8 Rheem Electric Tankless Water Heater Reviews: 220-240 Volt

Stop waiting to take a hot shower, and shower as long as you want to, with a Rheem electric tankless water heater.

Either replace your storage tank water heater, or add an electric tankless heater as a booster.

Or add a Rheem tankless heater to that cold bathroom at the far end of the house.

Rheem RTEX Electric Tankless Water Heaters. AllWaterProducts.com

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Introduction To The Rheem Electric Tankless Water Heater

Using a tankless water heater rather than a water heater with a tank has a number of advantages. With the most important one being that tankless water heaters supply limitless hot water on demand.

With a tanked water heater, you will only get the amount of hot water it holds in the tank at one time. Once that water is finished, you will have to wait for water in the tank to be heated up again before you will be able to use hot water again.

The Rheem RTEX series tankless water heaters are units powered by electricity that only heat water when it is required and provide hot water continuously and endlessly.

Tankless electric water heaters are also small. Which makes them easy to handle and install. Which is usually under the sink or on the wall. That way the floor space that would otherwise be wasted can be used for other purposes.

Tankless water heaters are not prone to wasting energy as opposed to models that use a tank which suffer from heat loss while they are not being used and hence waste energy. This means heating costs can be reduced by as much as 50%.

Their energy efficiency is generally around 99%. While tanked heaters, depending on the model, have an energy efficiency of between 60% and 70%. The initial cost of tankless water heaters is normally higher than tank models. Although the operating cost is much lower, and they tend to last much longer.

In this article, we’ll take look at 8 different models of Rheem electric tankless water heaters. The models reviewed all operate on 240 volts, which is much better at providing more water at a higher temperature rise than the 120 volt products.

Tankless Water Heater Selection Criteria

When selecting a tankless water heater, one of the first specifications you need to look at is the maximum flowrate measured in gallons per minute (GPM). The table below shows the average GPM used by different applications.

Average Water Usage in Gallons per minute (GPM)

Average household fixtures water usage in gallons per minute. AllWaterProducts.com

This is in itself, however, is not enough. The average water usages shown above does not take into account the temperature of the water being fed into the water heater. Any model water heater is rated at how much it can increase the temperature of the water coming into it at the flowrate specified. This is shown in the table below:

Rheem RTEX tankless water heaters GPM per temperature rise

Looking at the table above, we can for example, see that the model RTEX-04 water heater can increase the water temperature by 48°F at 0.5 GPM. If you live in an area where the ground water temperature is 70°F, this means you will get hot water at 70°F + 48°F = 118°F.

As 105°F is generally seen at hot enough, this unit will be suitable for a single standard hand sink. If you however want to run it for a washing machine that runs at 1.0 GPM, your outlet temperature will only be 94°F (70°F + 24°F), which may not be hot enough.

Only the regions in the very southern part of the US have ground water with a temperature of 70°F. So, you could only use the model RTEX-04 water heater for a single standard hand sink in areas with a ground water temperature of 57°F or more, (105°F – 48°F = 57°F). Which is still only the southern part of the country as shown in the map below:

Approximate Ground Water Temperatures U.S.A. EPA
Puerto Rico and Hawaii Ground Water Temperatures Are 75 Degrees

According to a customer who has spoken to Rheem’s customer service, the Rheem products are rated with a groundwater temperature of 70°F. This means Rheem’s recommendation for how many bathrooms or fixtures any of the products are suitable for are highly overrated.

Based on the description above, this should be obvious, and you can calculate the correct requirements based on the example given above.

To summarize:

  • Calculate the maximum GPM required. Do this by adding up all of the water using devices in the home based on the average water usage table.
  • Determine the incoming groundwater temperature and the temperature rise needed to get to at least 105°F.
  • Use these figures to determine which unit is big enough for the GPM needed to run.
  • Always buy the next size up from what you think you need. That way you won’t be disappointed in the heater’s performance. One of the most common complaints by customers is that they don’t get the expected flowrate from the heater that they selected.

Installation Criteria

As the heater types reviewed in this article are all electric, you will not have to install a gas line or gas burner. Also, as there are also no exhaust gases, venting is not required.

This means both the installation as well as the maintenance of the system will be cheaper. If a single unit is not able to meet the demand of your home, multiple units can easily be installed. Also, they can serve hot water either combined or individually.

