Rheem RTEX-18 Review – Everything You Need To Know

One could take one of two routes to get the best tankless heater for their home. The first includes browsing the internet for the best-rated models, checking out their specs, reading the reviews left behind by their users, and purchasing the model that performs above par on all counts.

The second route includes checking out our Rheem RTEX-18 review. That is it. This route is so small because this model is one of the top-rated on the market. Its features, which include digital temperature control, low energy consumption, and an endless supply of hot water, are the reasons why.

Read on to check out our unbiased review of Rheem RTEX-18. 

Rheem RTEX-18 Review – Pros and Cons

Rheem RTEX-18
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Here are the positive and negative points of Rheem RTEX-18:


  • Advanced self-modulation
  • Durable copper heating elements
  • Has a digital temperature display


  • Only a 1-year warranty on parts

Rheem RTEX-18 Review – Features

Here are the worth-noting features of Rheem RTEX-18:

  • Advanced self-modulation

One of the best features of this model is its ability to self-modulate. It takes into account your water usage and then increases/decreases its energy consumption based on the same. Let us explain in simple terms how it’s able to do that.

This tankless heater is equipped with an output regulator and a temperature sensor. The former takes into account the hot water demands of your household over a specific time period. It then relays that information to the latter which adjusts power to meet the demand accordingly.

  • External digital thermostatic control

You can gauge the user-friendly nature of this model by merely adjusting its temperature through the external digital thermostatic control. That is one feature that you don’t usually get in budget models, as they give you a knob to increase/decrease their temperature.

Rheem RTEX-18 is different. It combines digital controls with LED display to make operating this model a seamless experience for its users. You can increase or decrease the temperature in increments of 1*F right from the top right corner of this machine. Isn’t that great!

  • Variable flow rate

Most tankless water heaters you see on the market offer temperature increase that varies with the water you’re getting out of them. That’s also the case with Rheem RTEX-18. You can count on this model to increase your cold water temperature by up to 82*F at a flow rate of 1.5GPM. 

Increase the flow rate any further and the temperature increments would start getting smaller. For instance, if you’re operating this model at its full flow rate of 4.4 gallons per minute, the temperature difference between the water entering this unit and the one leaving it would be <30*F.

  • Activates on low flow rates

Unlike some other models whose reviews you can find on our website, the RTEX-18 doesn’t require you to open all hot water taps for it to start working. This model resumes the heating operation the moment your hot water demand touches the 1GPM mark.

This feature of the RTEX-18 offers you two benefits. The first is in the form of water savings as you are no longer bound to consume more hot water than you need just to turn the heater ‘on’. The second benefit comes in the form of energy savings. We have explained it in more detail below. 

  • Energy-efficient operation

Rheem models are known worldwide for their energy-efficient operation. Their residential models are especially famous for the way they dial down their energy consumption while operating at full flow. You might find it useful to know then that this model isn’t any different. 

All the unique selling points of this tankless water heater that we described above combine to make it 99.8% energy efficient. That means that the upward pressure that it would place on your monthly electricity bills would be close to negligible. 

  • Durable heating element

Veteran plumbers tell us that any heater – tankless or tank – is as good as the sturdiness of its heating element. Models that cost astronomically upfront but have a durable heating element should be preferred over those that have a smaller price tag but lackluster heating element, as per them.

Thankfully for your brain cells, the RTEX-18 doesn’t force you to make an either/or decision. It does that by offering the best of both worlds. This tankless heater has two durable copper heating elements which, although rugged, haven’t jacked up its asking price. 

  • Easy to install

Here’s a tip for users who are going to buy a tankless water heater for the first time: check out its bottom connections if you want to know how easy or otherwise the installation procedure would be. Models that have ¾-inch NPT water connections at the bottom are those that are the easiest to install.

The Rheem RTEX-18 offers exactly these connections. Though the universal fit of the connections doesn’t mean you can install the heater on your own. You might end up voiding the warranty if you decide to do that. Leave it to a licensed plumber to take advantage of these standard connections. 

  • Multiple warranties

Here’s one area where this model has disappointed us. Its warranty periods are present, but their length isn’t the best on the market. That isn’t something we didn’t expect – as most Rheem models have the same warranty period. Still, it’s something we think Rheem should look to improve upon.

The 5-year warranty on the heating chamber might look great on paper but isn’t that huge in normal practice. Then comes the laughable 1-year parts warranty which, given the asking price of this model, isn’t enough by any stretch of the imagination. 

Final Verdict

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Our review of the Rheem RTEX-18 tells you everything you need to know about this highly-advanced tankless water heater. It delivers on most of the promises it commits and goes above and beyond them with its self-modulation, digital temperature control, and display and durable copper elements. Don’t think you can ask more from a model intended for residential usage. 

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Rheem RTEX 18 tankless water heater
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