Rheem RTEX-13 review: excellent at the job it is not sold for
At $267 and one 60-amp breaker it is the cheapest and by far the easiest electric tankless install in this comparison. It is also the one most often bought for a job it cannot do.
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The Rheem RTEX-13 is a good product with a bad number printed on it, and almost everything people dislike about it traces back to that number.
The listing says 4.8 GPM. Work backwards: 13 kW is 44,358 BTU per hour, and 4.8 GPM at that output implies a temperature rise of 18.5 °F. That is taking 70 °F groundwater and delivering it at 88.5 °F. Nobody showers at 88 °F. The figure is arithmetically defensible and practically useless.
What 13 kW actually gives you is a very capable single-fixture heater, and at $267 on one breaker it is the best value in this comparison — as long as you buy it for that.
| Specification | Rheem RTEX-13 |
|---|---|
| Output | |
| Rated power | 13 kW at 240 V |
| Heat output | 44,358 BTU/hr |
| Advertised flow rate | 4.8 GPM Manufacturer figure. It is a real number at a very small temperature rise, and not the number you will see in winter — the rows below are the same unit at rises you will actually meet. |
| Real flow rate | |
| At 35 °F rise (Florida, summer) | 2.5 GPM |
| At 50 °F rise (mid-Atlantic) | 1.8 GPM |
| At 70 °F rise (northern winter) | 1.3 GPM A standard shower head is 1.8 to 2.5 GPM. Divide this row by that to get the honest fixture count. |
| Electrical | |
| Current draw | 54 A at 240 V |
| Breakers required | 1 × 60 A double-pole |
| Service implication | Needs 54 A of spare capacity in the panel On a 100 A service this is usually a panel upgrade before it is a water heater purchase. |
| Physical | |
| Dimensions (W × H × D) | 8.25 in × 12.62 in × 3.6 in |
| Weight | 4 lb |
| Mounting | Wall |
| Heat exchanger material | Copper |
| Footprint note | Smallest unit here by a wide margin 8.25 inches wide genuinely fits in a vanity cabinet, which is what makes it a point-of-use unit rather than a whole-home one. |
| Control | |
| Maximum set temperature | 140 °F |
| Modulation | Self-modulating |
| Display | Digital LED thermostat, 1 °F increments |
| Plumbing | |
| Connections | 3/4 in NPT |
| Ownership | |
| Warranty | 5 years on the heat exchanger, 1 year on parts |
| Model number | RTEX-13 |
One breaker is the feature
Everything else here is a three-breaker appliance. The EcoSmart ECO 24 wants 100 amps across three 40s; the Stiebel Tempra 29 Plus wants 120 amps across three 50s. Both frequently trigger a subpanel, sometimes a service upgrade, and the electrical work routinely costs more than the heater.
The RTEX-13 draws about 54 amps and takes one 60 amp double-pole breaker.
That single difference changes the shape of the project:
- A panel with two free spaces can usually take it. Three-breaker units need six.
- The conductor run is one cable, not three, which is most of the labour.
- Typical installed electrical cost is $150 to $450, against $500 to $2,000 for the larger units.
For a garage sink, a workshop, a cabin, or a bathroom at the far end of a house where the hot water takes ninety seconds to arrive, this is the correct product. You are solving a local problem locally, and the whole job stays small.
the electrical requirements for electric tankless units covers the load calculation if you want to confirm your panel before ordering.
What 13 kW really delivers
The arithmetic that governs every tankless heater:
GPM = BTU output ÷ (temperature rise × 8.34 × 60)
At 44,358 BTU per hour:
- 35 °F rise — southern summer — 2.5 GPM
- 50 °F rise — mid-Atlantic spring — 1.8 GPM
- 70 °F rise — northern winter — 1.3 GPM
A standard shower head is 1.8 to 2.5 GPM; a low-flow head is 1.5. So in the south, in summer, this unit runs one shower. In the north, in winter, it does not quite run one shower at full flow — you would notice, and you would blame the heater.
