The most efficient tankless water heater depends on your fuel, incoming water temperature, and how many fixtures you want to run at once. Among these options, the Rinnai RX199iN lists the highest Uniform Energy Factor (UEF) at 0.98. The Rheem propane model is also a strong high-efficiency choice, while the compact Orbek electric unit may suit homes with the right electrical capacity. Compare efficiency alongside flow rate and installation needs before choosing.
Table of Contents
Quick Comparison: Efficient Tankless Water Heaters
| Product | Fuel | Maximum Listed Flow | Efficiency Information | Best Fit |
|---|---|---|---|---|
| Rheem RTG Series 7.0 GPM Natural Gas | Natural gas | 7.0 GPM at a 35°F rise | 0.82 UEF | Homes needing indoor whole-house hot water |
| Rinnai RX199iN Condensing Smart Sense | Natural gas or propane | Up to 11.1 GPM | 0.98 UEF | Large households seeking high efficiency and flow |
| Orbek 14kW Electric Tankless | Electric | Up to 3.38 GPM | Advertised as 98% efficient | Smaller hot-water needs with adequate electrical capacity |
| Rheem RTGH Series 9.5 GPM Liquid Propane | Liquid propane | 9.5 GPM at a 35°F rise | 0.93 UEF | Large homes using propane |
| ThermoMate Condensing Natural Gas | Natural gas | Up to 6 GPM | 0.86 UEF | Smaller households wanting a condensing gas model |
Flow rates and efficiency figures come from the supplied product specifications. A heater’s actual output can vary with incoming water temperature, installation, and household demand. The Orbek’s advertised efficiency percentage is not a UEF figure, so it cannot be compared directly with the UEF ratings above.
Our Picks: Best Most Efficient Tankless Water Heater reviews
Rheem RTG Series: A Whole-House Natural Gas Option
This indoor natural gas model is designed for whole-house use. Its listed capacity is 7.0 GPM at a 35°F temperature rise, with 160,000 BTUs of output. Rheem says that flow can support two to three simultaneous showers, though the real number of fixtures depends on how cold your incoming water is and how much hot water each fixture uses.
The unit has a wall-mount design, a digital control panel with temperature adjustments to within ±1°F, and built-in freeze and flame-failure protection. It is a non-condensing model with a listed 0.82 UEF. That is lower than the UEF figures for the Rinnai, Rheem propane, and ThermoMate models in this roundup, so this is not the best pick if your main goal is the highest listed efficiency.
It may make sense if your home already uses natural gas and you want a whole-house heater with a moderate-to-high flow rating. Before buying, confirm the unit’s gas, venting, and installation requirements with a qualified installer. The supplied specifications recommend direct venting and professional attention to the installation.
Pros:
- 7.0 GPM listed flow at a 35°F temperature rise
- Indoor, wall-mounted design saves floor space
- Digital temperature control with ±1°F adjustment
- Includes freeze and flame-failure protection
Cons:
- Its 0.82 UEF is below several other models here
- Indoor gas installation and venting need careful planning
Verdict: Choose this Rheem if you want an indoor natural gas whole-house unit, but consider the higher-UEF choices if energy efficiency is your top priority.
Rinnai RX199iN: Highest Listed UEF and High Flow
The Rinnai RX199iN is the efficiency leader in this group by its listed UEF: 0.98. It is a condensing gas heater rated for up to 11.1 GPM, with the product description saying it can serve as many as seven fixtures at once. That is a maximum figure, not a guarantee for every home; colder incoming water and higher demand can reduce the flow of hot water.
Its Smart Sense gas control system supports natural gas or propane, and the unit is described as suitable for indoor or outdoor installation. Outdoor installation requires the RX-OVC vent cap. The compact, suitcase-sized design may help where space is tight, and the Quick Flush feature is intended to simplify flushing. Wi-Fi control requires a separate Controlr module.
This model has the strongest combination of listed UEF and maximum flow among the options here, but it is a more involved installation than a simple appliance swap. Rinnai recommends professional installation. As a condensing heater, it also requires proper handling of condensate; confirm drainage and venting details with the installer and product instructions.
Pros:
- Highest listed UEF in this comparison at 0.98
- Up to 11.1 GPM listed flow for higher household demand
- Supports natural gas or propane
- Indoor or outdoor installation options, with the specified outdoor vent cap
Cons:
- Professional installation is recommended
- Wi-Fi control requires a separate module
Verdict: The Rinnai is the best match if you want the highest listed UEF and substantial flow, provided its installation requirements and fuel setup fit your home.
