Quick Answer: The Rheem RTEX-18 is an 18 kW, 240 V electric tankless heater that needs 75 A on two 40 A double-pole breakers and two 8 AWG wire runs. Rheem’s own sizing table gives it 1.8 GPM at 105°F from 37°F groundwater and 4.4 GPM from 77°F groundwater, so it runs one low-flow shower in a northern winter and about two showers in the Gulf states. It listed at $449.00 (Big Frog Supply) and $523.99 (FactoryPure) on November 11, 2026, both out of stock, and carries a 5-year leak, 1-year parts warranty with no labor coverage.
Last updated November 11, 2026: specifications checked against Rheem’s RTEX-18 and RETEX-18 product pages, Rheem’s pro (RH-TE-WH-SS Rev. 1, 7/26) and retail (THD-PTE 18-36 Rev. 1, 11/24) spec sheets, the Performance tankless electric installation manual, Rheem’s tankless electric warranty (EX52 Rev 12/19) and its dormant-restart bulletin; prices against dealer listings that day.
Rheem sells the same 18 kW electric tankless heater under two model numbers: the RTEX-18 (Professional Classic, through plumbing wholesalers) and the RETEX-18 (Performance, at Home Depot). Depending on which page you read, it delivers 1.5, 1.8, 3.2, 3.51, 4.4, 5.0 or 7 GPM. All of those numbers can be correct at once, because each one assumes a different groundwater temperature and set point. This review goes through Rheem’s spec sheets, installation manual and warranty, works out what the heater delivers where you live, and compares it with the other 18 to 20 kW electric units on the market. We did not test the heater ourselves; every figure below is from the manufacturer documents or dealer listings named next to it.
Rheem RTEX-18 specs at a glance
| Spec | Rheem RTEX-18 / RETEX-18 | Source |
|---|---|---|
| Type | Electric tankless, self-modulating, indoor wall-hung | Rheem spec sheets, manual |
| Power | 18 kW at 240 V, single phase; two 9 kW elements | Rheem spec sheets, manual |
| Current draw | 75 A maximum | Rheem spec sheets |
| Breakers and wire | 2 x 40 A double-pole; 2 x 8 AWG (75°C) plus a ground per circuit | Rheem spec sheets, manual |
| Recommended service panel | 150 A | Rheem spec sheets |
| Efficiency | 0.94 UEF | Rheem spec sheets |
| Heating chambers | 2, copper heat exchanger, copper immersion elements | Rheem spec sheets |
| Flow | 0.3 GPM to activate; 7 GPM maximum through the unit | Rheem manual |
| Rated delivery | 1.8 to 4.4 GPM at 105°F, or 1.5 to 2.9 GPM at 120°F, by groundwater zone | Rheem pro and retail spec sheets |
| Temperature range | 80°F to 140°F in 1-degree steps | Rheem product pages |
| Water pressure | 25 to 150 psi | Rheem spec sheets |
| Connections | 3/4 in NPT brass, inlet and outlet on the bottom | Rheem spec sheets |
| Size and weight | 17 x 14 x 3.6 in; 14.78 lb shipping weight | Rheem spec sheets |
| Venting | None required | Rheem spec sheets |
| Warranty | 5 years leaks, 1 year parts, no labor | Rheem warranty EX52 |
By the numbers
- 1.8 GPM: what Rheem’s pro spec sheet gives the RTEX-18 at 105°F with 37°F groundwater (Zone 1). Rheem’s own fixture table puts a shower at 2.0 GPM, so in the coldest zone it cannot hold 105°F on one standard shower.
- 4.4 GPM: the same heater with 77°F groundwater (Zone 9), per the same table. The 2.4-times spread comes entirely from inlet temperature.
- 75 A on two 40 A breakers: the RTEX-18’s draw per Rheem, against 54 A on one 60 A breaker for the 13 kW RTEX-13 and 100 A on three 40 A breakers for the 24 kW RTEX-24.