This is not a Rheem defect. 13 kW is 13 kW, and any manufacturer's 13 kW unit lands in the same place. It is a sizing question, and it is the question the listing does not help you ask. Our the GPM calculator does the arithmetic against regional groundwater temperatures, and how to size a tankless water heater explains why household size is the wrong starting point.
Build and daily behaviour
The chassis is 8.25 inches wide, 12.6 inches tall and about 3.6 inches deep, at roughly 4 lb. That is small enough to sit inside a vanity cabinet or on the wall of a utility cupboard without planning around it — a meaningful practical difference from the 17 inch EcoSmart.
Control is a digital LED thermostat in 1 °F increments up to 140 °F. That ceiling is higher than the EcoSmart ECO 24's 130 °F, which matters if a dishwasher without a booster heater is in the picture.
Self-modulation is present and works, but there is a limit to what modulation can do with only 13 kW to allocate. On the larger units, opening a second tap is absorbed by spare capacity. Here there is very little spare capacity, so a second draw is felt almost immediately. That is a consequence of the size, not the control system.
The 5 year heat exchanger and 1 year parts warranty is the weakest cover in this comparison. Against a $267 purchase that is a defensible trade — this is not a unit you would pay to repair — but it is worth noting next to EcoSmart's lifetime terms.
What a point-of-use install actually solves
The reason to fit a small heater at a fixture is rarely that the main heater is inadequate. It is that the water takes too long to arrive.
A 3/4 inch copper pipe holds roughly 0.023 gallons per foot. A 60 foot run from a basement tank to an upstairs bathroom is therefore holding about 1.4 gallons of cooling water that has to be pushed out of the way before anything warm reaches the tap. At a 1.5 GPM basin flow that is a 56 second wait, every time, and all of it goes down the drain.
Three consequences, and they are the ones people actually notice:
- Wasted water. A household running that fixture six times a day discards roughly 8.4 gallons daily — over 3,000 gallons a year — before anyone has washed anything.
- Wasted energy. All of that water was heated. It cools in the pipe and is thrown away.
- Behaviour. People stop using hot water at that fixture, which is its own quiet cost.
A 13 kW unit mounted under the basin removes the run entirely. Hot water is produced eighteen inches from the tap and arrives in about two seconds. That is what this product is for, and measured against that job rather than a whole-house one, 1.3 GPM in winter is not a limitation — a basin tap does not use more than that.
The plumbing is simple: tee into the existing cold supply, feed the heater, and take the outlet to the hot tap. The existing hot line can be capped or left connected as a backup. It is the electrical side, and only the electrical side, that requires a professional.
Where it sits against the alternatives
Against the RTEX-18 — $267 against $415, one breaker against two, 1.3 GPM against 1.8 at a winter rise. Neither is a whole-home heater in a cold climate. If you are heating one fixture the 13 is the right buy and the extra money is wasted; if you are covering a small warm-climate house, see the Rheem RTEX-18 review: priced against a bigger heater, but read the EcoSmart comparison first.
Against the EcoSmart ECO 24 — a different class of product. $420 against $267, 100 amps against 54, 2.3 GPM against 1.3 in winter. If your panel can take 100 amps and you need whole-home coverage, the EcoSmart ECO 24 review: the heater is the cheap part is the relevant unit. If it cannot, the choice was never between these two.
Against keeping your tank — genuinely worth considering. If the problem is a long wait for hot water at one fixture, this heater solves it for $267 plus wiring while the tank keeps doing the heavy lifting. That hybrid arrangement is common and sensible, and it sidesteps the sizing problem entirely. tankless versus tank running costs has the ten-year comparison.
Who should not buy this
- Anyone trying to heat a whole house with it, in any climate. The arithmetic does not permit it.
- Anyone replacing a 40 or 50 gallon tank one-for-one. You will be disappointed within a week.
- Anyone north of the mid-Atlantic wanting shower service. 1.3 GPM in January is not a shower.
- Anyone who needs long warranty cover. One year on parts is the shortest here.
What works
- One 60 amp double-pole breaker. Every other unit here needs two or three. This is the difference between an afternoon and a subpanel.