Orbek 14kW: Compact Electric Heating for Limited Demand
The Orbek is a compact 240-volt electric tankless heater rated for up to 3.38 GPM. Its small cabinet can fit in tighter spaces than a storage tank, and it has an LED display for setting water temperature from 86°F to 140°F. The company advertises 98% efficiency, but that figure is not presented as UEF, so it should not be treated as directly equivalent to the gas models’ UEF ratings.
Electrical capacity is the main decision point. The supplied requirements call for a 240V, 60-amp, two-pole breaker, one set of 6 AWG wires, and a household panel with at least 150 amps of total capacity. Have an electrician check your panel and wiring before purchase. The listed maximum flow also depends on the temperature rise needed, so a colder climate or several simultaneous hot-water draws may be a poor match.
The unit includes protection for leakage, overheating, and dry heating, and the product information says it is ETL certified. It may suit a small home, a point of use, or a household with modest hot-water demand and sufficient electrical service. Its claimed ability to supply two to three points should be assessed against your actual inlet temperature and flow needs.
Pros:
- Small, space-saving electric design
- LED temperature display with a broad setting range
- Listed maximum flow of up to 3.38 GPM
- Includes several stated safety protections
Cons:
- Requires a 60-amp, 240-volt circuit and adequate panel capacity
- Maximum flow may be limiting for larger households or colder incoming water
Verdict: Consider the Orbek for modest hot-water needs only after confirming that your electrical panel can support its stated requirements.
Rheem RTGH Series: High-Capacity Propane Heating
This indoor liquid propane model is built for higher whole-house demand. Its listed output is 199,900 BTUs, and its maximum flow is 9.5 GPM at a 35°F temperature rise. Rheem describes it as suitable for three to four simultaneous showers, but your results will depend on incoming water temperature and the flow rate of your fixtures.
The unit uses condensing technology and has a listed 0.93 UEF, making it a higher-efficiency option than the non-condensing Rheem RTG model in this roundup. Its digital control adjusts temperature to within ±1°F, and the product description lists freeze protection and multiple safety features. The manufacturer also lists a 15-year heat exchanger warranty and a five-year parts warranty.
This is a logical choice for a larger home that uses liquid propane and needs more flow than the ThermoMate or Orbek provides. It is not the top listed UEF option—that is the Rinnai—but it offers a strong combination of propane compatibility, capacity, and efficiency. Confirm installation, venting, condensate handling, and warranty terms with a qualified professional before committing.
Pros:
- High listed flow of 9.5 GPM at a 35°F rise
- 0.93 UEF condensing design
- Designed for larger households with multiple hot-water draws
- Digital temperature control and listed freeze protection
Cons:
- Uses liquid propane rather than natural gas
- Indoor installation calls for professional planning
Verdict: Pick this Rheem if your home uses propane and needs high flow; choose the Rinnai instead if the highest listed UEF is your main goal.
ThermoMate Condensing: Mid-Range Natural Gas Capacity
The ThermoMate is a condensing natural gas heater rated for up to 6 GPM and 114,000 BTUs. Its listed UEF is 0.86. The product description says it can serve two showers at 2 GPM each when the incoming water is 65°F and the output is 105°F. That example is useful for context, but colder inlet water or more simultaneous demand can lower the available flow.
Controls include a 3.5-inch touchscreen for adjusting the output from 90°F to 140°F and viewing operating information. The product information lists several safety protections and freeze protection that activates at 41°F. Because this is a condensing heater, it produces mildly acidic condensate; the supplied description recommends draining it and suggests a condensate neutralizer kit, which is not included.
Its 6 GPM maximum puts it below the higher-capacity Rinnai and Rheem propane models, but above the Orbek’s listed maximum flow. It may suit a smaller household seeking a natural gas condensing unit, as long as the installation can accommodate venting and condensate drainage. The 0.86 UEF is a useful comparison figure, though it is below the Rinnai and Rheem propane models’ listed UEF.
Pros:
- Condensing design with a listed 0.86 UEF
- Up to 6 GPM listed flow
- Touchscreen displays temperature and operating information
- Includes stated freeze and other safety protections
Cons:
- Condensate drainage is required, and a neutralizer kit is not included
- Maximum flow is lower than the Rinnai and Rheem propane models
Verdict: The ThermoMate is worth considering for a smaller natural gas household that wants a condensing unit and can plan for condensate disposal.
