Rheem RTEX-18 review: priced against a bigger heater

The Rheem RTEX-18 is a well-made heater in an unfortunate position, and one number explains most of this review.

It costs $415.21. The EcoSmart ECO 24, sitting beside it in the same search results, costs $419.99 — four dollars and seventy-eight cents more — and delivers 24 kW to this unit's 18 kW. That is a quarter more hot water for the price of a coffee.

Everything good about the RTEX-18 has to be weighed against that fact, and there is genuinely good here: it is the best-built of the two Rheems, the most heavily reviewed unit in this comparison, and it asks for two breakers where its rivals ask for three.

Two breakers instead of three is the real argument#

This is the case for buying the RTEX-18, and it is stronger than it first appears.

Electric tankless heaters are usually discussed in amps, but panels run out of two things independently: capacity and physical spaces. A double-pole breaker occupies two slots. Three of them occupy six.

Plenty of houses have a 200 amp service with adequate spare capacity and a panel that is physically full. In that situation:

• RTEX-18 — 75 amps, two double-pole breakers, four slots. Often fits.
• EcoSmart ECO 24 — 100 amps, three double-pole breakers, six slots. Often does not, and a subpanel plus feeder adds $600 to $1,500.

If that is your panel, the RTEX-18 is not $5 worse value than the EcoSmart — it is $600 better, and the output difference is the price of avoiding the subpanel. That is a legitimate reason to buy this heater, and it is the only one.

breaker and wire sizing for tankless heaters covers how to check both capacity and spaces before you order anything.

Checking your own panel takes five minutes and needs no tools. Open the panel door — the cover stays on — and count the empty positions in the breaker columns. A double-pole breaker spans two adjacent positions in the same column, so four scattered singles are not the same as two usable pairs. Then read the main breaker at the top: it will say 100, 150 or 200 amps, and that is your service size.

Two adjacent pairs free and a 200 amp main, and this unit is straightforward. Fewer than four adjacent positions, and you are into either a tandem-breaker rearrangement or a subpanel, at which point the EcoSmart's third breaker stops being a disadvantage and you should buy the larger heater instead.

What 18 kW delivers#

The arithmetic is the same for every unit in the category:

GPM = BTU output ÷ (temperature rise × 8.34 × 60)

18 kW is 61,419 BTU per hour:

• 35 °F rise — southern summer — 3.5 GPM
• 50 °F rise — mid-Atlantic — 2.5 GPM
• 70 °F rise — northern winter — 1.8 GPM

Rheem advertises 4.4 GPM, which corresponds to a rise of about 28 °F. As category marketing goes that is less egregious than the RTEX-13's 18.5 °F basis, but it is still a summer number.

The practical reading: in a warm climate this covers two low-flow shower heads or one comfortable one plus a sink. In a cold climate it is a single shower, and running the kitchen tap during it will be noticed. Our our tankless sizing calculator gives the number for your own groundwater temperature, and our tankless sizing guide explains the method.

The 4.2 star problem#

With 2,191 ratings this is the most thoroughly reported unit in the comparison, and it carries the lowest average — 4.2, against 4.4 for the RTEX-13 and 4.5 for the EcoSmart ECO 24.

That is worth taking seriously, but not at face value, because the RTEX-13 sitting above it is a smaller heater from the same manufacturer with the same warranty and the same control system. Build quality is not what separates them.

What separates them is what each gets bought for. At $267 and one breaker, the RTEX-13 reads like a point-of-use unit, so people buy it for a sink or a cabin and it does that well. At $415 and 18 kW, the RTEX-18 reads like a whole-home unit, so people buy it to replace a 50 gallon tank — and then meet 1.8 GPM in February.

The negative reviews are, overwhelmingly, sizing complaints. They are also fair: nothing in the listing tells a buyer in Ohio that this heater will not run their shower in winter.

Scale is the maintenance job nobody mentions#

Electric tankless heaters are sold as maintenance-free and they are not. They are lower-maintenance than a tank, which is a different claim.

The mechanism is straightforward. Calcium and magnesium precipitate out of water as it is heated, and in a tankless heater that happens against the surface of the element, which is the hottest metal in the system. Scale forms there first and insulates the element from the water it is meant to be heating. Two things follow: output falls, and the element runs hotter than designed, which shortens its life.

How fast this happens depends almost entirely on water hardness:

• Under 7 grains per gallon — soft to moderately hard. Descale every 2 to 3 years.
• 7 to 12 gpg — hard. Annually.
• Over 12 gpg — very hard. Twice a year, or fit a softener and stop fighting it.

The job itself is a circulation pump, a bucket and a few pints of food-grade white vinegar or a proprietary descaler, run through the unit for 45 minutes. It costs almost nothing provided isolation valves with service ports were fitted at installation. If they were not, the same job means draining the system and breaking pipework, which turns a 45 minute task into a plumber's visit.

Fit the valves during installation. They cost around $50 as a pair and they are the difference between maintenance that happens and maintenance that does not.

This applies equally to every electric unit in this comparison. It appears here because with 2,191 ratings this is the unit whose long-term reviews most often describe declining performance after two or three years — which is very rarely a failure, and very often scale.

Where it sits against the alternatives#

Against the EcoSmart ECO 24 — the comparison that defines this product. Same money, 25% less output, one fewer breaker. Unless the breaker count is genuinely your constraint, the EcoSmart ECO 24 review: the heater is the cheap part is the better purchase and it is not close.

Against the RTEX-13 — $148 more for 5 kW more, and a second breaker. If you are heating one fixture, the Rheem RTEX-13 review: excellent at the job it is not sold for does it for less with an easier install. The step up only makes sense if you genuinely need two fixtures in a warm climate.

Against the Stiebel Eltron Tempra 29 Plus — $695 against $415, and 28.8 kW against 18. The Stiebel is a different tier: more output, and Advanced Flow Control, which weakens flow rather than losing temperature when capacity is exceeded. In a cold climate that behaviour matters more than the kilowatts. See Stiebel Eltron Tempra 29 Plus review: the one that tells you the truth.

Against gas — if there is a gas line at the property, this comparison is uncomfortable for every electric unit here. A Rinnai RE180iN review: the flow rate is real, the venting is the bill gives 4.2 GPM at the winter rise where this manages 1.8, without touching the panel. the gas-versus-electric tankless comparison works through that decision.

Who should not buy this#

• Anyone with six free panel spaces and 100 amps of headroom. Buy the EcoSmart ECO 24 for the same money.
• Anyone heating a single fixture. The RTEX-13 is cheaper, smaller and easier to wire.
• Anyone in a cold climate expecting whole-home service. 1.8 GPM is one shower.
• Anyone on 100 amp service. 75 amps will usually fail the load calculation without delivering enough to be worth the fight.

The verdict#

This is a competent heater that is difficult to recommend, because the product beside it on the shelf costs the same and does more.

The exception is real and worth restating: if your panel has four free spaces and not six, the RTEX-18 is the largest electric tankless heater you can install without a subpanel, and that makes it the right buy despite the arithmetic. Check your panel before you check the price — for once, that is the order that decides this purchase.

For everyone else, the money goes further $5 up or $148 down.