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Rheem

Rheem 18kW 240V Tankless Electric Water Heater, Gray

  • Based on 2,521 reviews
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Style: Whole Home, 18 kw


Features

  • External digital thermostatic control with LED display (+/1 degree accuracy)
  • Most advanced self-modulation, adjust power to meet hot water demand
  • Durable Copper immersion two heating elements, field Serviceable. Flow Rate- up to 4.4 GPM
  • Simple installation bottom 3/4 inch NPT water connections
  • Threaded for easy replacement, simple installation, digital temperature display

Description

This tankless electric water heater is ideal for mid-range whole-home applications. This unit is ideal for single shower applications in the Midwest. The Rheem Performance tankless electric water heaters offer a continuous, endless supply of hot water. A lower kilowatt unit can provide hot water on demand for single or multiple point-of-use applications while a higher kilowatt unit could provide hot water for a whole house depending on the inlet ground water temperature. Tankless electric water heaters save space, save energy, save money, and require no venting. The sleek, compact size, 90 percent smaller than a tank, allows you to save space and place the unit wherever you want, including under a sink. Tankless units only draw energy when hot water is flowing, while traditional tanks are constantly drawing energy to keep the water in the tank at the desired temperature. Rheem's self-modulating technology can save Up to 50 percent on your water heating bill.

Brand: Rheem


Capacity: 18 Gallons


Power Source: AC Power


Product Dimensions: 14.5"W x 18.25"H


Special Feature: LED Display


Brand: ‎Rheem


Product Dimensions: ‎14.5"W x 18.25"H


Special Feature: ‎LED Display


Color: ‎Gray


Wattage: ‎18000 watts


Voltage: ‎240 Volts


Maximum Flow Rate: ‎4.4 Gallons Per Minute


Maximum Temperature: ‎140 Degrees Fahrenheit


Heat Output: ‎18 Kilowatts


Efficiency: ‎99.8%


Style: ‎Whole Home, 18 kw


Maximum Operating Pressure: ‎150


Mounting Type: ‎Wall


Is Electric: ‎Yes


Manufacturer: ‎Rheem


Item Depth: ‎18.25 inches


UPC: ‎020352685335


Global Trade Identification Number: ‎35


Model: ‎Professional Classic Tankless Electric RTEX-18


Item Weight: ‎14.78 pounds


Item model number: ‎RTEX-18


Is Discontinued By Manufacturer: ‎No


Manufacturer Part Number: ‎RTEX-18


Special Features: ‎LED Display


Amperage: ‎75 Amps


Date First Available: January 19, 2017


Frequently asked questions

The Rheem 18kW 240V tankless electric water heater can provide a maximum flow rate of up to 4.4 gallons per minute at a 35°F temperature rise. The flow rate may vary depending on your home's incoming water temperature.

This tankless water heater requires 240V electrical connection, and it's recommended to be installed by a professional. It's designed for easy replacement of your existing water heater and it comes with a digital temperature control in 1-degree increments for complete control of hot water.

Yes, the Rheem 18kW tankless water heater is highly energy efficient. It's designed to heat water only when it's needed, eliminating the need for a storage tank and reducing energy consumption.

Yes, this product comes with a 5-year warranty for heating chamber and 1-year warranty for parts. Please register the product to activate the warranty.

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Top Amazon Reviews

🚀 Abunda's Overview

This is our summary and key points to consider based on customer reviews.

The Rheem Tankless Water Heater was generally well received, with users particularly praising its compact design and nearly instant hot water supply. Noteworthy also were its energy-saving qualities and the fact that it never runs out of hot water. Some users, however, expressed concerns about installation difficulties and sufficient hot water supply for simultaneous multiple use (such as two showers at once). A few users also mentioned some confusion regarding the unit's wiring, and one cautioned that the unit might not work as effectively for filling bathtubs. As a whole though, most reviewers found the product to be well worth the purchase.

Pros

  • 👍 Compact design frees up space
  • 💰 Significant energy saver
  • 🔥 Provides nearly instant hot water
  • 💧 Unlimited hot water supply

Cons

  • 👎 Difficulty with self-installation; may require a professional
  • 🛠 Some confusion around the wiring
  • 🚿 May not provide sufficient hot water for multiple simultaneous uses
  • 🛁 Might not supply enough hot water for filling bathtubs

Should I Buy It?

If you are looking for a compact, energy-efficient solution for your hot water needs, and do not mind possibly hiring professionals for installation, then yes, the Rheem Tankless Water Heater is a sound purchase. However, if you have a larger household or often require multiple simultaneous uses of hot water, you might want to consider a larger model.

