Best High Efficiency Gas Water Heater: 5 Rheem Tankless Models Compared

Choosing the best high efficiency gas water heater means matching fuel type, hot-water demand, and installation location—not just picking the model with the biggest flow rate. The five Rheem tankless heaters below offer different capacities and configurations, including outdoor units, indoor condensing models, and a model with built-in recirculation. Compare their specifications carefully, since flow rates depend on the temperature rise your home needs and the right choice also depends on whether you have natural gas or propane.

Quick Comparison

Our Picks: Best High Efficiency Gas Water Heater reviews

1. Rheem RTG 8.4 GPM Natural Gas Outdoor Tankless Water Heater

This outdoor natural gas model is designed to supply hot water on demand for a whole home. Its rated output is 8.4 gallons per minute at a 35°F temperature rise, with a 180,000 BTU/hr input. Rheem lists it for homes that may need to run several hot-water fixtures at once, though actual capacity depends on incoming water temperature and total household demand.

The non-condensing design has a listed 0.82 Uniform Energy Factor (UEF). The outdoor cabinet is intended for wall mounting, and placing the unit outside avoids the need for indoor venting. It also has a digital temperature control with stated adjustments of ±1°F, plus freeze and flame-failure protection. The supplied product information calls for professional installation, an important consideration for gas connections and safe setup.

This is a sensible fit if your home uses natural gas, you have a suitable outdoor installation location, and you want a whole-house tankless unit without indoor venting. It has a lower listed UEF than the indoor condensing Rheem options below, so shoppers focused on the highest stated efficiency may prefer those models. The product description’s energy-savings figure is a comparison with a 50-gallon electric tank heater, not a guarantee of savings in every home.

Pros:

• 8.4 GPM rated flow at a 35°F temperature rise
• Outdoor installation avoids indoor venting
• Digital temperature control and listed safety protections
• 15-year heat exchanger and 5-year parts warranties are stated in the product information

Cons:

• Requires natural gas and a suitable outdoor location
• Its 0.82 UEF is below the listed 0.93 UEF of the indoor condensing models

Verdict: Choose this model if you want an outdoor natural gas tankless heater with substantial stated capacity and do not need the highest UEF in this group.

2. Rheem RTG 7.0 GPM Liquid Propane Outdoor Tankless Water Heater

This Rheem RTG model is the propane-fueled outdoor option in the lineup. It is rated at 7.0 GPM at a 35°F rise and has a 160,000 BTU/hr input. That is less stated capacity than the natural gas RTG model above, so it may be a better match for a smaller household or for homes with more modest simultaneous hot-water needs.

The heater is non-condensing, with a listed 0.82 UEF. Its outdoor design avoids indoor venting, and the description lists a digital temperature control, freeze protection, and flame-failure protection. The product title specifies liquid propane, so confirm that fuel matches your home before ordering; the supplied description contains inconsistent fuel wording in one place.

Like other tankless models, its real-world flow depends on how much the incoming water must be heated. A colder supply can reduce the flow available at the desired hot-water temperature. This unit is worth considering when you use propane and want an outdoor installation, but households with several hot-water demands at once may want to compare it with the higher-capacity indoor propane model.

Pros:

• Propane option for homes without natural gas service
• Outdoor installation avoids indoor venting
• 7.0 GPM rated flow at a 35°F rise
• Listed digital controls and multiple safety features

Cons:

• Lower stated flow and BTU input than the other RTG and RTGH models here
• Requires a suitable outdoor location and professional installation

Verdict: This is the outdoor propane pick for households whose demand fits its 7.0 GPM rating and who do not need the extra capacity of the larger indoor model.

3. Rheem RTGH 9.0 GPM Natural Gas Indoor Condensing Tankless Heater

The RTGH is an indoor natural gas condensing model rated at 9.0 GPM at a 35°F rise. Its 180,000 BTU/hr input is paired with a listed 0.93 UEF, the highest stated efficiency among the first four models in this comparison. Condensing technology and the higher UEF make it a strong option for shoppers who want to prioritize the efficiency specification while keeping a natural gas connection.

