The Rheem RTEX-18 is an 18 kW, 240V electric tankless heater that Bob Vila's Glenda Taylor installed in a small commercial building and rated 4.8 out of 5. It mounts flat on a wall at 13.9 by 17 by 3.6 inches and delivered hot water at the sink within a few seconds. It is a small-capacity unit that needs two 40A double-pole breakers and a 150A service panel, so it suits light demand rather than a house with several bathrooms.
Full review
Installed in a building with no utility room
Bob Vila's Glenda Taylor installed the RTEX-18 in a small commercial building she and her husband were remodeling, where there was no room for a tank heater. She wired two 40A double-pole breakers into a 200A panel (Rheem calls for at least 150A), ran cold water in and hot water out to a PEX manifold, and branched to two sinks. She describes the plumbing and wiring as straightforward, while noting that the hoses and manifolds were bought separately and that hiring a professional is often the safest route.
The unit measures 13.9 inches wide, 17 inches high and 3.6 inches deep, and she mounted it on a wall, which she contrasts with the 4 to 6 square feet of floor a tank heater takes.
Hot water in seconds, sized for light demand
A flow sensor starts the heating as soon as a hot tap opens, and in Bob Vila's installation the water at the faucet was hot within a few seconds. Temperature is set with a knob on the front; she settled on 120 degrees F.
Bob Vila is clear that the RTEX-18 is a relatively small-capacity heater, rated at up to 4.4 GPM in the warmest zone. For a large home with several bathrooms it suggests a higher-capacity model and points to Rheem's sizing tool.
Running cost
The unit's EnergyGuide sticker estimates $288 a year to operate. After a couple of weeks of use, Bob Vila's reviewer had not seen a noticeable increase in the building's electricity usage.
Strengths
- +Compact 13.9 x 17 x 3.6 inch wall-mounted body, per Bob Vila
- +Hot water reached the faucet in a few seconds in Bob Vila's installation
- +Temperature set with a single front knob; Bob Vila settled on 120 degrees F
- +EnergyGuide sticker estimates $288 a year to operate, per Bob Vila
Watch-outs
- −Requires a 150A service panel and two 40A double-pole breakers
- −Small capacity: Bob Vila advises a higher-capacity model for a large home with several bathrooms
- −Professional installation suggested; plumbing parts such as hoses and manifolds are bought separately
How it compares
At up to 4.4 GPM in the warmest zone, per Bob Vila's spec table, the RTEX-18 is a smaller electric unit than the Stiebel Eltron Tempra 36 Plus (7.5 GPM) and well below the rated flow of the gas Rinnai RU199iN (11 GPM) and Takagi T-H3-DV-N (10 GPM). Unlike the gas units it needs no venting, but it does need two 40A double-pole breakers.
Rating sources
Our 4.5 score is the average of these published ratings, each adjusted for how strictly its publisher scores. More about methodology.