    • Rheem tankless water heater
Style: Whole Home, 18 kw
Love this tankless heater, it works great! Never run out of hot water while showering! There's only 2 of us in our home so not sure how it would work with a family but I'm sure a bigger size or maybe another mounted where hot water is needed most?? Would definitely buy again!!
Reviewed in the United States on January 31, 2026 by JAX
    • If you know what you're getting, you'll love it!
Style: Whole Home, 24 kw
Review Image by 2LiveDrew 2LiveDrew
Review Image by 2LiveDrew 2LiveDrew
There are many factors to consider before buying an electric tankless heater. By far the most important -- at least for me -- is whether the ground water temperature in your area is consistently warm enough to give the water heater a chance to succeed. I live in North Central Florida where the groundwater temperature is a rock-solid 72F (~22C) year-round. This is the ideal setup for tankless electrics; if you are far north this heater will likely fall short at the exact worst time, i.e. when it's winter and you could really use a nice, hot shower. I recommend looking closely at the map provided on Rheem's website and product pages to determine if this product is right for you. Second caveat, if you have a natural gas hookup, it is worth researching whether a gas-powered unit would be more efficient and/or less costly. There are considerations beyond this, of course, but those are the key ones that will determine whether you will be a happy camper or have cold water poured on those dreams of long, hot showers. If you're still considering after reading the above, then the short of it is (drumroll) ... I'm very satisfied and fully recommend making the jump to tankless! Here's the long-winded version: There is about 25- to 30-feet of pipe to travel between the water heater and the showerhead in my bathroom, and it takes about 25 seconds (well, 22.93 ... yes, I used a stopwatch) for the shower water to reach max temp. I did not use a stopwatch on my tank heater, but I'd guestimate it takes about 10 seconds longer for the tankless to deliver hot water than the tank unit it replaced. The main benefit, again, is that the tankless will continue to deliver hot water indefinitely while the tank is finite -- so, no more playing beat the clock and if you get in the water and realize you forgot your razor or something. The temperature coming out of the showerhead lines up with the temperature on the digital display for the main shower. There is a slight, but noticeable (I'd say 2F-4F/1C-2C) drop from the maximum temperature for the shower furthest away from the water heater. There is enough overhead that you can crank it a few degrees higher -- just be sure your shower handle can settle on something that won't scald you if you go above the recommended! I can also verify that the throughput on the model I bought (24 kW) is enough to handle two showers running at the same as well as start a load of laundry on hot -- I'm sure it could handle more, but there's not a scenario I can imagine where I would need it to. All in all, the flow is as good or better than the water heater it replaced. The dimensions listed on the product page are accurate. As far as your monthly bill, this hasn't made a significant impact although it may in the future. Here's some data from my most recent utility bill compared to the same month last year. Keep in mind the following figures are the daily averages listed on my bill for the whole home, not just the heater, although it is the only major appliance we have replaced over the past 12 months. April 2025 = 33.90 kWh electric and 0.06 KGA water (whole-home daily average) April 2024 = 31.34 kWh electric and 0.03 KGA water (whole-home daily average) Based on the above info (and the prior two months), it's essentially a wash -- we're using more power but only because we're taking longer showers because we can actually enjoy the heat now rather than worrying whether we're leaving enough water for everyone else! I'm sure that our water usage will level out once the novelty wears off, but that might be a while :) Now for the elephant in the room: Installation. If you can't say right now, with confidence and without any further research, whether you could get away with a DIY install, I'll answer the question for you: No, you can't. The plumbing end of the equation is easy -- the only hard part was ditching the tank unit. Beyond that, all you will need is a few fittings and a little pipe to hook it all up, plus the proper hardware to secure it to the wall. Any homeowner who would be comfortable ripping out a vanity sink/faucet and installing a new one should be able to do this portion themselves. The electrical hookup is another story. First, you will need 200-amp service to make this viable; if you don't, save yourself the headache. Second, depending on the distance between your electrical box and your water heater, you may need to have an auxiliary breaker box installed, as we did. Just as water temperatures vary regionally, so do electrician's rates. In our area we paid $850 to have a licensed electrician and his journeyman knock it out -- it took them about three hours and that includes the cost of the pictured auxiliary box. I would expect that rate to be higher in major metros but hopefully it gives a ballpark for what you can expect to pay beyond the sticker price. ... show more
Reviewed in the United States on May 27, 2025 by 2LiveDrew 2LiveDrew
    • Tankless Hot Water install
Style: Whole Home, 24 kw
I replaced a 55 gallon hot water tank with this unit, I bought a little larger unit than what the ratings said I needed to be safe. Make sure you check your electrical panel to make sure you have room for 3 x 40 double pole 40 amp circuit breakers. I didn’t have enough room, had to add a sub panel plus had to run 9 x 8 gauge wires the unit requires. I’m pretty handy, but asked a certified electrician to help me with the pre wiring. To be honest, I would have been afraid to try and do this myself even with all my experience. Installation was pretty simple after you have the wiring installed. Use 1 inch piping from your electrical panel to the tankless electrical unit or you won’t be able to pull the nine 8 gauge wires through the pipe. I also suggest having an electrical panel with a main breaker so you don’t have to shut the whole house down for the final install. Additionally i installed a surge protector to hopefully protect the unit from power surges which appears to have delicate electronics inside. We’ve only had this unit in operation for 4 days so far, but the water is really hot, have had no issues at this point and I love the added space gained without having the tank. Time will tell if we save 50% on energy, but from all my research it makes sense even with us living in a warm climate. There are several tankless unit manufacturers with great ratings, I went with Rheem because if I have an issue, they are based in the USA and hopefully that means English speaking in my time zone….. Hope my review is helpful to you. Update: it’s been 7 months, still works great, very happy with this decision even though I can say I’ve seen significant energy savings ... show more
Reviewed in the United States on May 6, 2025 by Amazon Customer

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