The Rheem company manufactures different types of appliances such as air conditioners, heat pumps, furnaces and water heaters. Rheem’s RTEX Professional Classic series tankless electric water heaters provide on demand, uninterrupted hot water.
Electric Tankless Water Heaters
The Rheem Professional Classic series water heaters are tankless electric units. The smallest size is the RTEX 04 which is a 3.5kW model with a maximum flow rate of 0.9 GPM. The largest model, the RTEX 36, provides a flow rate of up to 8.8 GPM. These water heaters have an ON/OFF dial control and a digital temperature display.
RTEX 04
The Rheem RTEX 04 is the only model in the series which works with 120-volt electrical current. It has one internal heating chamber and 1/2-inch NPT water connections. It has to be hard-wired into the electrical panel with 10 AWG wire and a 30A circuit breaker.
Its hot water flow rate is up to 0.9 GPM, but this depends on the temperature of the cold water. The colder the water, the lower the flow of hot water will be. This model is small, so it might only handle a single faucet in warmer climates. We would not buy it unless you needed a 120V tankless water heater. Its flow rate is barely enough for a standard hand sink faucet.
RTEX 06
This is a 6-kilowatt, 240-volt tankless water heater with a maximum flow rate of 1.5 GPM. It is enough for a standard hand sink or a kitchen sink, but not both at the same time. This model is too small to provide hot water for a shower.
RTEX 13
We have skipped the RTEX 08 and RTEX 11 models and gone straight to the RTEX 13. It is a 13kW, 240V tankless water heater with two heating chambers. Its installation requires 1/2-inch NPT water fittings, number 6 wires (6 AWG) and a 60A circuit breaker. However, it is not big enough to heat water for more than one faucet or shower at a time. If you turn on more than one tap, then the water temperature will drop. The RTEX 13 might be enough for one bathroom, but it is too small for multiple bathrooms and kitchens.
RTEX 18
This model has larger 3/4-inch water connection fittings and two heating chambers. It requires 8 AWG wire and two 40A circuit breakers. Its maximum flow rate is 4.4 gallons per minute. This model might provide enough hot water for a shower and a bathroom faucet running at the same time. However, the water temperature might drop if you are taking a shower and the dishwasher uses water or the kitchen sink is turned on full blast. A bath tub requires a flow rate of 4 GPM or more, so the RTEX 18 probably does not have enough heating power to fill a bathtub with hot water.
RTEX 24
This 24kW 240V tankless water heater has three heating chambers and 3/4″ NPT water fittings with a flow rate of up to 5.9 gallons per minute. It needs three 40A circuit breakers in the electrical panel. This model can provide enough hot water for a shower or for filling a bath tub.
RTEX 27
The 27-kilowatt model is very similar to the 24kW model, except that it has a flow rate of up to 6.6 GPM. It can probably handle two water outlets running at the same time. Still, this depends on the inlet water temperature, the total flow rate and the distance between the water heater and the faucet/shower.
RTEX 36
This is the largest model in the Professional Classic series. It has four heating chambers and a flow rate of up to 8.8 gallons per minute. This is the unit you need if you want to provide hot water for a large house with more than one bathroom. However, its list price is over $1000. The cost of installation will also add a few hundred dollars extra. You need the correct size electrical panel, circuit breakers and wiring to power this 36-kilowatt tankless water heater.
Conclusion
These Rheem RTEX tankless electric water heaters have both advantages and disadvantages.
The installation is the biggest disadvantage because you will probably hire a plumber and electrician to install the unit. It is very important for the safety and performance of the electric water heater that the installation is done correctly. If the wires are not the right size or if the electrical panel cannot handle the load, then you will experience problems. The lights might flicker when the unit turns on, or the water temperature could fluctuate while you are showering. These problems can be avoided with the right installation.
The benefits are that it needs less space because it has no tank. It can also be installed closer to the point of use, thereby reducing the waiting period for hot water. If you have chosen the right Rheem RTEX model for your needs, then it will provide constant hot water without running cold (shop now on Amazon).
You might also read our ECO electric tankless water heater review. These water heaters are made by the Ecosmart company and has similar features, sizes and performance.