- Up to 34%: the energy saving Rheem’s selling guide quotes from the Department of Energy for one tankless heater serving a home with up to 2 bathrooms in some climates, against up to 50% for point-of-use heaters at each outlet.
- $449.00 vs $449.00: the RTEX-18 (out of stock) and Eemax’s 18 kW HomeAdvantage II HA018240 (in stock) at Big Frog Supply on November 11, 2026. The two share their electrical ratings, flow table and warranty document.
Why the RTEX-18 has so many GPM ratings
An 18 kW element turns electricity into about 61,400 BTU of heat an hour (18 x 3,412). By the standard water-heating formula, GPM equals BTU per hour divided by 500 times the temperature rise. Every flow figure published for this heater is that one number divided by a different rise:
| Where it appears | Flow claim | Assumption behind it |
|---|---|---|
| Rheem RTEX-18 product page | 1.8 to 3.2 GPM | 105°F output, groundwater 37°F to 67°F (Zones 1 to 7) |
| Rheem pro spec sheet (RH-TE-WH-SS) | 1.8 to 4.4 GPM | 105°F output, groundwater 37°F to 77°F (Zones 1 to 9) |
| Rheem RETEX-18 product page; FactoryPure listing title | Up to 4.4 GPM | 105°F output from 77°F groundwater (a 28°F rise) |
| Rheem retail spec sheet (THD-PTE 18-36) | 1.5 to 2.9 GPM | 120°F output, groundwater 37°F to 77°F |
| Home Depot US listing title (item 300800620) | 3.51 GPM | About a 35°F rise, by our math (61,400 / (500 x 35)) |
| Home Depot Canada listing (RETEX-18 FC) | 1.5 to 5.0 GPM | 5.0 GPM equals a 25°F rise in Rheem's manual |
| Rheem manual; RETEX-18 page | 7 GPM | Maximum flow through the unit, not hot-water output (18°F rise) |
The two Rheem tables do not contradict each other. The pro sheet answers “how much 105°F water comes out”, which is roughly what reaches a showerhead. The retail sheet answers “how much 120°F water comes out”, which is then mixed with cold at the tap. Mix 1.5 GPM of 120°F water with 37°F cold water down to 105°F and you get about 1.8 GPM at the shower, the pro sheet’s figure. The 3.51, 5.0 and 7 GPM numbers are the ones to ignore unless your groundwater is unusually warm. Home Depot Canada’s listing also describes the heater as serving “1 shower” with a “household size less than 2” and a “5 gallon” tank capacity, which is meaningless for a tankless heater.
What the RTEX-18 delivers where you live
Rheem’s sizing guide splits the US into nine groundwater zones. This table merges the zone rows from both Rheem spec sheets. The “showers” column is our arithmetic at Rheem’s 2.0 GPM per shower.
| Rheem zone | Groundwater | GPM at 105°F (pro sheet) | GPM at 120°F (retail sheet) | 2.0 GPM showers at once |
|---|---|---|---|---|
| Zone 1 | 37°F | 1.8 | 1.5 | Less than 1 |
| Zone 2 | 42°F | 2.0 | 1.6 | 1 |
| Zone 3 | 47°F | 2.1 | 1.7 | 1 |
| Zone 4 | 52°F | 2.3 | 1.8 | 1 |
| Zone 5 | 57°F | 2.6 | 1.9 | 1 plus a 0.5 GPM faucet |
| Zone 6 | 62°F | 2.9 | 2.1 | 1 plus a 0.5 GPM faucet |
| Zone 7 | 67°F | 3.2 | 2.3 | 1 plus a bathroom faucet |
| Zone 8 | 72°F | 3.7 | 2.6 | About 2 at reduced flow |
| Zone 9 | 77°F | 4.4 | 2.9 | 2 |
Groundwater also changes with the season, as Rheem’s sheet notes, so size for your coldest month. Rheem sells an optional IFR 3-4 outlet flow regulator that caps flow so the water stays hot. Its table recommends a 2.0 GPM insert for the RTEX-18 in Zones 2 through 7, 3.0 GPM in Zones 8 and 9, and no insert in Zone 1. Our flow rate guide and sizing guide cover how to add up fixtures.
