Rheem RTEX-18 Review

Read time: 5 minutes Last Updated: 09/18/2019

Rheem RTEX-18

  • High energy efficiency rating at 99%
  • Excellent temperature range consider smaller unit dimensions
  • Twin copper immersion heating chambers
  • Only requires two 40 amp breakers to operate correctly
  • Difficult to mount without professional support
  • Some rough handling during shipping may cause immediate part failures
Dimensions & Capacity:
14.5” x 18.25” x 3.5”
Fuel Type:
Flow Rate & Water Pressure:
4.4 GPM
Heating Capacity:
80 °F to 140 °F
Energy Efficiency Ratings & Certifications:
Price and Warranty:
Under $500, 5-year warranty
Display Type:
Material Composition:
Copper and other non-corrosive metals
Our Review

The Rheem RTEX-18 has flown under the radar in some DIY circles, which is really too bad given that it contains many of the high-end components of its larger cousins without also carrying a heftier price tag. As it stands, this models 4.4 GPM is still plenty capable of serving an average-sized home’s regular hot water needs without running dry even once.

This model’s external digital thermostatic control system is another major credit to its name. Though it looks similar to digital interfaces used on competing models, it provides an unmatched level of integration with this unit’s self-modulating temperature control modules. As such, a Rheem RTEX-18 will always be able to adjust their unit’s output temperature without needing to crack out the owner’s manual every time.

Read more about our how our ratings broke down for the RTEX-18 below or checkout how it compares to other whole house electric water heaters.

Why Might I Want this Specific Tankless Water Heater?

There’s no question about it. The tankless water heater market today is crowded with models in every shape, size, and capacity imaginable (not to mention some of the cheap knockoff models). As such, it’s becoming harder and harder for the average homeowner to succinctly find a tankless water heater to fit their average simultaneous hot water usage needs. Luckily, high-performing midrange models like the Rheem RTEX-18 are still readily available.

Even on its face, Rheem is one of the most trusted brands in the tankless water heater manufacturing business. Thousands of homes across the country make use of their many reliable water heating modules each and every day. Rheem can also claim a fair number of repeat customers, many of whom are repeatedly impressed at the brand’s ability to implement more durable and efficient components into each new model without hiking prices.

The Rheem Rtex-18 fits into this tradition well. For example, with a 4.4 GPM on average, this model is more than capable of efficiently delivering an ample volume of hot water across two to three in-home outlets at once. Moreover, this model’s self-modulating power control makes it easy to use the least amount of electricity while still attaining an optimally satisfying (and safe) water temperature.

Key Specifications and Features to Look for in a Tankless Water Heater

Dimensions & Capacity

Understanding a prospective unit’s dimensions is essential to the installation process, which may be complicated if you don’t leave enough space for all necessary connection lines. While you may expect the Rheem RTEX-18 to have a low capacity as a result of its compact size, but it can actually output quite a few gallons per minute without needing to store any water internally.

Fuel Type

Standard tankless water heaters today are powered by either electricity, natural gas, or propane. While each fuel source has its own advantages, the Rheem Rtex-18 runs on electricity (which is considered to be the most popular due to its simplified installation process).

Flow Rate & Water Pressure

Flow rate (measured in gallons per minute or GPM) describes how much water can pass through a given unit in a minute’s time. This metric also affects water pressure, which is necessary for a sanitary and efficient water distribution system. The Rheem RTEX-18 clocks in at a respectable 4.4 GPM, which is actually higher than average for units in its size class.

Heating Capacity

Heating capacity describes the top temperature to which a unit’s heating components can warm direct ground water. The Rheem RTEX-18 can heat water to temperatures between 80 °F and 140 °F, a range typically only seen in larger or more expensive units.

Energy Efficiency Ratings & Certifications

Some high-performing tankless water heaters today receive an energy efficiency rating or regulatory certifications to further solidify their ability to save users money on their future utility bills. The Rheem RTEX-18 carries a remarkable 99% energy efficiency rating, indicating that only 1% of its overall energy usage is not applied directly to water heating.

Price and Warranty

Of course, price comparisons are always necessary when deciding between a set of similarly capable units. Add-on value, such as the Rheem RTEX-18’s 5-year warranty, should be taken into account when considering the long term investment value of picking a certain unit over another.

Display Type

The most user-friendly tankless water heaters available today come with modern digital displays to make it easy to modify the operational temperature with precision. The Rheem RTEX-18 can be counted among this modern group, while some of its competitors still use antiquated analog interfaces.

Material Composition

Those looking to pick up a tankless water heater for the long-term should certainly opt for a model with proven durability. This includes the use of corrosion-resistant internal components, such as those used in the Rheem RTEX-18.

Different Uses for a Tankless Water Heater

Decentralized In-Home Water Heating

Tankless water heaters see a lot of use among homeowners who are looking to move on from their bulky tank water heater and decentralize their water heating system, in turn. Units like the Rheem RTEX-18 make this easy due to their modular nature, which allows them to be installed in a heavy use location such as a kitchen or bathroom. This keeps the desired output point and production point close together, providing more convenience and temperature consistency overall.

Cutting Water and Electric Bills

Tankless water heaters like the Rheem RTEX-18 are also prized by their owners for the manner in which they immediately save homeowners on their utility bills. This is primarily the lack of tank on a tankless water heater. Without, the unit doesn’t store water that isn’t being used, nor is it heating said stored water. As a result, your future electricity and water bills will feel more commensurate with your realistic usage.