Rheem RTEX 18 Review:
Electric Tankless Water Heater Review and Buying Tips

  • Reviews: 4.5 out of 5 from 775+ global Amazon ratings

  • Power: 18 kW/140 degrees

  • GPM: Up to 4.4 GPM

  • Efficiency: 99.8%

  • Type: POE | 240V | 2x 40A breakers

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Rheem RTEX models are small electric tankless heaters that are part of the brand’s Professional Classics series. These patented water heating systems deliver hot water using reliable heating elements to produce an unlimited supply in both point-of-use and whole-house applications.

See our full rundown of the Best Electric Tankless Water Heaters.

Rheem RTEX 18 Tankless Water Heater

Despite their awesome reputation, not all tankless water heaters are energy efficient or provide continuous hot water. However, the RTEX-18 has garnered lots of interest and positive reviews, becoming one of the most popular water heaters online.

We wrote this review by searching through hours of content on the Rheem RTEX series. We’ve compiled product specifications on key features like water flow and heat output, previous customer reviews, and the opinions of expert reviewers to bring you a comprehensive verdict on this electric tankless water heater.

The Rheem RTEX-18 is available from Amazon with Prime shipping in the United States. Check out the product detail page for more information.

Rheem RTEX-18: Overview

The Rheem RTEX-18 is a highly durable heater that makes for a sensible investment. It’s designed with Rheem’s trademark reputation for strength and efficiency and should operate for long periods of time without issues for most consumers.

Rheem’s tankless heaters feature impressive self-modulating technology that ensures the copper heating elements don’t heat above the desired water temperature. It also means that the heater can deal with periods of high demands, by rationing supply.

Users can achieve flow rates from 2 gallons per minute all the way up to 4.4 GPM in ideal conditions, which is enough for an average-sized home and to urn two appliances simultaneously.

Rheem supplies their heater with a 5-year Guarantee which reflects the brand’s confidence that your unit will last a long time. Finally, the redesigned digital user interface and LED display means that programming the heater is a cinch.

Heater features:

  • Minimal maintenance.
  • No stored water tanks, stale hot water, or heat loss.
  • Brass-topped copper heating elements for increased durability
  • Thread-mounted heating elements that are easy to remove and replace.
  • A 5-year warranty and 1 extra year for electrical components
  • Bottom 3/4″ NPT water connections
  • Dial Control with adjustable digital temperature display

Energy efficiency ratings/certifications:

  • The Rheem RTEX-18 achieves a highly impressive 99.8% energy efficiency (among the best tankless electric water heaters).
  • Only 1% of total energy consumed is not spent on water heating directly.
  • Electric tankless whole-house models are renowned for their efficiency.

Rheem RTEX-18 certifications:

Intertek ETL C US Electric Autodesk No Lead UL

Flow rate/water pressure:

The Rheem RTEX series can easily keep up with hot water demand thanks to its high flow rate to power ratio. The Rheem RTEX-18 clocks in at a respectable 4.4 GPM, which is higher than average for units in its size category. Even when used in cold climates, most people should still be able to receive above 2 GPM, which is enough to run a single appliance at a time without function loss.

Heating capacity:

Like the majority of other high-quality whole house water heaters, The Rheem RTEX-18 is can heat water at temperatures up to 140 degrees. Heating capacity defines the surface temperature of a building unit where the heaters can warm direct water.

Fuel type:

As an electric water heater, The Rheem RTEX- 18 runs on mains electricity. At 18 kilowatts, it can be powered with a single power source but requires 240 volts and 2x 40 amp circuit breakers at a minimum to function without the risk of dimming lights or blown fuses. As an electric unit, the RTEX doesn’t require venting.

Heater applications:


  • Bathroom sink, showers, and tubs
  • Kitchen sinks and dishwashers
  • Detached buildings, cabins
  • workshops and barns


  • Boat kitchens and bathrooms
  • RV kitchens and bathrooms


  • Small offices, office lunchrooms, and restrooms
  • Foodservice sinks
  • Hotel kitchens and bathrooms

Display type:

Rheem RTEX-18 is among the most modern-looking tankless heaters. An LED digital display helps control and change operating temperature with precision. This is a worthwhile shopping factor, as some competitors to the Rheem still use antiquated analog interfaces.

Material Composition:

The Rheem RTEX 18 has proved durable across its lifecycle. The brand uses anti-corrosive internal parts including robust copper immersion heating elements with brass tops.


Customer reviews for Rheem RTEX-18: Highlights

  • DIY reviewers report few issues when installing the Rheem RTEX-18, as long as you are comfortable with fitting water connections and additional circuit breakers if necessary.
  • Many comments highlighting the Rheem RTEX’s ability to run two or more fixtures including a hot shower, washing machine, and kitchen sink at the same time.
  • Reaching 4.4 GPM requires ideal incoming water temp, but even at a lower flow, the Rheem is still capable of serving weekly hot water needs.
  • The external digital thermostat of this model is large and easy to read.

Customer reviews: Any downsides?

  • Some customers say their device had temperature fluctuations after 1-year of use. (This may be due to a corroding heating element).
  • The thermostat may show slight inaccuracies for some users.
  • If you aren’t very familiar with installing heating or electrical equipment, then the tankless water heater is somewhat difficult to install. Always use a licensed electrician if unsure.
  • Some customers report increases in hot water wait times up to one minute after long periods of use. (This can usually be fixed by replacing the elements in the water heater).

Read all reviews for the Rheem RTEX-18 on the Amazon product page >

Brand reputation: Are Rheem tankless water heaters any good?

Rheem is one of the most recognizable names in tankless heaters, having pioneered new heating technologies since the 1930s. All Rheem RTEX electric water tanks are covered with a 5-year warranty and an additional 1 year for electrical components.

Installation: Product manuals and guides

What size breaker do I need for a Rheem tankless water heater?

  • Rheem recommends using two 40 amp circuit breakers for the RTEX-18.
  • A single 60 amp breaker may also work.

What temperature should a Rheem tankless water heater be set at?

  • With no standby heat loss, you can set the water temperature at your family’s preference without wasting money
  • Rheem estimates bill savings of up to 34% when the heater is set at the same temperature as a previous tank installation.
  • Savings can increase to 50% when the RTEX-18 is set up as a point-of-use heater.

How to install a Rheem electric tankless water heater

Other Rheem RTEX tankless electric water heaters

Tankless electrical hot water heaters from the RTEX series are designed to be used as either indoor point-of-use (POU) models for one fixture, or point-of-entry (POE) models to heat water for a whole house.

Here’s the full Specification Sheet on the RTEX heater range.

Rheem currently produces the RTEX series in multiple sizes, with power outputs from 3 kW to 36 kW. Depending on their size, heaters can function at between 120 and 240V and from 29 to 54 Amps (RTEX-4 is the only electric and tankless Rheem heater that can run on 120V).

The largest RTEX tankless electric water heater carries four heat exchangers, which are capable of producing hot water at a very high flow rate of 8.8 GPM. All units have digital thermostatic controls that enable precise temperature controls from 80 – 140 Fahrenheit.

Bottom line – Should I buy this specific tankless water heater?

Rheem is the most trusted brand in tankless water heaters, with the RTEX-18 being the most popular heater in the RTEX range. Over 250,000 American homes utilize this reliable unit every day. The RTEX-18’s self-modifying technology gives you easy access to hot water without wasting energy, while brass-topped heating coils help keep your system running at an optimal level for longer. We think the RTEX-18 is a good fit for most homes with 2 bathrooms.

Read more details about Rheem Tankless Electric Heaters on the Brand’s Amazon page >