This unit is designed for indoor installation with direct venting, so you will need to plan for the required vent route and have the installation checked by a qualified professional. The product information also lists low-NOx emissions compliance, a digital temperature control with ±1°F adjustments, and freeze and other safety protections. It is described as suitable for homes with multiple bathrooms, but the 9.0 GPM rating is measured at a specific temperature rise—not a promise of that flow under every home’s conditions.

Compared with the outdoor RTG natural gas model, this heater has a higher listed flow rate and UEF but requires an indoor location and direct venting. Its 180,000 BTU/hr input is lower than the 199,900 BTU/hr propane RTGH model, so fuel type and local installation needs should guide the comparison as much as capacity.

Pros:

• 0.93 UEF is the highest listed for the first four models
• 9.0 GPM rated flow at a 35°F temperature rise
• Indoor direct-vent design with stated low-NOx compliance
• 15-year heat exchanger and 5-year parts warranties are stated in the product information

Cons:

• Requires natural gas, indoor space, and direct venting
• Professional installation adds planning and installation requirements

Verdict: Pick the RTGH if you have natural gas and want an indoor condensing model with a high stated UEF and a 9.0 GPM rating.

4. Rheem RTGH 9.5 GPM Liquid Propane Indoor Condensing Heater

This indoor RTGH is the higher-capacity propane option among Rheem’s RTG and RTGH models here. It has a stated 9.5 GPM flow at a 35°F rise and a 199,900 BTU/hr input. The condensing design has a listed 0.93 UEF, making it a noteworthy choice for propane homes that need more stated flow than the 7.0 GPM outdoor RTG model offers.

Installation is indoors with direct venting, so check the available location and venting path before choosing it. The description lists a digital LED temperature control with ±1°F adjustments, freeze protection, and a multi-point safety system. It also states 15-year heat exchanger and 5-year parts warranties. As with any on-demand heater, the rated flow is tied to the specified temperature rise; your incoming water temperature and the number of fixtures running at once affect what you can expect.

Compared with the natural gas RTGH, this propane model has a slightly higher stated flow rate and a higher BTU input, while both list a 0.93 UEF. It is not the best fit if your home is set up for natural gas, and it does not include the recirculation and connected controls featured on the IKONIC model.

Pros:

• 9.5 GPM rated flow at a 35°F rise
• 0.93 UEF condensing design
• Propane fuel option with indoor direct venting
• Listed digital temperature control and stated warranty coverage

Cons:

• Requires liquid propane and an indoor direct-vent installation
• Higher flow ratings depend on the home’s temperature rise and simultaneous demand

Verdict: Choose this model if you need a higher-capacity indoor propane heater and your home can accommodate its direct-vent installation.

5. Rheem IKONIC 11.2 GPM Natural Gas Tankless Heater with Recirculation

The IKONIC is an indoor natural gas condensing tankless heater with a stated 11.2 GPM capacity and 199,900 BTU input. Its standout feature is built-in recirculation, which is designed to reduce the wait for hot water at a faucet or shower. The product description says its customizable recirculation can save an average of 11,000 gallons of water per year by reducing the time water runs while you wait; actual results will depend on the home and how the system is set up.

This model also includes EcoNet Wi-Fi technology for setting adjustments, energy and water use tracking, and maintenance alerts through a mobile device. Hot-start programming is designed to keep the heater ready between back-to-back uses. The compact design and stated installation flexibility may be useful for replacement projects, but the product information does not provide a UEF or warranty term, so compare those details with the other models and confirm them before buying.

The IKONIC has the highest stated flow rate in this group and adds smart controls and recirculation that the other listed models do not specify. Those features may be valuable in a large home or where a long wait for hot water is a regular frustration. It is also the most feature-focused option here, so the additional controls may matter less if your priority is simply a basic tankless heater matched to your fuel and flow needs.

