A high-efficiency hot water heater can reduce standby energy use by heating water when you need it, rather than keeping a tank hot all day. The right choice depends on more than efficiency: fuel type, indoor or outdoor placement, hot-water demand, and the temperature of your incoming water all affect how well a tankless model will work in your home. These five options range from large-capacity Rheem units to a more compact ThermoMate model, with different installation needs and flow rates.
Table of Contents
Quick Comparison
| Product | Fuel | Installation | Maximum flow rate | Efficiency detail |
|---|---|---|---|---|
| Rheem RTG 9.5 GPM Natural Gas Outdoor | Natural gas | Outdoor | 9.5 GPM at a 35°F rise | 0.82 UEF |
| Rheem RTGH 9.5 GPM Propane Indoor | Liquid propane | Indoor, direct vent | 9.5 GPM at a 35°F rise | 0.93 UEF |
| Rheem RTG 7.0 GPM Propane Outdoor | Liquid propane | Outdoor | 7.0 GPM at a 35°F rise | 0.82 UEF |
| ThermoMate CZ422NG Natural Gas Indoor | Natural gas | Indoor | Up to 5 GPM | 0.81 UEF; listing also states up to 98% thermal efficiency |
| Rheem RTGH 9.0 GPM Natural Gas Indoor | Natural gas | Indoor, direct vent | 9.0 GPM at a 35°F rise | 0.93 UEF |
Tankless flow rates depend on the temperature rise needed and how many fixtures are running at once. Check the product details and have a qualified installer confirm the right size and setup for your home.
Our Picks: Best High Efficiency Hot Water Heater reviews
1. Rheem RTG 9.5 GPM Natural Gas Outdoor: Best for High Demand
This outdoor natural gas tankless heater is the highest-flow option in this group, with a listed capacity of 9.5 GPM at a 35°F temperature rise and an output of 199,900 BTU per hour. Rheem says it can support three to four simultaneous showers under the stated conditions, making it a candidate for larger households with several bathrooms.
Its outdoor design avoids the need for indoor venting, and the metal cabinet is made for wall mounting. The digital control allows temperature adjustments in 1°F increments. The listed 0.82 UEF is lower than the 0.93 UEF of the two indoor condensing Rheem models below, so this unit may suit someone who values high flow and outdoor placement over the highest UEF in this selection.
Before choosing it, confirm that your climate and installation location are suitable for an outdoor heater, and ask a qualified professional to check gas supply and sizing. The product listing states that professional installation is required and includes freeze protection, but outdoor exposure and local conditions still matter.
Pros:
- High listed flow rate for a whole-house model
- Outdoor installation avoids indoor venting
- Digital temperature control adjusts in 1°F increments
- Listed 15-year heat exchanger and 5-year parts warranties
Cons:
- Listed 0.82 UEF is below the 0.93 UEF of the condensing Rheem models
- Requires a suitable outdoor location and professional installation
Verdict: Choose this model if you have natural gas, want a high-capacity outdoor heater, and can verify that its flow rate fits your home’s demand.
2. Rheem RTGH 9.5 GPM Propane Indoor: Best for High Capacity and UEF
This indoor Rheem runs on liquid propane and combines a listed 9.5 GPM flow rate at a 35°F rise with a 199,900 BTU-per-hour output. Its 0.93 UEF is the highest listed value among the five choices here. Rheem describes it as a condensing model with direct venting, intended for whole-home use and larger households.
The digital control adjusts the set temperature in 1°F increments. The product information also lists freeze protection, multiple safety features, CSA certification, a 15-year heat exchanger warranty, and a 5-year parts warranty. Its indoor placement makes it important to plan for the required venting and the space needed for installation.
This is not a natural gas model, so it is only a fit for homes set up for liquid propane. Its high flow rating is tied to a 35°F rise; colder incoming water or several fixtures running together can change the usable flow. Have a professional confirm that the unit’s capacity and venting requirements fit your home.
Pros:
- Listed 0.93 UEF
- High 9.5 GPM flow rate at a 35°F rise
- Direct-vent indoor design
- Listed 15-year heat exchanger and 5-year parts warranties
Cons:
- Uses liquid propane rather than natural gas
- Indoor installation requires suitable direct venting
Verdict: This is a strong option for a propane-equipped home that needs high flow and wants the highest listed UEF in this lineup.
