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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
Power Source: AC Power
Special Features: LED Display
Wattage: 18000 watts
Voltage: 240 Volts
Flow Rate: 4.4 Gallons Per Minute
Maximum Temperature: 140 Degrees Fahrenheit
Heat Output: 18 Kilowatts
Efficiency: 99.8%
Maximum Pressure: 150
Mount Type: Wall
Is Electric: Yes
Capacity: 18 Gallons
Item Dimensions W x H: 14.5"W x 18.25"H
Item Weight: 14.8 Pounds
Item Depth: 18.25 inches
Material Type: Stainless Steel
Brand Name: Rheem
Model Number: RTEX-18
Manufacturer: Rheem
UPC: 020352685335
Global Trade Identification Number: 35
Manufacturer Part Number: RTEX-18
Included Components: Fittings, Aerators
Unit Count: 1.0 Count
Item Type Name: Water Heater
Color: Gray
Style Name: Whole Home, 18 kw
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.
Style: Point-of-Use, 6.5 kw
Works good so far.
Reviewed in the United States on May 17, 2026 by Mo
Style: Whole Home, 18 kw
Great system, easy to install. Was a 1/2 day project but over all I'm very pleased on the outcome. The temperature is very easy to set and instant hot on demand was a plus. I did have to add a sub panel
Reviewed in the United States on May 3, 2026 Reviewed in the United States on May 3, 2026 by Paul Gallardo
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
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Reviewed in the United States on May 6, 2025 by Amazon Customer
Style: Whole Home, 18 kw
I'll leave the original review below. It's been a little over 2+ years and have had to replace every single part and then finally replace the entire unit. Then the new one brand new out of the box did not work properly then finally fail 1yr and 2 months after purchase. This unit should be removed from sale and recalled that is how dangerous they are. If you have this unit installed I would highly recommend take it out and replacing with something else. It almost burned my house down! Just take a look at the pictures in this review and see for yourself. Okay full detail review below. 1st I'd like to say Rheem makes good products but this is not one of them. Here's why: 1st unit failed only less than 1 year of use. The elements exploded inside and needed to be replaced. Fine no problem, kill breaker, shut water off, unscrew 4 wires, take out element replace, re-screw wires, done in 30 minutes. Far beats dealing with a leaking water heater tank with 2 inches of water in your basement. Or lugging a 50lb broken tank up your stairs to replace at the Lowes or Home depot. Ok so I put the new elements in and it's heating again. Not more than a few months later the wire harness started arcing and fused together melting off and almost catching fire. I happened to see the arcing when the unit was on and didn't realize this was not part of how it operates. Thought it was just how it is. It tripped the breaker thankfully. I contacted the company and they sent me a new wire harness. I replaced it, still the unit wouldn't heat. So they sold me new inlet and outlet water sensors and a new flow sensor. That didn't fix it. So they said the motherboard must have fried. So I replaced the motherboard. That didn't fix it. So I purchased a completely new one. I mount the new one hooked it up and the water heater would stop heating randomly during a shower. So I set it to bypass mode 125 degrees and said "heck with the temp controls just give me heat" So a brand new unit I cannot even set the temperature. But I figured 125 is hot enough. Okay moving on. Fast forward to a few days ago, Sept 2nd 2023 brown colored water shot out of my shower head and I knew right away the heater blew. It made 1 year. I open the element rods and sure enough the 1st one exploded worse than any before. Okay no problem, let me swap it with a spare. So I did but.. I had to remove the entire unit from the wall and shake and empty out all the guts that exploded sitting inside the element housing. Still not that hard. So I did it see picture of sink.. EZPZ right? Nope, after repressurizing, a weld must have cracked and a slow drip came down the 1st pipe element housing. I thought maybe its not tight enough. So I tightened it more and the crack go worse and went from a small drip to a full stream. I confirmed it was coming from a welded joint not the threads on the element rod. Okay now the entire unit is trash and needs to be junked all because as I was tightening the element rod it put stress on the joints and cracked them. This is horrible quality in the build and should not happen. So I put the old one back in and swap every part. Wire harnesses, motherboard, elements and finally the sensors. Oh but the sensor's I tightened too much and they broke through the housing which also makes the entire unit useless as that cannot be repaired. So I threaded bolts into their holes and set it to bypass. I ordered a Mizudo instead which does not have copper housings, instead cast aluminum and does not have it's heating rods mixed with the water like this and many unit's have. So far it's been excellent. I highly recommend not purchasing these Rheem electric water heaters for 2 reasons. Fire hazard and quality. They constantly break. Amazon should remove this from their store before someone's house burns down and someone gets hurt or killed. Original Review Below I love this water heater. I really don’t have any complaints. Super easy to install. Takes up like no space. No more water tank to explode damaging my basement. It takes longer to get hot water at the furthest end of the house but it’s so worth it. Once it’s hot it stays hot and I can run a shower plus 2 faucets before I start losing heat. This one took 2 dbl 40amp breakers and it’s powerful enough to heat multiple faucets simultaneously. We never take more than 1 shower at a time anyway with a family of 4. Hasn’t been a problem keeping up with our hot water needs at all. And for the price? This is a great purchase. Ditch the tank! You’ll thank yourself later.