Most homes will however need to have an additional electrical circuit breaker box installed to be able to install the needed circuit breakers. Most everyone will also have to install new heavier gauge wiring that is capable of running the products.

The table below shows the technical specifications of the various models, complete with the circuit breaker required and the recommended wire size (gauge).

Rheem electric tankless water heaters specifications.

The heaters you decide to install should also be placed as close to the point of use as possible.

If there are bathrooms on opposite ends of the house, it would be better to have two smaller heaters. Rather than trying to only use one larger heater. This would cut down on heat lost over travel time from the heater to the point of use. Also, it would reduce the amount of time until hot water is felt at the point of use.

If you don’t do this, the heater you select may be undersized. As the water coming out of the heater will lose some of its heat and arrive at the service point colder than expected. This is not something that is normally considered. So, keep this in mind when determining which model is suitable for your unique situation.

Note: One interesting point that often comes up in the FAQs of the various products is that many people apparently only have 208 volts available instead of 240 volts. This is barely usable, and will reduce the amount of hot water by at least 25% (as stated by Rheem).

People really need to have a full 240 volts available to achieve the temperature increase as specified.  

One customer had wired 240 volts, but had a poor wire connection, so was only putting out 235 volts. When he fixed the wire connection, he got 240 volts and the amount of hot water increased.

Based on this, before making your selection, make sure which voltage you have available in your home . As this could have a big impact on how hot the water is that you have available.

Features Comparison Table: Rheem 220-240 Volt Tankless Heaters

 RTEX-36RTEX-27RTEX-24RTEX-18RTEX-13RTEX-11RTEX-08RTEX-06
Push-button ON/OFF control with LED temperature displayNoNoNoNoNoNoNoYes
ON/OFF Dial Control with adjustable digital temperature displayYesYesYesYesYesYesYesNo
Self-modulating power controlYesYesYesYesYesYesYesNo
Maximum Flow Rate in GPM8.86.65.94.43.172.681.951.5
Water connection¾” NPT¾” NPT¾” NPT¾” NPT½” NPT½” NPT½” NPT½” NPT

Let’s now have a look at some more details of the different Rheem electric tankless water heaters, their features and what customers have to say about them.

Before we start, it should be noted that the RTEX series of products reviewed in this article are newer up-dates to the RTE series of products. This article is only about the RTEX series.

Before buying any of the Rheem RTEX models, keep in mind that these are meant for indoor use and they can’t be installed outdoors. They must be installed per codes and manufacturer recommendations by qualified installers and electricians for safe and reliable operation, and for the 5-year warranty to be valid.

Rheem electric tankless water heaters will enable you to enjoy the convenience and comfort of continuous hot water on demand. The whole-home electric models are excellent for vacation homes, homes with luxury bathrooms and homes with busy families. While the point-of-use electric models are ideally suited for a single shower or faucet.

These water heaters are recommended for use in applications such as bathrooms, kitchen sinks, washing machines, tubs, showers, small offices, garage sinks. Also, in combination with other types to act as a booster, in cottages, homes, apartments etc.

When evaluating user reviews, recommendations are often found to install water flow regulators to control the flow rate of incoming water and therefore the outlet water temperature.

As confirmed by users, these units are relatively simple to install, as long as the instructions provided by the manufacturer are followed.

The negative reviews we found mostly have to do with water fluctuation and modifications of the electrical systems in the home. That’s, including the installation of new distribution panels, electrical wires and circuit breakers. For these services contact a local electrician.

In one of the negative reviews, the customer complained about having to do regular maintenance to clean scale from the elements. Although not commonly found to be a problem, inlet water quality does play a role in the correct operation, longevity and amount of maintenance that needs to be performed.

Water hardness and excessive chemical and mineral elements in the water can damage these units. If you do have a water quality problem at your home, it is advisable to install water filtration and softening systems to protect your tankless water heater.

RTEX-36 Rheem Electric Tankless Water Heater 240V With 4 Heating Chambers

RTEX-36 is the largest tankless and electric water heater from Rheem.

When visually compared to the other whole-house models below, it has similar features. However, it has four heating elements that allow hot water to be produced at up to 8.8 GPM.

This Rheem tankless electric water heater is suitable for whole-house water heating as it has 4 heating elements and 36 kW of power.

It can therefore supply more than one fixture with hot water, even large tubs.