3. Rheem RTG 7.0 GPM Propane Outdoor: Best for Moderate Whole-Home Demand
This outdoor propane model is rated for 7.0 GPM at a 35°F rise, with a 160,000 BTU-per-hour output. Rheem lists it for two to three simultaneous showers under the specified conditions. Its lower capacity than the 9.5 GPM models may make it a better match for a home with more moderate hot-water demand.
The non-condensing design has a listed 0.82 UEF. Since it installs outdoors, it avoids indoor venting, while its metal cabinet is designed for wall mounting. Temperature adjustments are made through a digital LED control, and the listing includes freeze and flame-failure protection. This model also has the same listed warranty terms as the other Rheem options: 15 years for the heat exchanger and 5 years for parts.
Make sure your home uses propane and that the heater is sized for your climate and number of fixtures. The product description contains conflicting fuel references, so confirm the exact fuel connection and installation requirements with the manufacturer or installer before ordering.
Pros:
- Outdoor installation does not require indoor venting
- 7.0 GPM listed flow rate for moderate household demand
- Digital temperature control and listed safety protections
- Listed 15-year heat exchanger and 5-year parts warranties
Cons:
- Lower listed flow rate than the 9.0 and 9.5 GPM Rheem models
- Listed 0.82 UEF is below the indoor condensing models’ 0.93 UEF
Verdict: Consider this model if you have propane, prefer outdoor installation, and do not need the highest flow capacity in this group.
4. ThermoMate CZ422NG 5 GPM Natural Gas Indoor: Best Compact Option
The ThermoMate CZ422NG is a smaller indoor natural gas tankless heater with a listed maximum flow of 5 GPM and an output of 102,000 BTU. The listing says it can serve two showers at once under its stated conditions, including a 105°F outlet temperature with 65°F inlet water. Its dimensions are listed as 21 inches high, 13 inches wide, and 7 inches deep, which can help when installation space is limited.
Its controls include a 3.5-inch LED display with a stated temperature range of 90–140°F. The product information lists a copper heat exchanger, a 0.81 UEF, and up to 98% thermal efficiency. Those are different efficiency measures, so compare the UEF with other units when considering energy performance. It also lists several safety protections and a 120V power connection.
At 5 GPM, this model has less capacity than the larger Rheem units. It may fit a smaller household or a home with more modest simultaneous hot-water use, but the actual flow depends on inlet temperature and demand. The listing specifies indoor installation and a vent connection, so installation should be checked by a qualified professional.
Pros:
- Compact listed dimensions for an indoor installation
- Natural gas fuel and up to 5 GPM listed flow
- LED display with a 90–140°F temperature setting range
- Product information lists multiple safety protections
Cons:
- Lower maximum flow than the Rheem whole-home models
- Indoor venting and a 120V power connection are required
Verdict: Choose the ThermoMate if you need a compact natural gas unit and its 5 GPM capacity matches your household’s likely demand.
5. Rheem RTGH 9.0 GPM Natural Gas Indoor: Best High-Flow Natural Gas Pick
This indoor Rheem model uses natural gas and is rated at 9.0 GPM at a 35°F temperature rise, with an output of 180,000 BTU per hour. Its listing describes condensing technology, direct venting, and a 0.93 UEF. Rheem says it is designed for whole-home use and can support three to four simultaneous showers under the listed conditions.
The digital LED temperature control adjusts in 1°F increments. The product information also lists freeze protection, multiple safety features, CSA certification, and warranty coverage of 15 years for the heat exchanger and 5 years for parts. Compared with the 9.5 GPM propane model, this one offers slightly less listed flow but uses natural gas, which may be the more convenient fuel for homes already connected to it.
It is an indoor unit, so direct-vent installation must be planned and checked before purchase. As with any tankless heater, the listed flow is not a guarantee of the same output in every home; incoming water temperature and simultaneous use affect performance.
Pros:
- Listed 0.93 UEF
- High 9.0 GPM flow rate at a 35°F rise
- Uses natural gas and has a direct-vent indoor design
- Listed 15-year heat exchanger and 5-year parts warranties
Cons:
- Requires indoor direct-vent installation
- Listed flow is slightly below the 9.5 GPM Rheem options
Verdict: This is a sensible high-capacity choice for a natural-gas home that can accommodate indoor direct venting.
