Choosing the best outdoor natural gas tankless water heater means matching its flow rate and heating capacity to your home—not simply picking the model with the biggest numbers. The options below range from compact units for everyday use to higher-output models for homes with several bathrooms. Before you buy, check your incoming water temperature, expected hot-water use, gas supply, and local installation rules. A qualified professional can help confirm the right size and safe setup.
Quick Comparison
| Product | Maximum flow rate | BTU input | Notable feature |
|---|---|---|---|
| Rheem RTG Series Outdoor Natural Gas Tankless Water Heater | 8.4 GPM | 180,000 BTU | Digital temperature control and freeze protection |
| Westinghouse Outdoor Natural Gas Tankless Water Heater | 5.1 GPM | 120,000 BTU | Temperature stability rated to ±1°F |
| Ranein Outdoor Natural Gas Tankless Water Heater | 8.5 GPM | 190,000 BTU | High stated capacity with freeze protection |
| Rinnai RE140eN Outdoor Natural Gas Tankless Water Heater | 5.3 GPM | 140,000 BTU | LED screen with service reminders and error codes |
| Rinnai REP199eN Smart-Circ Outdoor Natural Gas Tankless Water Heater | 7.6 GPM listed in product details | 199,000 BTU | Built-in recirculation pump with Smart-Circ scheduling |
Maximum flow figures are product specifications, not a guarantee that every fixture can run at full flow at the same time. Actual output depends on factors such as incoming water temperature and the temperature rise your home needs.
Our Picks: Best Outdoor Tankless Water Heater Natural Gas reviews
1. Rheem RTG Series: High Capacity for Larger Hot-Water Demands
The Rheem RTG Series is a high-output outdoor unit rated at 8.4 GPM and 180,000 BTU per hour. Its listed capacity makes it a candidate for larger households that may draw hot water from several places at once. Rheem specifies 8.4 GPM at a 35°F temperature rise, so your home’s colder incoming water could reduce the flow available at your chosen hot-water temperature.
This is a non-condensing model with a metal cabinet for outdoor wall mounting. The product information lists built-in freeze protection, flame-failure protection, CSA certification, and an LED control panel with temperature adjustments in 1°F increments. Outdoor installation avoids the need for a conventional exhaust vent through the home, but the unit still requires a suitable location and professional installation.
The stated capacity is a strength for households with heavier demand, but the unit’s output should be checked against your local winter water temperature and planned use. The product information also calls for professional installation. Its listed warranties are 15 years for the heat exchanger and five years for parts; confirm the full terms before purchase.
Pros:
- High stated output of 8.4 GPM and 180,000 BTU
- Temperature control with 1°F adjustment increments
- Built-in freeze and flame-failure protection
- Outdoor design with a metal cabinet for wall mounting
Cons:
- Professional installation is required
- Actual flow can be lower than the maximum when a larger temperature rise is needed
Verdict: Choose the Rheem if you want a high-capacity outdoor unit and have an installer confirm that its output and gas supply suit your home.
2. Westinghouse: A Mid-Range Option With Temperature Stability Features
The Westinghouse is rated for up to 5.1 GPM and 120,000 BTU. That places it below the higher-output Rheem and Ranein models, while offering a more moderate capacity for homes with everyday hot-water needs. The manufacturer describes it as suitable for use with showers, sinks, dishwashers, and laundry appliances, but the number of fixtures you can run comfortably will depend on their combined flow and your required temperature rise.
Its self-modulating system adjusts gas and water flow to help maintain a stated temperature stability of ±1°F. The outdoor design includes freeze protection and a variable-speed combustion fan. The product information specifies standard 3/4-inch NPT residential plumbing connections and a 120V AC power connection, so plan for power as well as gas and water connections.
One important cold-weather detail is that the manufacturer advises fully draining the system in temperatures below 5°F or during extended periods of non-use. Freeze protection is useful, but it should not be treated as a substitute for following the manual’s winterizing instructions.
Pros:
- Stated capacity of 5.1 GPM and 120,000 BTU
- Temperature stability rated to ±1°F
- Includes freeze protection and a variable-speed combustion fan
- Product details specify standard 3/4-inch NPT plumbing connections
Cons:
- Lower maximum flow than the higher-output models in this group
- Requires a 120V AC connection and may need draining in very cold conditions or during long periods without use
Verdict: The Westinghouse is worth considering for moderate household demand if its lower flow rating fits your needs and you can provide power and follow its cold-weather instructions.