Rheem’s manual gives the other side of the same curve: at 1.5 GPM the RTEX-18 can raise water 82°F, at 2 GPM 61°F, at 3 GPM 41°F and at 4 GPM 31°F. If your inlet water is 50°F and you want 120°F (a 70°F rise), that table puts you between 1.5 and 2 GPM.
Wiring: what the 18 kW actually needs
| Model | kW | Max amps | Breakers | Wire | Panel Rheem recommends |
|---|---|---|---|---|---|
| RTEX-13 | 13 | 54 | 1 x 60 A double-pole | 1 x 6 AWG | Not listed |
| RTEX-18 | 18 | 75 | 2 x 40 A double-pole | 2 x 8 AWG | 150 A |
| RTEX-24 | 24 | 100 | 3 x 40 A double-pole | 3 x 8 AWG | 200 A |
| RTEX-27 | 27 | 113 | 3 x 40 A double-pole | 3 x 8 AWG | 200 A |
| RTEX-36 | 36 | 150 | 4 x 40 A double-pole | 4 x 8 AWG | 300 A |
Rheem’s manual requires each wire pair on its own double-pole breaker with a separate ground conductor per circuit. That makes the RTEX-18 the largest Rheem electric tankless model that still fits a 150 A panel by Rheem’s own recommendation. Rheem’s spec sheet adds that switching from an electric tank to electric tankless often requires a panel upgrade. Our electrical requirements guide covers load calculations and panel upgrades.
On 208 V (common in apartments and commercial buildings), Rheem lists separate “208V performance” pages for both models, and its manual warns that the 240 V elements produce less heat at lower voltage. Eemax’s spec sheet for its 18 kW HomeAdvantage II gives 13.5 kW and 65 A at 208 V, and the same (208 / 240) squared math gives 75% of rated power for the RTEX-18. On 208 V, read every flow figure above as about a quarter lower.
RTEX-18 vs RETEX-18: pro and retail versions
| RTEX-18 (Professional Classic) | RETEX-18 (Performance) | |
|---|---|---|
| Sold through | Plumbing wholesalers and online dealers | Home Depot ("Exclusive to the Home Depot" on Home Depot Canada) |
| kW, amps, breakers, wire | 18 kW, 75 A, 2 x 40 A, 2 x 8 AWG | Same |
| UEF, chambers, cabinet | 0.94, 2 copper chambers, 17 x 14 x 3.6 in | Same |
| Rated delivery on the spec sheet | 1.8 to 4.4 GPM at 105°F | 1.5 to 2.9 GPM at 120°F |
| Headline flow on Rheem's product page | 1.8 to 3.2 GPM | Up to 4.4 GPM |
| Warranty | 5 yr leaks, 1 yr parts (EX52) | Same document |
| Price checked November 11, 2026 | $449.00 Big Frog Supply; $523.99 FactoryPure (both out of stock) | Home Depot blocked our reads; not confirmed |
Buy whichever is in stock at the lower price. Nothing in Rheem’s documents separates the two beyond the sales channel and how the flow is quoted.