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Reviewed in the United States on January 22, 2021 Reviewed in the United States on January 22, 2021 by F. Digiovanni
Style: Whole Home, 18 kw
These systems are amazing. The future is in your hand when you hold one. I fully recommend. Please, however read on to understand them better. Customer support is awesome and available. This 18kw unit did NOT worked in my home, and I went thru two of the 18kw units before I determined the problem thinking the first one was not working and the problem . Please read on. You will want one and never look back if you read on.. I guarantee You WILL want this one of these water heaters, however, please read on. These WH’s are complicated. They work awesome in the lab environment they were created in. If your home does not match that lab setting or you can’t recreate it. It will not work. I went thru three of them to solve my problem. YOU WILL NEED. Flow restrictions, preasesure regulators, water softener, volt/amp meter filters(?), and a lot of patience tuning it to work. Once tuned however, they are flawlessly perfected at heating water instantly. Wow! the future is in these devices. Why was mine so difficult? I’m the last house from the power company. Literally at the end of the line. And I put in the 18kw unit. They are all super easy to install in both wiring and plumbing, but, what appeared to be my problem was there wasn’t enough physical energy to run it properly. Duel 40 amp req. So it wouldn’t work unless the flow restriction was almost closed. I tweeted and tweeted every possible measure to get it to work. I’m on well water too, so that incoming cold temperature affects the system and set up on a 40/60 pressure switch. The water rate It was moving was to fast and I never could get both sides(heating chambers) of the water heater to come on. It just would NOT work. It would come on for a sink at half open but when you tried to shower. It would turn off and say E5. Oh how I grew to hate that display. A simple to read display and simple to adjust temperature dial. I literally went from Flintstones to The Jetsons in appearance. About the 13kw unit. I had to get instead; WHICH SOLVED MY PROBLEM. It’s a little more than half the power necessary to function. Single 50 amp It is doing its job beautifully now. I have almost full water pressure again and it seems sold at 130 deg. But again, several hours spent tuning so it did work. If the flow rate is to high and water temperature to low it will not work. You have to tune them with additional devices to stabilize them so that units performance in the lab environment it was created in and it needs in your home. I have replaced two tanks now. One electric in a home and one propane in a camper. Worth it! Worth IT! WORTH IT! As for durability; install a water softener or they probably won’t last long without regular maintenance descaling it. I believe fully in this new technology and as well this 18kw unit but if you don’t understand advanced plumbing and advanced electrical troubleshooting if you’re a do-it-yourselfer, hirer someone who is because the electrical power supply will Kill you if you don’t understand it. Let this advice be not a discouraging experience, let it be that your knowledge now of some of the possibilities YOU might encounter moving in the direction of The Jetsons.
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Reviewed in the United States on December 6, 2025 by Biz Trailer
Style: Whole Home, 24 kw
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.
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Reviewed in the United States on May 27, 2025 Reviewed in the United States on May 27, 2025 by 2LiveDrew
Style: Whole Home, 24 kw
Just installed. Works well. Hope it will last longer than the off brand one we had before. Very easy installation due to being a replacement for the same style unit. Electrical, plumbing and mounting locations all the same.
Reviewed in the United States on March 29, 2026 Reviewed in the United States on March 29, 2026 by K.Barron
Style: Whole Home, 24 kw
Did a lot of research, not to mention I've had a few tankless water heaters. The other ones I had were gas, this was the first Electric tankless I purchased. I narrowed the choice to Rheem and Stiebel. Both tankless heaters are good. Stiebel is a bit expensive. Pro's about Stiebel, you only need two breakers. Whereas Rheem (this model and larger) needs 3 breakers. I put this in our 1200 sq ft, manufactured home in South Florida, the home has 2 full bathrooms and dishwasher and as of now there are no issues with heating and maintaining the water flow. The unit is easy to adjust the temperature of the water. I'm one of those DIY persons and hooked the unit up myself, electrical and water lines. The home came with a large tank water heater taking up a lot of room. As you can see in the photo using the tankless heater it allows us to have extra room to store misc. items. I'll pop back in and give a 3 month review. UPDATE: 02/2026: 7 month Review: No issues with the Rheem RTEX-24 Electric tankless water heater. Keeps up with the water demand never running out of hot water.
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Reviewed in the United States on September 13, 2025 Reviewed in the United States on September 13, 2025 by PETE