Rheem RTEX-36 Electric Tankless Water Heater. AllWaterProducts.com

4 baths? No problem – Click Here

This model is equipped with a self-modulating power control that uses a computer logic control to meet the demand for hot water by sending the right power to the heating elements.

It also features an external adjustable digital thermostat complete with LED temperature display for easy monitoring and control.

Customer feedback shows that the unit is suitable for large homes with 3 to 4 bathrooms, plus a dishwasher and washing machine, and 4 people living there.

One review recommended using shower heads with low gallon per minute consumption. Thus making the RTEX-36 suitable for more than 3 showers, and 2 to 3 faucets.

There were also some interesting comments from a seller of Ecosmart that has switched to become a Rheem seller due to the superior support Rheem provided.

Also, despite his plumber friends telling a customer he needed a gas unit to ensure consistent hot water volume, he managed to prove them wrong by using this unit.

Although one customer claimed that the money saved on electricity for a normal hot water heater had paid for his unit, he unfortunately did not state the period over which this was achieved. Suffice to say, he did see a saving in energy costs.

RTEX-27 Rheem Tankless Water Heater With 3 Heating Chambers @ 240 Volts

The RTEX-27 tankless water heater looks the same as the RTEX-36 model, has the same features, but only uses 3 heating elements instead of 4.

This model generates 27 kW and produce 6.6 GPM of hot water.

Customers confirm that it has worked very well since it was installed. Also, that it is suitable for running 2 tubs, washer, dishwasher, kitchen sink and two lavatory faucets, all at the same time.

Rheem RTEX-27 Tankless Water Heater. AllWaterProducts.com

Replaces an 80 gallon tank style water heater – Click Here

One customer replaces a faulty 80 gallon hot water tank with this unit and has nothing but praise for the RTEX-27.

The same customer mentions that he had to install extra circuit breakers and heavier wiring for about $450. That was to have 3 new lines connected to the system (i.e. one line and breaker per heating element).

Although the temperature was initially set too hot at 140°F they now run it at 125°F. Which is perfect for 4 full bathrooms, a kitchen, and a laundry, even with 3 showers running at the same time.

One customer complained that after using the unit for eight months, the hot water stopped running. The problem turned out to be hard water in the area, resulting in calcium building up on one of the heating elements. Which was slowing down the water flowrate at the hot water faucets.

This problem can be prevented by installing appropriate filtration and water softener systems to prevent any deposits from clogging up the system.

This customer also mentions that before the calcium build up problem, they couldn’t run the faucet on their soaking tub fully open. As this unit is however only rated for 6.6 GPM, and the tub was rated for 10 GPM, this makes sense.

Another customer mentioned the same problem that while filling their 80 gal Jacuzzi tub they were not able to run the hot water fully as the heater maxed out at 5 GPM. As this is lower than the specified 6.6 GPM, one can only presume that the inlet water temperature in this specific case is low.

Other reviews for this model make mention of being advised by friends and professionals to use a gas fired tankless water heater rather than an electric one. However, after doing the proper calculations as described in this article and taking inlet water temperature into account, the customer decided on this model and is very satisfied with his decision.

The last review for this model worth mentioning obtained an estimate for a tankless gas water heater, fully installed. However, they found out it was extremely expensive due to requiring plumbing, gas, electrical and venting.

When comparing whole house tankless electric water heaters, this customer researched several brands, including EcoSmart, Rheem and Stiebel Eltron.

When selecting this specific Rheem model, the customer did so due to the homes in their area typically having 200 Amp service from the street. So, they were only able to buy the 27kw unit, as the 36kw requires 300 Amp service.

RTEX-24 Rheem Tankless Water Heater: 3 Heating Chambers

RTEX-24 tankless water heaters are designed to provide power of up to 24 kW and water flow of 5.9 GPM, which is very popular.

These use larger ¾” water connections, located at the bottom of the unit.

The temperature of the hot water can be adjusted in the range from 80°F to 140°F using the built-in dial-knob and seen on the small screen.

In the FAQs for this unit, one customer asked if it was possible to only connect 2 of the elements and effectively use it as a RTEX-18, but with the option to upgrade later, i.e. connect the 3rd element later.

Rheem RTEX-24 Electric Tankless Water Heater. AllWaterProducts.com

Very popular and good value – Click Here

Although this may well be possible, one has to question the reason for doing so. I can only presume it is to save energy costs. If this is the case, there is no need as this model uses advanced self-modulation which will automatically reduce the power consumption when demand is lower.