Pros:

• 11.2 GPM stated flow rate and 199,900 BTU input
• Built-in recirculation is designed to reduce hot-water wait time
• EcoNet Wi-Fi supports settings control and usage tracking
• Hot-start programming is designed for back-to-back use

Cons:

• The supplied product information does not state a UEF or warranty term
• Requires natural gas; an outdoor setup requires the noted conversion kit

Verdict: Consider the IKONIC if you want high stated flow plus recirculation and connected controls, and verify the efficiency and warranty details before making your final choice.

How to Choose a High Efficiency Gas Water Heater

Match the fuel to your home

Start by confirming whether your home uses natural gas or liquid propane. These products are fuel-specific, and a natural gas model is not a direct substitute for a propane model. Check the heater’s product details and have an installer confirm compatibility with your gas supply before ordering.

Estimate your peak hot-water demand

Think about how many hot-water fixtures may run at the same time, such as showers, sinks, or a dishwasher. Compare that demand with the heater’s flow rate, but read the temperature-rise condition too. The listed RTG and RTGH flow rates are measured at a 35°F rise; a greater rise in water temperature can change the flow available. A qualified installer can help size a unit for your incoming water temperature and household use.

Choose indoor or outdoor installation

An outdoor model can avoid indoor venting, but it still needs an appropriate outdoor location and proper installation. Indoor models in this group use direct venting, which requires a suitable route and space. Do not choose by location alone: ask an installer to review the site, venting needs, and the manufacturer’s installation requirements.

Compare efficiency figures carefully

UEF is a useful specification when it is provided. In this lineup, the RTG outdoor models list 0.82 UEF, while the RTGH indoor condensing models list 0.93 UEF. The IKONIC product information provided here does not list a UEF. Treat advertised energy-savings claims as comparisons under stated conditions, not as a promise of what your utility bills will be.

Consider recirculation and control features

Recirculation may reduce the time you wait for hot water at a distant tap, and the IKONIC includes built-in recirculation and Wi-Fi controls. Consider whether those features solve a real problem in your home. For simpler needs, a model with digital temperature adjustment and the right fuel and capacity may be a better fit.

Plan for professional installation and upkeep

Gas tankless heaters involve fuel connections, water connections, and—in indoor models—venting. The product descriptions call for professional installation or recommend it. Before buying, ask about installation requirements, maintenance, warranty terms, and any local code requirements. Confirm the exact warranty and included components for the specific model you are considering.

Frequently Asked Questions

What does a tankless gas water heater do?

A tankless heater warms water as it is needed instead of storing a full tank of hot water. These Rheem models are described as on-demand units for whole-home use, but their ability to serve multiple fixtures depends on the model’s capacity and the home’s temperature-rise needs.

Is a higher GPM rating always better?

Not necessarily. A higher rating can support more simultaneous hot-water use under the stated test condition, but it may not be the best fit if your home does not need that capacity. Compare the rating at the temperature rise shown and consider your household’s peak demand.

Which model has the highest stated UEF?

The RTGH natural gas and propane indoor condensing models each list a 0.93 UEF. The outdoor RTG models list 0.82 UEF. The supplied information for the IKONIC does not include a UEF, so check its full specifications before comparing efficiency.

Should I choose an indoor or outdoor tankless heater?

That depends on the space and installation options at your home. The RTG models are designed for outdoor installation, while the RTGH models use indoor direct venting. Have a qualified installer assess the location and venting requirements before you decide.

Does the IKONIC recirculation feature save water?

Its recirculation system is designed to reduce the wait for hot water, which can reduce water running down the drain while you wait. The product information states an average annual water-savings figure, but your results will vary with your home, usage, and system setup.

Can I install one of these heaters myself?

The product descriptions call for professional installation or recommend it. Since installation involves gas and, for indoor models, direct venting, consult a qualified installer who can confirm the right model and installation plan for your home.

Final Recommendation

For a balance of high stated efficiency and whole-home capacity, the Rheem RTGH 9.0 GPM natural gas model is a strong option if your home has natural gas and supports indoor direct venting. Choose the 9.5 GPM RTGH if you use propane and need more capacity, or the outdoor RTG models if outdoor installation better suits your property. If reducing hot-water wait time and using connected controls are priorities, compare the IKONIC’s recirculation features while confirming its full efficiency and warranty details before buying.