3. Ranein: The Highest Stated Flow Rate in This Comparison
Ranein lists this outdoor natural gas model at up to 8.5 GPM and 190,000 BTU, the highest stated flow rate among these five options. The product description says it is intended to serve four to six outlet points. Treat that as a general capacity claim rather than a promise that all fixtures will receive full hot-water flow at once; a plumber can size the unit for your actual water use and local conditions.
The unit has a compact listed footprint of 24.3 by 15.1 by 8.1 inches and uses a fan beneath the unit that adjusts speed as conditions change. Its product details also list freeze protection and standard 3/4-inch G connections. For long periods without use in cold weather, the manufacturer advises draining remaining water through the pressure-relief valve.
There are two key limits to consider. The product information says this model is not recommended for areas above 2,000 feet, and it does not provide the same warranty details in the supplied specifications as some other picks. Confirm the altitude guidance, local service options, installation requirements, and warranty terms before choosing it.
Pros:
- Highest stated maximum flow in this comparison at 8.5 GPM
- 190,000 BTU input for substantial hot-water demand
- Compact listed dimensions for an outdoor wall-mounted unit
- Includes freeze protection and a variable-speed fan
Cons:
- Not recommended by the manufacturer for areas above 2,000 feet
- Requires draining in cold weather during long periods without use
Verdict: Consider the Ranein for high stated capacity if your home is within its altitude guidance and a qualified installer verifies the unit is suitable for your climate and gas supply.
4. Rinnai RE140eN: A Compact Choice for Moderate Demand
The Rinnai RE140eN is an outdoor, non-condensing model rated at 5.3 GPM and 140,000 BTU. Its smaller, suitcase-sized design may suit homes where installation space is limited, while its stated output is more modest than the higher-flow Rheem and Ranein options. Rinnai describes it as capable of serving several fixtures, but your incoming water temperature and fixture demand will affect real-world performance.
An LED screen displays service reminders and error codes, which can make routine maintenance needs or faults easier to identify. The product information lists residential coverage for the heat exchanger, parts, and labor, with registration required within 90 days of installation for the stated labor coverage. Check the warranty terms and registration instructions before purchase.
Rinnai recommends professional installation and points buyers toward qualified technicians. Wi-Fi control is not included; the product details identify a separate Controlr module for that function. Compared with the larger units here, the RE140eN gives up some maximum capacity, but may be a better match if your hot-water demand is moderate and you value a compact design.
Pros:
- Compact outdoor design
- Stated output of 5.3 GPM and 140,000 BTU
- LED screen provides service reminders and error codes
- Listed heat-exchanger, parts, and labor warranty coverage
Cons:
- Lower maximum flow than the high-capacity options
- Wi-Fi control requires a separate module
Verdict: The RE140eN is a sensible option for moderate demand if its 5.3 GPM rating fits your household and you plan to use a qualified installer.
5. Rinnai REP199eN Smart-Circ: Built-In Recirculation for Faster Hot-Water Access
The Rinnai REP199eN pairs a 199,000 BTU input with a built-in recirculation pump. Its Smart-Circ feature is designed to learn hot-water use patterns and schedule recirculation accordingly. That can be appealing if you want hot water to reach a fixture sooner, but the benefit depends on the home’s plumbing setup and how the recirculation system is installed.
There is a small specification difference to note: the product title lists 7.9 GPM, while the detailed product description says up to 7.6 GPM and mentions six fixtures. Confirm the current rating and sizing guidance with the manufacturer or installer before buying. Even at the listed capacity, the actual flow available depends on your required temperature rise and the demands of other fixtures.
Like the RE140eN, this model is an outdoor non-condensing unit with a compact design and a recommendation for professional installation. The product details list a separate Controlr module for Wi-Fi control. The recirculation system is its main distinction from the other picks, so it is most compelling when reducing the wait for hot water is a priority—not simply when you want the highest possible flow rate.
Pros:
- Built-in recirculation pump with Smart-Circ scheduling
- 199,000 BTU input
- Compact outdoor design
- Product details list heat-exchanger, parts, and labor warranty coverage
Cons:
- Product title and detailed description give different maximum flow figures
- Wi-Fi control requires a separate module, and professional installation is recommended
Verdict: Choose the REP199eN if built-in recirculation is important to you, but verify its flow rating and confirm that your plumbing can support the system.