How the RTEX-18 compares with other 18 to 20 kW electric heaters
| Model | Power | Breakers | Warranty | Price checked November 11, 2026 |
|---|---|---|---|---|
| Rheem RTEX-18 | 18 kW / 75 A | 2 x 40 A | 5 yr leaks, 1 yr parts | $449.00 Big Frog (out of stock); $523.99 FactoryPure (out of stock) |
| Eemax HomeAdvantage II HA018240 | 18 kW / 75 A | 2 x 40 A | Covered by the same EX52 document as Rheem | $449.00 Big Frog (in stock); $518.36 Plumbing-Deals (in stock) |
| EcoSmart ECO 18 | 18 kW / 75 A | 2 x 40 A | Limited lifetime, product registration required (EcoSmart) | Big-box listings blocked our reads |
| Stiebel Eltron Tempra 20 Plus | 19.2 kW / about 80 A | 2 x 40 A | Not checked for this review | $729.00 Big Frog (new); $529.00 FactoryPure (manufacturer reconditioned) |
| Rheem RTEX-13 (step down) | 13 kW / 54 A | 1 x 60 A | 5 yr leaks, 1 yr parts | $365.00 Big Frog (in stock) |
| Rheem RTEX-24 (step up) | 24 kW / 100 A | 3 x 40 A | 5 yr leaks, 1 yr parts | $582.59 Big Frog (out of stock) |
- Eemax HA018240. Eemax is a Rheem brand; its spec sheet lists the same 18 kW, 75 A, two 40 A breakers, 0.3 GPM activation, 7 GPM maximum and an identical temperature-rise table (61°F at 2 GPM, 41°F at 3 GPM), in a 17 x 14 x 3.75 inch case. Rheem’s warranty document names Eemax, Rheem and Richmond together. When the RTEX-18 is out of stock, the HA018240 at the same $449.00 is the closest substitute. Our Eemax review covers the rest of that line.
- EcoSmart ECO 18. EcoSmart is also a Rheem brand, and its product page lists the same 75 A, two 40 A breakers, 150 A panel recommendation and 17 x 14 x 3.635 inch cabinet as the RTEX-18. It also lists a limited lifetime warranty with product registration, against 1 year on parts for the RTEX-18, which is the main reason to choose it at a similar price. See our EcoSmart guide.
- Stiebel Eltron Tempra 20 Plus. It has 7% more power on the same two 40 A breakers plus Stiebel’s Advanced Flow Control, which cuts flow to hold temperature when demand exceeds capacity. At $729.00 new it costs $280 more than the RTEX-18’s Big Frog price. Big Frog listed the larger Tempra 24 Plus at $699.00 the same day, $30 less than the 20 Plus. See our Stiebel Eltron review.
The warranty fine print
| Term | What Rheem's EX52 warranty says |
|---|---|
| Parts (mechanical or electrical failure) | 1 year from original purchase |
| Leaks | 5 years from purchase, or until the owner occupies a new home with the heater installed, whichever is later |
| Labor | Not covered; removal and repair costs are on the owner |
| Start date | Purchase date with the original receipt; otherwise the manufacture date |
| Remedy | Repair or replace, at Rheem's choice; RMA number required before any return |
| Excluded | Dry fire; freezing; pressure outside the published range; flood, fire, lightning; heater moved from its original location; install not to code or the manual; non-approved sealant |
| Same document covers | Eemax, Rheem and Richmond tankless electric heaters (EX52 Rev 12/19) |
The manual adds that once the heater is installed you should not return it to the store, and that Rheem can deny claims for damage from water outside its quality limits. Rheem’s 2025 selling guide calls it “a 5-year heating chamber warranty with a 1-year parts limited warranty”, which matches the certificate’s leak and parts terms. For comparison, Rheem’s gas IKONIC carries 15 years on the heat exchanger and 1 year of labor (see our IKONIC review).
The most common way to lose coverage on an electric tankless heater is a dry fire: power on while air is trapped in the chambers. Rheem’s dormant-restart bulletin says to switch the breakers off after any water shutoff, open the hot taps with aerators removed, run water at full flow for 5 minutes, and only then turn the power back on.
Installation notes
- Indoors only, out of freezing temperatures; Rheem’s manual says freeze damage is not covered. See our freeze protection guide.
- Clearances: 12 inches above and below, 6 inches in front and to the sides, and flammable materials at least 24 inches from the heater and the hot outlet pipe.
- Mounting: upright with the connections at the bottom, on 4 screws at least 1 inch long, level.