How to Choose an Efficient Tankless Water Heater
Match the Fuel to Your Home
Start with the fuel already available and the fuel you want to use. The Rheem RTG and ThermoMate are natural gas models, the Rheem RTGH uses liquid propane, and the Orbek runs on electricity. The Rinnai supports natural gas or propane according to its product information. Fuel changes can add cost and installation work, so confirm compatibility before choosing based on efficiency alone.
Compare UEF Carefully
Uniform Energy Factor, or UEF, provides a standard efficiency measure for comparing many water heaters. Among the listed UEF values here, the Rinnai is highest at 0.98, followed by the Rheem propane at 0.93, ThermoMate at 0.86, and Rheem natural gas at 0.82. Orbek’s advertised 98% efficiency is not identified as UEF, so it is not an apples-to-apples comparison. A higher rating can indicate greater efficiency, but your fuel cost, usage, and installation also affect operating expense.
Size for Simultaneous Hot-Water Use
Think about what may run at the same time, such as showers, a dishwasher, or a sink. Compare that combined demand with the heater’s flow rating, but pay attention to the stated temperature rise. A flow figure at a 35°F rise does not guarantee the same output when the incoming water is colder. Check sizing guidance for your location and household rather than relying only on a maximum GPM number.
Check Electrical and Gas Requirements
Tankless heaters can have substantial utility requirements. The Orbek needs a 240-volt, 60-amp, two-pole breaker, 6 AWG wiring, and a panel with at least 150 amps of total capacity, according to its supplied specifications. Gas units need a compatible fuel supply and proper venting. Have a qualified electrician or plumber check your home before ordering so you can account for upgrades and installation constraints.
Plan for Venting and Condensate
Installation details vary by model. The Rheem natural gas unit is listed for indoor installation with direct venting, while the Rinnai can be installed indoors or outdoors with its specified outdoor vent cap. Condensing models also need a plan for condensate. The ThermoMate product description specifically says condensate must be drained and recommends a neutralizer kit. Follow the selected model’s installation instructions and local requirements.
Consider Controls, Protection, and Service
Temperature controls, freeze protection, diagnostic displays, and flushing features can make ownership more convenient, but they do not replace proper sizing or maintenance. Compare the features that matter to your home, and ask an installer about service access, flushing, warranty conditions, and required maintenance before deciding.
Frequently Asked Questions
Which tankless water heater has the highest listed UEF?
The Rinnai RX199iN has the highest listed UEF in this comparison at 0.98. UEF is only one factor; fuel costs, installation, and your hot-water demand also affect overall value.
Is a tankless water heater always more efficient than a tank model?
Not necessarily in every home or situation. Tankless units heat water on demand, but real-world energy use depends on how much hot water you use, the unit’s efficiency, and the cost of electricity or gas. Compare operating costs for your home rather than assuming one type will always cost less.
How do I know what flow rate I need?
Add the flow needs of the fixtures you expect to use at the same time, then compare that total with the heater’s output at the temperature rise your home requires. A unit’s maximum GPM may not apply when the incoming water is cold or several fixtures are running.
Can I install a tankless water heater myself?
Installation can involve gas or high-capacity electrical connections, venting, water connections, and, for condensing models, condensate drainage. The Rinnai product information recommends professional installation. Check the specific product instructions and local requirements, and use qualified professionals where needed.
Does the Orbek’s advertised 98% efficiency mean it is more efficient than the gas models?
Not based on the figures provided. The Orbek’s 98% claim is not identified as UEF, while the gas models list UEF ratings. Because the figures use different or unclear measures, they cannot be compared directly from the supplied information.
Do condensing tankless heaters need a drain?
Condensing models produce condensate that must be managed. The ThermoMate description specifically calls for draining mildly acidic condensate and recommends a neutralizer kit. Check the installation instructions for whichever model you select.
Final Recommendation
For the highest listed UEF and the greatest listed flow in this group, the Rinnai RX199iN is the strongest all-around efficiency pick, assuming its fuel and installation requirements suit your home. The Rheem propane model is a compelling alternative when you need high output and use liquid propane. For smaller needs, the ThermoMate offers a mid-range natural gas option, while the Orbek is best reserved for homes with modest demand and enough electrical capacity. Confirm sizing, utility requirements, and installation costs before making your final choice.