How to Choose an Outdoor Natural Gas Tankless Water Heater
Estimate Your Peak Hot-Water Demand
Start by listing the fixtures that may run at the same time, such as a shower, kitchen sink, or washing machine. Add their expected hot-water flow to estimate peak demand. A product’s maximum GPM is only a starting point; running several fixtures together may reduce the hot-water flow available to each one.
Account for Incoming Water Temperature
Tankless heaters must raise incoming water to your chosen temperature. Colder groundwater calls for a larger temperature rise, which can lower the flow a unit can deliver. Check local groundwater temperatures and the manufacturer’s performance information for your area. A flow figure given at a particular temperature rise, such as the Rheem’s listed 8.4 GPM at 35°F, should not be assumed to apply under every condition.
Match the Unit to Your Gas Supply
BTU input helps show how much heating capacity a unit can provide, but the home’s gas line must also be able to supply the appliance properly. Have a qualified installer check the existing gas service, line size, connection, and any local requirements. Do not choose a unit based on BTU or GPM alone.
Check Outdoor Location and Installation Rules
Outdoor installation can avoid running a standard exhaust vent through the home, but it still requires careful placement. Confirm that the unit can be mounted in a suitable location with the clearances and protection required by its instructions and local code. These models call for professional installation, so ask a licensed installer to check the site before you buy.
Plan for Freezing Weather
Freeze protection is useful, but cold-weather care differs by model. Some manufacturers advise draining the unit during very cold conditions or extended periods without use. Read the installation and winterizing instructions for your specific heater, and ask your installer how to protect it during a power outage or long absence.
Decide Whether Recirculation Is Worth It
A built-in recirculation pump, like the one in the Rinnai REP199eN, is intended to reduce the wait for hot water by moving water through the plumbing system. It adds a feature beyond basic on-demand heating, but its value depends on your home’s pipe layout and use habits. Ask an installer whether your plumbing can use the system as intended.
Compare Warranty Terms and Service Access
Warranty coverage can vary by component and may depend on registering the unit or meeting other conditions. Read the full warranty rather than comparing only the heat-exchanger term. It is also wise to check whether a qualified technician can install and service the model in your area.
Frequently Asked Questions
What size outdoor natural gas tankless water heater do I need?
The right size depends on the hot-water flow you need at the same time and the temperature rise required in your location. List your likely simultaneous uses, then have an installer compare that demand with the manufacturer’s performance data for your incoming water temperature.
Can a tankless water heater supply more than one shower at once?
It may, but the answer depends on the model’s actual output under your conditions and the total flow of the showers and other fixtures. A product’s maximum GPM does not guarantee that it can supply that flow at every temperature rise.
Does an outdoor tankless water heater need venting?
These products are designed for outdoor installation, which can avoid the need for a conventional exhaust vent through the home. Correct outdoor placement and installation still matter. Follow the manufacturer’s instructions and local codes.
Can an outdoor gas tankless water heater freeze?
Yes. Freeze protection may help, but it does not remove the need to follow the manufacturer’s cold-weather directions. Depending on the model and conditions, draining the unit may be advised during very cold weather or extended periods without use.
Does a natural gas tankless water heater need electricity?
Some models require an electrical connection for controls or other components. The Westinghouse product information specifies a 120V AC connection. Check the installation requirements for the exact model you are considering.
Is professional installation necessary?
The supplied product information recommends or requires professional installation for several of these units. A qualified installer can check gas supply, plumbing, location, electrical needs, and local code requirements before the heater is put into service.
What is the advantage of a built-in recirculation pump?
A recirculation pump can help reduce the wait for hot water at fixtures by circulating water through the plumbing. The Rinnai REP199eN includes a pump with Smart-Circ scheduling. Its usefulness depends on your home’s plumbing layout and how you use hot water.
Final Recommendation
For a home with heavier hot-water demand, the Rheem RTG Series offers a strong balance of high stated flow, temperature controls, and listed safety features. The Ranein has the highest stated flow in this group, but its altitude limit and cold-weather instructions deserve close attention. For moderate demand, compare the Westinghouse and Rinnai RE140eN against your household’s actual needs. Choose the Rinnai REP199eN if built-in recirculation is a priority, and confirm its flow specification before purchasing. In every case, have a qualified professional size and install the unit for your home.