- $267, the lowest price in this comparison by a wide margin, and the most stable.
- 8.25 inches wide and 4 lb. It genuinely fits inside a vanity cabinet, which is what a point-of-use heater has to do.
- 1,404 Amazon ratings at 4.4 stars — the most thoroughly reported unit here after the RTEX-18, and better rated than it.
- 140 °F maximum setpoint and 1 °F digital increments, both better than the EcoSmart's 130 °F ceiling.
What doesn't
- The 4.8 GPM on the listing requires an 18.5 °F temperature rise. That is 70 °F groundwater warmed to 88 °F. It is not a shower, it is a tepid rinse, and no useful application runs at that rise.
- 1.3 GPM at a 70 °F rise. In a northern winter this will not run a single standard shower head at full flow.
- 5 year exchanger, 1 year parts — the shortest cover here, against EcoSmart's lifetime and Stiebel's seven years.
- Sold in the same listings and search results as whole-home units with no meaningful distinction drawn, which is how most of the negative reviews happen.
- No modulation headroom. At 13 kW there is very little capacity in reserve, so any second draw is felt immediately.
The verdict
Buy the RTEX-13 for one fixture and it is excellent — cheap, small, easy to wire, well rated by 1,400 owners, and better specified than its price suggests.
Buy it because the box says 4.8 GPM and you will join the minority of those owners writing one-star reviews in January, and they will not be wrong.
The score reflects both facts. The hardware deserves better than 7.2; the way it is sold does not. Decide what you are heating before you decide what to buy, and this becomes one of the easiest recommendations in the category.
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Check price on Amazon Opens Amazon in a new tab. Price and availability change — check before you buy.Questions people actually ask
What breaker does the Rheem RTEX-13 need?
One 60 amp double-pole breaker, carrying about 54 amps at 240 volts. That is the single biggest practical advantage of this unit. The EcoSmart ECO 24 needs three 40 amp breakers and 100 amps of capacity; the Stiebel Tempra 29 Plus needs three 50s and 120 amps. Many panels that cannot accommodate those can accommodate this, and the wiring job is a fraction of the cost.
Can the Rheem RTEX-13 heat a whole house?
No, and this is the central thing to understand before buying. 13 kW is 44,358 BTU per hour, which is 2.5 GPM at a 35 °F rise and 1.3 GPM at a 70 °F rise. A standard shower head uses 1.8 to 2.5 GPM. Even in Florida in summer this unit is one shower and nothing else; in a northern winter it will not fill one shower at full flow. It is a point-of-use heater and it is a good one.
Why does Rheem advertise 4.8 GPM?
Because 13 kW does deliver 4.8 GPM — at a temperature rise of about 18.5 °F. That would mean taking 70 °F incoming water up to 88.5 °F. The figure is arithmetically true and practically meaningless, because no one wants 88 °F water. Every manufacturer in this category quotes flow at a flattering rise; Rheem's is the most flattering here, and Stiebel Eltron's the least.
Rheem RTEX-13 or RTEX-18?
Different jobs. The RTEX-13 is a point-of-use unit on one breaker at $267. The RTEX-18 is a small whole-home unit on two breakers at $415, delivering 1.8 GPM at a 70 °F rise against the 13's 1.3. If you are heating one fixture, the extra $148 and the second breaker buy you nothing. If you are trying to cover a small warm-climate house, neither is really enough and the EcoSmart ECO 24 is the better spend.
Is the Rheem RTEX-13 good for a cabin or a garage?
This is exactly what it is for. A single sink or shower, intermittent use, and often a panel with very little spare capacity — all three play to its strengths. The 8.25 inch chassis mounts almost anywhere, the single 60 amp breaker is realistic even in a small subpanel, and at $267 it costs less than plumbing a supply line back to the main heater would.
Does the RTEX-13 need a dedicated circuit?
Yes. A 60 amp double-pole breaker serving nothing else, with conductors sized for a continuous load. It is not a plug-in appliance and there is no configuration in which sharing a circuit is acceptable. Our guide to electric tankless wiring covers the conductor sizing and the load calculation.