A number of customers mention that they use this model in areas with ground water temperatures of as low as 40°F to 52°F without any problems. These mentions however don’t specify which appliances are being used.

Another question a customer asked is how long it takes the water to heat up. The answer to this one varies from about 5 to 10 seconds, to 30 to 45 seconds. Depending on how far the unit is away from the water outlet.

This emphasizes the point that where possible, the unit should be installed as close to the outlet as possible. Even if it sometimes means installing 2 smaller units nearby, rather than using a larger unit further away.

If you are replacing an existing tank heater, the amount of time it takes for the new water heater to deliver hot water should be no different than it was with the old water heater.  

RTEX-18 Rheem Electric Tankless Water Heater With 2 Heating Chambers @ 240V

RTEX-18 electric tankless water heaters have been designed with an output power of up to 18 kW and water flows of 4.4 GPM.

A number of reviews for this unit mention that the electric bill has decreased substantially from the standard 50 gallon hot water heater the customer had before. With one mentioning a figure of $20 per month.

The reviews on how much water the unit can deliver at which flowrates and with what ground water temperature are mixed.

On the one hand, a water temperature from between 42°F to 70°F seems to not present any problems, although the number of appliances mentioned in those reviews are in line with the flowrate we would calculate using the methods described.

There are however also a number of reviews that state that the maximum usable flowrate is 1.5 GPM. However, these reviews don’t mention the inlet water temperature.

We did however also pick up that one customer expected the outlet temperature to be 120°F as opposed to the 105°F used in most calculations and sizing tools. Having said that, one customer reports achieving 130°F. Although he does add that he has to reduce the flowrate to get that.

Techniques used to increase the temperature includes restricting the flowrate manually, to installing inline flow limiters, flow regulators, or using low-flow shower heads. Two showers with 1.5 GPM heads were mentioned in this regard.

One interesting question was whether this unit could be used to heat a pool. The answer there is a definite no, as the chlorine in the pool would damage the unit.

On the same topic, some customers mention installing filtration and/or water softener systems for their systems. A 5 micron filter, and the Waterboss 900 water softener were mentioned in this regard.

One customer reported an interesting problem in that when he used dimmable LED light bulbs, these flickered badly whenever the tankless water heater was turned on. This problem is caused by the heater using so much power that it causes a voltage drop. The root cause of this problem is that the customer was using a 200 amp distribution panel. So, a larger panel will probably not have this issue.

Rheem RTEX-13 240V Tankless Heater, Shower and Sink Capable

The Rheem RTEX-13 water heaters are smaller units and can be used in both single and multi-application since the maximum water flow can reach up to 3.17 GPM.

When comparing to the above bigger models, you will find that RTEX-13 uses two elements of 6.5 kW each. This model also uses a ½” NPT side water connection, rather than a ¾” NPT bottom connection like the bigger models.

When using a water heater of this size, it is no longer possible to run the whole home from it. However, depending on the inlet water temperature, one or two devices should work fine.

Rheem RTEX-13 Electric Tankless Water Heater. AllWaterProducts.com

Get 1.0 to 3.17 GPM anywhere in the USA – Click Here

Most of the customer questions dealt with this issue and providing you understand the relationship between flowrate, how much the unit increases the temperature and the temperature of the inlet water, you should be able to size your unit appropriately.

Also remember our advice to always select a unit one step up from what your calculations indicate you should use.

One customer asks if this unit can be used in conjunction with a gas hot water tank to heat water further so that one never runs out of hot water. This type of application is called a booster. It is actually a common use for small tankless water heaters such as this one.

One of the questions for this unit was if it can be installed in a manufactured home. A plumber that has installed several of these units in manufactured homes answered that question. Since manufactured homes are generally wired to the minimum code, one of the biggest things to consider is the wire size. That’s because these units pull a significant amount of electricity.

Rheem RTEX-11 Great Booster For Distant Bath

The Rheem RTEX-11 can still be used in both single and limited multi-applications since the maximum water flow can reach up to 2.68 GPM. When comparing to the above bigger models, you will find that RTEX-11 uses two heating elements of 5.5 kW and a ½” NPT side water connection as does the RTEX-13.