- Plumbing: flexible water heater hoses recommended; hold the unit’s fitting with a second wrench so the flow sensor isn’t damaged; no soldering at the unit; ball valves on the inlet and outlet; pipe rated to at least 150°F within 3 feet.
- Relief valve: not required by UL 499, but required in Massachusetts and Kentucky per Rheem, and recommended with flex or high-temperature CPVC pipe.
- Leaks: Rheem recommends a drained drip pan or an automatic leak detector and shutoff valve.
- Hard water: Rheem’s manual says hard water must be treated or the heat exchanger flushed regularly. Our descaling guide covers the process.
CPSC’s recall database lists no recall of any Rheem, Eemax or EcoSmart tankless water heater, checked November 11, 2026. The only Rheem water-heater recall it lists is a 2016 repair program for electric tank heaters sold at Home Depot.
Who should buy the RTEX-18, and who shouldn’t
- Buy it for a 1-bath home, apartment or ADU in the southern US (Zones 6 to 9, groundwater 62°F or warmer), where it covers a shower plus a faucet or two showers.
- Buy it as a single-bathroom heater anywhere, including a cold-climate addition or a primary suite, paired with a 1.8 to 2.0 GPM showerhead.
- Buy it if your panel is 150 A: it is the largest Rheem electric model that fits Rheem’s 150 A recommendation.
- Step up to the RTEX-27 or RTEX-36 for a 2-bath home in a colder zone, if your panel can take 113 to 150 A: at 52°F groundwater Rheem’s pro sheet rates them 3.5 and 4.6 GPM at 105°F, against 2.3 GPM for the RTEX-18. Our best electric tankless roundup compares the whole-home models.
- Step down to the RTEX-13 ($365.00 at Big Frog) for a single sink or a warm-climate shower on one 60 A breaker.
- Look at gas if you need two showers at once in a cold climate; our electric vs gas comparison covers the trade-offs.
The bottom line
The Rheem RTEX-18 is an honest 18 kW heater with an inconsistently labeled flow rating. Rheem’s own tables put it at 1.8 GPM in the coldest US zone and 4.4 GPM in the warmest at 105°F, on two 40 A breakers and a 150 A panel. That makes it a warm-climate small-home heater or a one-bathroom heater anywhere, not a cold-climate whole-house unit. It costs $449.00 at the dealer we found listing it lowest, the same as Eemax’s HomeAdvantage II with matching ratings, but carries only 1 year on parts and no labor. If a longer warranty matters, Rheem’s EcoSmart ECO 18 lists a limited lifetime warranty with registration. For the rest of Rheem’s lineup, see our best Rheem tankless water heaters guide.
Sources: Rheem product pages for the Professional Classic RTEX-18, RTEX-18 (208V performance), Performance RETEX-18 and RETEX-18 (derated 208V performance) (rheem.com, November 11, 2026); Rheem Professional Classic RTEX spec sheet RH-TE-WH-SS Rev. 1 (7/26); Rheem Performance RETEX spec sheet THD-PTE 18-36 Rev. 1 (11/24); Rheem tankless electric selling guide TKE-401 Rev. 5 (03/25), which cites DOE’s “Advantages of Tankless or Demand-Type Water Heaters”; Rheem Performance tankless electric installation and owner’s manual; Rheem limited warranty for tankless electric water heaters EX52 Rev 12/19; Rheem bulletin “Best Practices to Restart a Dormant Tankless Electric Water Heater”; Eemax HomeAdvantage II specification sheet and product page (eemax.com); EcoSmart ECO 8, ECO 11, ECO 18, ECO 27 and ECO 36 product pages (ecosmartus.com); Home Depot Canada RETEX-18 FC listing (SKU 1001070391); Home Depot US listing title for item 300800620; dealer listings at Big Frog Supply, FactoryPure and Plumbing-Deals (November 11, 2026); CPSC recall database (saferproducts.gov, November 11, 2026).