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Rheem RTEX-27 Review – Self Modulating Tankless Water Heater

Rheem is one of the top three tankless water heater companies in the market. They have different models of water heaters for different purposes. RTEX 27 has been designed for residential water heating purposes. In this article, we will review Rheem RTEX 27 as comprehensively as possible to find out its positives and negatives if any.

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Specification- Rheem RTEX 27

Product Dimension (inch)18.25 x 17.63 x 3.5
Product Weight17.8 pounds
Maximum Flow Rate6.6 GPM
Fuel TypeElectricity
Installation Indoor
Voltage240 Volts
Power27 KW
Temperature Range80°F – 140°F
Rheem RTEX 27 Tankless Residential Water Heater


Flow Rate

The maximum flow rate of Rheem RTEX 27 is 6.6 GPM. Well, this rating has some terms and conditions. You will get this speed only when the temperature rise is less than 31°F. For example, if the groundwater temperature of your region is 80°F and you set the output water temperature to 110°F, then you may get an output water speed of 6.6 gallons per minute. As per the technical specification sheet of this model, below is the relationship between temperature rise and output water speed of Rheem RTEX 27:

Temp RiseFlow Rate
92° F2.0 GPM
74° F2.5 GPM
62° F3.0 GPM
46° F4.0 GPM
37° F5.0 GPM
31° F6.0 GPM


Therefore, you need to know the temperature of groundwater because the input water temperature largely determines the performance of the on-demand water heater. For example, if the groundwater temperature is 40°F then you will get a flow rate of about 3 GPM with lukewarm output water. On the other hand, if the groundwater temperature is 75°F, then the flow rate will be about 6 GPM with the same output water temperature. That means, keeping all conditions the same, only the groundwater temperature difference can increase or decrease the water heater performance by double or more. Below is a list of average groundwater temperature of different regions-

Ground Water TempArea/Region
35 °FMinnesota, Canada
37°FMinnesota, Maine, Montana, Canada
42°FMinnesota, Maine, Montana, North Dakota, NY, Canada
47°FIowa, Nevada, North Dakota, South Dakora, Washington
52°FIndiana, Kansas, South Dakota, Washington, Virginia
57°FTenessee, Kentucky, North Carolina, Virginia
62°FArkansas, California, Georgia
67°FAlabama, Mississippi, Louisiana, Georgia
72°FTexas, Florida, California
77°FFlorida, Texas

Coverage- Good for a large family

Is Rheem RTEX 27 good for a big family? The answer is yes. But check the input water temperature. If you are living in a very cold region, then you might need to have an additional water heater to support your hot water needs. Let me clarify the capacity of RTEX 27 with an example. Below is the list of home fixtures with their required flow rates:

Fixture TypeReq. Flow Rate
Hand Sink0.5 GPM
Dishwasher1.5 GPM
Kitchen Sink1.5 GPM
Stnd. Shower2.0 GPM
Washing Machine1.5 GPM

From the list above, we find that a standard shower will consume 2.0 GPM water at full speed. Therefore, if you get 6.0 GPM water from RTEX 27, then you can run up to 3 showers at the same time. I’m sure it is a very rare situation that all your three bathrooms are engaged for a shower at the same time. Therefore, I think for a warmer region, Rheem RTEX 27 will be sufficient for a big family. But is the groundwater being very cold and you get a maximum flow rate of 3.0 GPM from the water heater, then you can still run it, but only one tankless water heater might not be sufficient for the peak hour use.

Installation- Indoor

This model of Rheem water heater has been designed for indoor installation. As it is an electricity run water heater, you do not have to worry about the probable risk of the dangers caused by the exhaust has. This water heater will not generate any vented gas at all.

Find our article on Rinnai V65iN indoor tankless water heater review.

Control over temperature

RTEX-27 has a very unique feature to control the output water temperature. The manufacturer calls it ‘Digital Thermostatic Control’. You can precisely change the temperature of the water within an accuracy range of only 1°F. The accuracy range seemed impressive to us.

Self-modulation System

A tankless water heater has two major challenges- it has to meet the demand for hot water and minimize energy consumption. To balance these two, Rheem RTEX 27 has a system that they call it ‘Self Modulation’. This system adjusts the electricity consumption as per the demand of hot water. In a lower demand, the water heater consumes minimum energy to save the cost.

Easy installation

RTEX 27 is renowned for its easy installation process. Most of the consumers rated this feature as the top one. Being an electricity run water heater, it is already easier to install than a gas-run one. The bottom ¾” NPT water connection system has made this unit even easier to install.


The price of this unit is equivalent to its nearest competitors such as Rinnai V65iN. This unit will cost you around 600 USD.


As an indoor water heater, Rheem RTEX 27 did a very good job in terms of size. Comparing its nearest competitor, Rinnai V65iN, the volume of Rheem 27 is almost half of the volume of Rinnai. Therefore, this unit is good to go in a small room. The dimension of this unit is 18.25 x 17.63 x 3.5 inches.

Pros and cons of RTEX 27

What we liked
  • On-demand water heater
  • Electricity run- no risk of gas leakage
  • 8 % energy efficiency
  • Higher output flow rate
  • Precise control over temperature
  • Easy installation process
  • Copper made heat exchanger
What we did not like
  • Although efficiency is high, electricity bills are higher than equivalent gas
  • No remote-control function included


Final words

For a big family, 6.6 GPM maximum water flow rate will be the best option available. Now considering the brand value, Rheem is one of the tops in terms of reputation as a water heater producer. But for a medium-sized family, this unit might be too good. Therefore, a product with a lower output rate can minimize the initial cost. Another important aspect that a user of the water heater should keep in mind is the installation cost. Rheem RTEX-27 will save a lot of money in the installation because of its easy setup process. Overall, I will give it an A+ as an electric tankless water heater.

1 thought on “Rheem RTEX-27 Review – Self Modulating Tankless Water Heater”

  1. The installation is not easier than a gas unit. I wanted to replace my electric 50 gallon hot water heater with a tankless model. I buy it and get it home and find out I need 3 – 40 amp circuits not the 1 that was connected to my existing heater. This should be mention in the description of the heater

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