The most efficient tankless water heater depends on fuel, household demand, and installation needs. The Rinnai RX160iN has the highest listed efficiency factor at 0.97 UEF, while the Rheem RTGH lists 0.93 UEF. For high flow, compare the MIZUDO and Rinnai RE199iN; for electric operation at several points of use, consider the ORBEK 14kW. The specifications below help distinguish efficiency claims from capacity and installation requirements.
| Product | Fuel | Listed capacity or efficiency | Best suited for |
|---|---|---|---|
| Rinnai RX160iN | Natural gas or propane | 9 GPM; 0.97 UEF | Flexible installation and high efficiency |
| Rheem RTGH | Natural gas | 8.4 GPM at 35°F rise; 0.93 UEF | Multi-bathroom homes |
| MIZUDO | Natural gas | Up to 10.3 GPM | Busy households wanting app control |
| Rinnai RE199iN | Natural gas | Up to 9.8 GPM | Large households needing high flow |
| ORBEK 14kW | Electric | Up to 3.38 GPM | Several nearby points of use |
Rinnai RX160iN Condensing Tankless Heater
Best for: Buyers prioritizing a high listed efficiency factor and fuel or placement flexibility. This condensing model lists 0.97 UEF and up to 9 GPM, with capacity described as serving up to six fixtures at once. Smart Sense supports natural gas or propane, and the unit can be installed indoors or outdoors.
Features include an easy-to-use interface and Quick Flush feature. Choose it if the ability to use either listed fuel and either installation location is useful. The outdoor installation requires an RX-OVC vent cap; WiFi control requires a Controlr module that is not included.
Rheem RTGH 8.4 GPM Natural Gas Heater
Best for: Families looking for an indoor natural-gas model with a stated efficiency figure and multi-shower capacity. The Rheem delivers up to 8.4 GPM at a 35°F temperature rise, using 157,000 BTU per hour. Its listed 0.93 UEF and condensing technology make it a measurable option to compare with other gas models.
The manufacturer describes it as suitable for three to four simultaneous showers and as using up to 34% less energy than a standard 50-gallon electric tank. It uses direct venting and is intended for indoor installation, so it is not the pick for an outdoor installation or a home without a suitable venting plan.
MIZUDO 199,000 BTU Natural Gas Heater
Best for: Busy households that want high stated flow and app control. MIZUDO lists up to 10.3 GPM from a 199,000 BTU input and describes support for hot-water use across multiple bathrooms, kitchen fixtures, laundry, and a dishwasher. Its 0.53 GPM low-flow activation can also suit fixtures with lower flow.
The heater lists ±1°F temperature control, an adjustable range up to 149°F, WiFi and app control, and anti-freeze protection. It is a strong capacity-focused comparison, but the 10.3 GPM figure is a maximum specification, not a guarantee that every home can run all fixtures at that rate. Check demand and installation requirements before selecting it.
Rinnai RE199iN Non-Condensing Heater
Best for: Large households seeking a compact indoor natural-gas unit with a high stated flow rate. This non-condensing model provides up to 9.8 GPM from 199,000 BTU and is described for whole-house use, including showers, laundry, and dishwashing. Its wall-mounted design saves space compared with a bulky storage tank.
An LED display provides diagnostic codes and service reminders. The listing also states a limited warranty of 15 years on the heat exchanger, five years on parts, and one year on labor. This model is non-condensing, so compare its specifications with condensing options if maximizing the listed UEF is your main priority; no UEF is provided in the supplied product details.
ORBEK 14kW Electric Tankless Heater
Best for: Buyers who need an electric unit for a limited number of nearby fixtures rather than maximum whole-house flow. ORBEK lists up to 3.38 GPM, 240V operation, and a stated 98% efficiency. It also advertises hot water in three seconds and ±1°F temperature control.
The manufacturer says the unit can serve two to three points of use, such as a bathroom and kitchen, and claims up to 60% less electricity use than traditional tank heaters. Installation requires a 240V/60A two-pole breaker, one set of 6 AWG wires, 1/2-inch NPT water connections, and a household panel with at least 150 amps total capacity. Avoid it if the electrical system cannot meet these requirements.
Buying Guide: How to Choose an Efficient Tankless Water Heater
Which efficiency number should you compare?
Compare the same type of efficiency measurement when available. UEF is listed for the Rheem (0.93) and Rinnai RX160iN (0.97), making those figures directly useful for comparison. The ORBEK lists 98% efficiency, but that is a different stated measure; do not treat it as directly interchangeable with UEF. The other supplied product details do not list a UEF.
How much hot water does your household need?
Estimate which fixtures may run at the same time, then compare the heater’s flow specification and the temperature rise needed in your area. A higher maximum GPM can help with simultaneous demand, but actual output varies with incoming water temperature and operating conditions. The Rheem’s 8.4 GPM is specifically stated at a 35°F rise; the other listed maximums should not be assumed to apply under identical conditions.
Should you choose gas or electric?
Gas models are the options here with higher stated whole-home flow capacities. The ORBEK is electric and lists lower flow, but may suit a smaller demand or selected points of use. Before buying, confirm available fuel, electrical-panel capacity, venting needs, installation location, and water connections. Have a qualified professional assess requirements that are not clear from the product specifications.
Are smart features and maintenance important?
The MIZUDO lists WiFi and app control, while the RX160iN supports a separately sold Controlr module. The RE199iN lists diagnostic codes and service reminders, and the RX160iN lists a Quick Flush feature. Compare these features with the maintenance and monitoring you want; smart control may require additional hardware.
Frequently Asked Questions
Which listed model has the highest UEF?
The Rinnai RX160iN lists a UEF of 0.97, higher than the Rheem RTGH’s listed 0.93. UEF is not provided for the other products.
Which heater has the highest stated flow rate?
The MIZUDO lists up to 10.3 GPM. Maximum flow alone does not establish output at every temperature rise or installation condition.
Can an electric tankless heater serve a whole house?
It depends on household demand and the model’s capacity. The ORBEK lists up to 3.38 GPM and describes use at two to three points, so compare that flow with simultaneous household needs.
Which model supports natural gas or propane?
The Rinnai RX160iN lists Smart Sense support for either natural gas or propane. Confirm fuel and installation requirements for the specific setup.
Does the Rinnai RX160iN include WiFi control?
The listing says WiFi control uses a Controlr module, which is not included.