One customer reports using the unit for 4 sinks that happen to be located close together. While another has installed it as a booster system for a shower that was too far away from the boiler to get decent hot water.

Rheem RTEX-11 Electric Tankless Water Heater. AllWaterProducts.com

Great hot water booster for a distant bathroom – Click Here Now

Another user reports only being able to run it with one shower with low flow shower head. Neither of these customers however make mention of the ground water temperature used for their installations.

There is also a recommendation to use Teflon tape on the non-tapered pipe fittings during installation, rather than pipe dope. That’s, because the latter liquefies on the hot side.

Another suggestion is to install valves on the unit to run de-scaler solution and also to filter the water before it goes to the unit.

Rheem RTEX-08 240V Single Shower Power Heater

Rheem RTEX-08 water heaters are smaller and can be used in single applications, and maybe multi-applications.

That depends upon if the inlet water temperature is high enough, since the maximum water flow can reach up to 1.95GPM.

The RTEX-08 uses one heating element and a side water connection.

The only customer comment of note is that this model works best for a 700-800 sq. ft. radiant floor heat set up and would not be big enough for a 1000 sq. ft. setup. The RTEX-11 would work best for 1000 sq. ft.

Rheem RTEX-06 220V Heater For One Sink, Or As A Booster Heater

The RTEX-06 tankless electric water heater is the smallest model from Rheem that runs from 240 V. It comes with 5.5 kW of power, generated by a single heating element.

It can supply one fixture with hot water flow up to 1.5 GPM. The RTEX-06 is a very basic model. It uses a push-button with an ON/OFF control, together with an external LED temperature display.

The Rheem RTEX-06 are not thermostatically controlled, so it only displays the temperature of the output water.

The unit will work for a sink in a popup camper, providing the electrical circuit can handle the current. As it needs a 30A breaker, it is probably a good use case if one is available.

As a very low flow shower is 1.5 gallons per minute and this is what the unit is rated for, it would be suitable for a shower providing the inlet water temperature is 80°F or higher. In colder areas, this unit is only really suitable for one small sink.

One customer asked if it would work for two showers and a Jacuzzi. By now you should agree that the answer to this one is definitely a resounding NO.

Besides using this unit for a small sink, this unit is also suitable for operating in conjunction with a hot water circulation system. Providing the purpose is only to raise or increase the temperature of the hot water circulatory system.

One customer reported that when he replaced a conventional water heater with a big tank with this unit, it paid for itself in about 6 weeks.

Jumping To Conclusions: Rheem Electric Tankless Water Heater Reviews

First, most houses do not have all their bathrooms and sinks close together. So, seriously consider buying two, or even three, of the smaller Rheem electric tankless water heater choices. This can save you money in up-grading your electric power service and also provide more bathing comfort.

This is especially true if you have a master bedroom that isn’t close to your current water heater.

Second, putting the RTEX-06 in a half bath is a great idea. You will get instant hot water without waiting for it to arrive from a distant heater.

As just mentioned, getting instant hot water depends upon how close the tankless heater is to where the water is used. If you just want to rinse an apple or wash your hands, you don’t need endless hot water. However, you do want it “right now”.

Third, besides the temperature of the water entering your house, how your house is organized will determine the number of heaters and their size.

For example: In my new house, that I am designing, I have back to back bathrooms and laundry area. My incoming water temperature is 42 degrees and I want 105 degree hot water.

The two showers and clothes washer could all be used at the same time. So, I am putting in the RTEX-36 that provides 4 GPM at my incoming water temperature. Each of the showers can use 2 GPM and the clothes washer uses 1 GPM. So, I can’t use all three at the same time. I can live with that but, maybe you can’t. You might decide you want a separate heater to handle the clothes washer and the laundry sink.

But what about the sinks in the bathrooms? They are each getting their own RTEX-06. So, I will get instant hot water in the sink, even if the showers are being used.

So, buy one or more of these 8 Rheem electric tankless water heater choices to keep you in hot water.

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Interested in 120 volt tankless heaters? Read this.

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8 Rheem Electric Tankless Water Heater Reviews: 220-240 Volt
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8 Rheem Electric Tankless Water Heater Reviews: 220-240 Volt
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3 endless showers at the same time? It's possible with the RTEX-36 Rheem electric tankless water heater. 2 small heaters might be better than one large heater - just do this...
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All Water Products
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