gas9.2 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. ThermoMate CZ750NG Natural Gas Tankless Water Heater | 8 GPM | Natural Gas | Yes | — | — | — |
| 2. Ranein Natural Gas Tankless Water Heater | 8.5 GPM | Natural Gas | — | — | 42 lb | 2-year |
| 3. Rinnai RE199iN Non-Condensing Natural Gas Tankless Water | 9.8 GPM | Natural Gas | — | — | 52.1 lb | 1-year |
| 4. Westinghouse Tankless Water Heater Gas Indoor | 7.9 GPM | Natural Gas | Yes | Yes | 39.3 lb | 2-year |
| 5. Rheem RTGH Series 9.0 GPM Natural Gas | 9 GPM | Natural Gas | Yes | — | 59.88 lb | 5-year |
ThermoMate CZ750NG Natural Gas Tankless Water Heater
It earns Best Overall with up to 8 GPM at a 33°F rise, 199,000 BTU, and temperature stability within ±1°F.
- Flow rate: 8 GPM
- Fuel: Natural Gas
- Temperature control
Good for
- Households that need up to 8 GPM for whole house use
- Buyers who want temperature stability within ±1°F
Skip it if
- You need more than 8 GPM of flow
- You need hot water for more than 5 fixtures at once
It delivers up to 8 GPM at a 33°F temperature rise, from a 72°F inlet to a 105°F outlet, powered by 199,000 BTU. An oxygen-free copper heat exchanger delivers up to 98% thermal efficiency, and water flow and temperature sensors hold stability within ±1°F. Freeze protection activates at 41°F, and 8 protections cover flame failure, overheating, over-temperature, freeze, dry-burn, blockage, high-temperature lockout, and excessive air pressure.
Ranein Natural Gas Tankless Water Heater
It earns Runner-Up with up to 8.5 GPM and 190,000 BTU for 4 to 6 outlet points in a large home.
- Flow rate: 8.5 GPM
- Fuel: Natural Gas
- 2-year warranty stated
Good for
- Families that need 4 to 6 outlet points at once
- Buyers who want a 42 lb wall-mounted heater
Skip it if
- You need more than 8.5 GPM of flow
- You live at an elevation over 2,000 feet
It delivers up to 8.5 GPM with 190,000 BTU for 4 to 6 outlet points. A blower under the unit adjusts speed with wind pressure and water temperature, and it weighs 42 lb. It carries a 2-year warranty, and in cold weather the pressure relief valve must be opened to drain water if the heater is unused for a long time.
Rinnai RE199iN Non-Condensing Natural Gas Tankless Water
It earns Highest Flow Rate with up to 9.8 GPM and 199,000 BTU for whole house use.
- Flow rate: 9.8 GPM, the highest here
- Fuel: Natural Gas
- 1-year warranty stated
Good for
- Households that need up to 9.8 GPM for multiple fixtures
- Buyers who want an LED display with diagnostic codes
Skip it if
- You want built-in WiFi app control
- You need more than 9.8 GPM of flow
This non-condensing heater delivers up to 9.8 GPM with 199,000 BTU for whole house use, including showers, laundry and dishwasher use. A wall-mounted design has an LED display with diagnostic codes and service reminders. It carries a 15-year heat exchanger warranty and a 5-year parts warranty, and professional installation by a qualified technician is recommended.
Westinghouse Tankless Water Heater Gas Indoor
It earns Also Great with up to 7.9 GPM, 180,000 BTU, and built-in WiFi app control.
- Flow rate: 7.9 GPM
- Fuel: Natural Gas
- Temperature control
- Phone app control
- 2-year warranty stated
Good for
- Households that need up to 7.9 GPM for 3 showers at once
- Buyers who want WiFi app and voice control
Skip it if
- You need more than 7.9 GPM of flow
- You need hot water for more than 3 showers at once
It delivers up to 7.9 GPM with 180,000 BTU for whole home use. WiFi app and voice control adjust temperature, monitor status, and give fault alerts, with no extra accessories required. Temperature stability stays within ±1°F, and it operates at water pressure as low as 15 PSI for homes with lower pressure. It carries a 2-year limited warranty for the complete unit and a 5-year warranty for the heat exchanger.
Rheem RTGH Series 9.0 GPM Natural Gas
It earns Also Great with up to 9.0 GPM at a 35°F rise, 180,000 BTU, and condensing technology.
- Flow rate: 9 GPM
- Fuel: Natural Gas
- Temperature control
- 5-year warranty stated
Good for
- Households that need up to 9.0 GPM for 3 to 4 showers
- Buyers who want condensing technology with a 0.93 UEF
Skip it if
- You need more than 9.0 GPM of flow
- You want built-in WiFi app control
It delivers up to 9.0 GPM at a 35°F rise, powered by 180,000 BTU, with condensing technology. Digital LED temperature control holds precision within ±1°F, and built-in freeze protection is included. It carries a 15-year heat exchanger warranty and a 5-year parts warranty, and mounting hardware and an installation manual are included.
How to choose
Match flow rate to fixtures
Flow rate in GPM decides how many fixtures run at once. The ThermoMate delivers up to 8 GPM for 3 to 5 fixtures, the Ranein delivers up to 8.5 GPM for 4 to 6 outlet points, the Rinnai delivers up to 9.8 GPM, the Westinghouse delivers up to 7.9 GPM for 3 showers, and the Rheem delivers up to 9.0 GPM for 3 to 4 showers. Compare the flow rate and fixture count on each pick before you choose.
Check fuel, venting and installation
All five picks use natural gas and are made for indoor installation. Professional installation is recommended for the Rinnai and Rheem picks. The Westinghouse pick requires standard 3/4-inch NPT residential plumbing. Check the fuel type, venting and installation notes for the pick you choose before buying.
Compare warranties by part
Warranty terms differ by part, so compare heat exchanger, parts and complete unit terms separately. The Ranein heater carries a 2-year warranty, the Westinghouse heater carries a 2-year limited warranty for the complete unit and a 5-year warranty for the heat exchanger, the Rinnai heater carries a 15-year heat exchanger warranty and a 5-year parts warranty, and the Rheem heater carries a 15-year heat exchanger warranty and a 5-year parts warranty.
FAQ
What flow rate do I need for a whole house?
Match flow rate to fixtures. The ThermoMate delivers up to 8 GPM for 3 to 5 fixtures, the Rinnai delivers up to 9.8 GPM, and the Rheem delivers up to 9.0 GPM for 3 to 4 showers.
Do these heaters need professional installation?
Professional installation is recommended for the Rinnai and Rheem heaters. The Westinghouse heater requires standard 3/4-inch NPT residential plumbing.
Which heater has WiFi app control?
The Westinghouse heater has built-in WiFi app and voice control, with no extra accessories required. The Rinnai Controlr module is sold separately.
What is the highest flow rate here?
The Rinnai RE199iN delivers up to 9.8 GPM, the highest flow rate among the picks. It is powered by 199,000 BTU for whole house use.