How to Choose a High-Efficiency Hot Water Heater
Match the fuel to your home
Start with the fuel available at your property. The options here include natural gas and liquid propane, and they are not interchangeable without the right appliance and setup. Confirm the exact fuel type on the product listing and have an installer check that your supply and connections are appropriate.
Size for flow and temperature rise
Look at both the flow rate and the temperature rise used to measure it. Several models list their GPM at a 35°F rise, while the ThermoMate gives a different example based on its stated inlet and outlet temperatures. If incoming water is colder, the heater must raise its temperature more, which can reduce the flow available. Consider how many showers, sinks, and appliances may run at the same time, then ask a qualified installer to size the unit for your conditions.
Compare UEF values carefully
Uniform Energy Factor, or UEF, is a useful way to compare listed efficiency. Among these choices, the two indoor Rheem condensing models list a 0.93 UEF, while the outdoor Rheem models list 0.82 and the ThermoMate lists 0.81. A higher UEF does not by itself tell you the total cost to operate: fuel rates, hot-water use, and installation all matter. The ThermoMate listing also gives a thermal efficiency figure; do not treat that measure as the same thing as UEF.
Choose indoor or outdoor placement
Outdoor units can avoid indoor venting, but they need an appropriate location and protection that fits local conditions. Indoor models require venting, and some need a direct-vent setup. Check the manufacturer’s installation instructions and local requirements before you choose based on convenience alone.
Check electrical and installation needs
Gas tankless heaters can still require electricity for controls or other components. The ThermoMate listing specifies a 120V connection. Ask the installer to confirm electrical access, gas capacity, water connections, venting, clearances, and any other requirements for your specific model. The product details recommend professional installation for the Rheem units as well.
Review warranty and safety details
Compare warranty coverage for the heat exchanger and parts, and read the conditions that apply to each. The Rheem models listed here include a 15-year heat exchanger warranty and 5-year parts warranty. Safety features differ by product, so check the manufacturer’s full documentation rather than relying only on a short feature list.
Frequently Asked Questions
What does UEF mean on a water heater?
UEF stands for Uniform Energy Factor. It is an efficiency measure that helps compare water heaters. A higher listed UEF indicates greater efficiency under the rating method, but actual energy use also depends on your household’s hot-water demand and local fuel costs.
Is a tankless water heater always more efficient than a tank model?
Not in every situation. Tankless units heat water on demand and do not store a full tank of hot water, but efficiency and operating cost depend on the model, fuel, installation, and how your household uses hot water. Compare the unit’s UEF and get an estimate based on your home.
How many showers can a 9.5 GPM model run at once?
The Rheem listings say the 9.5 GPM models can support three to four simultaneous showers at a 35°F rise. That is a stated capacity under specific conditions, not a promise for every household. Inlet water temperature and other fixtures running at the same time affect available flow.
Should I choose an indoor or outdoor unit?
Choose based on your home’s layout, climate, and installation options. An outdoor model avoids indoor venting but needs a suitable exterior location. An indoor model needs an approved venting setup and enough space. A qualified installer can help determine which placement works safely.
Can I install a gas tankless water heater myself?
These models have requirements for gas, water, venting, and sometimes electrical connections. The product information recommends or requires professional installation for the Rheem units. Use a qualified installer who can check local codes and the product’s installation instructions.
Which model is best for a smaller household?
The ThermoMate’s listed maximum is 5 GPM, below the other models in this selection, so it may be worth considering when household demand is more modest. Still, size the unit based on simultaneous use and incoming water temperature rather than household size alone.
Final Recommendation
For a home that needs high flow and has natural gas, the Rheem RTGH 9.0 GPM indoor model combines a strong listed capacity with a 0.93 UEF. If your home uses propane, the Rheem RTGH 9.5 GPM indoor model offers the highest listed flow and UEF in this group. For an outdoor setup, compare the Rheem RTG models against your fuel supply and household demand. Whichever you choose, have an installer confirm sizing, fuel, venting, and placement before you buy.
