Stiebel Eltron Tempra 29 Plus review: the one that tells you the truth
It costs two and a half times what a Rheem RTEX-13 does and it is worth it — for a specific person, in a specific climate, with a specific panel. Everyone else is overpaying.
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Almost every electric tankless water heater is sold on a flow rate measured at a temperature rise nobody experiences. The EcoSmart ECO 24 claims 5.8 GPM, which is true at a 28 °F rise. The Rheem RTEX-13 claims 4.8 GPM, which is not achievable at any temperature rise worth having.
Then there is this unit, whose listing says 3 GPM.
That looks like a weak number next to the others. It is in fact the most useful number in the category, because 28.8 kW delivering 3.0 GPM is exactly what the arithmetic gives at a 65 °F rise — a northern winter. Stiebel Eltron publishes the figure its competitors hope you never calculate.
| Specification | Stiebel Eltron Tempra 29 Plus |
|---|---|
| Output | |
| Rated power | 28.8 kW at 240 V |
| Heat output | 98,270 BTU/hr |
| Advertised flow rate | 3.0 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) | 5.6 GPM |
| At 50 °F rise (mid-Atlantic) | 3.9 GPM |
| At 70 °F rise (northern winter) | 2.8 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 | 120 A at 240 V |
| Breakers required | 3 × 50 A double-pole |
| Service implication | Needs 120 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) | 16.63 in × 14.5 in × 4.6 in |
| Weight | 19 lb |
| Mounting | Wall |
| Heat exchanger material | Copper |
| Control | |
| Maximum set temperature | 140 °F |
| Flow control | Advanced Flow Control — throttles flow to hold setpoint This is the feature that separates it from everything else here. Rather than let output temperature sag when demand exceeds capacity, it restricts flow to keep the temperature you asked for. You get a weaker shower instead of a cold one. |
| Modulation | Continuous, electronic |
| Display | LED with digital setpoint |
| Plumbing | |
| Connections | 3/4 in NPT |
| Ownership | |
| Warranty | 3 years parts and labour, 7 years on the heating element and exchanger |
| Model number | Tempra 29 Plus |
Advanced Flow Control is the reason to buy this
Every tankless heater has a capacity ceiling. What separates them is what happens when you hit it.
A conventional unit — the EcoSmart, both Rheems, essentially everything else at this price — keeps passing the same volume of water and simply fails to heat it enough. Open a second tap while someone is showering and the shower goes tepid, then cold. This is the single most common complaint about electric tankless heaters and it is not a defect; it is the designed behaviour.
The Tempra Plus does the opposite. When demand exceeds what 28.8 kW can heat to your setpoint, it restricts the flow instead. The shower weakens. It does not cool.
That is a genuinely different ownership experience, and it is worth being clear about the trade, because it is a trade. Some people find a dropping shower pressure more irritating than a wandering temperature, and nothing in the house warns you it is happening — you simply notice the water has gone soft while the dishwasher is filling. But a weak hot shower is recoverable by waiting; a cold one ends the shower.
In a warm climate where the unit rarely reaches its ceiling, this feature does very little and you are paying for nothing. In a cold climate where it is at the ceiling every winter morning, it is the whole product.
The arithmetic, and where 28.8 kW actually lands
The physics is fixed. A BTU raises one pound of water one degree Fahrenheit, water weighs 8.34 lb per gallon:
GPM = BTU output ÷ (temperature rise × 8.34 × 60)
28.8 kW is 98,270 BTU per hour, delivered at effectively 100% efficiency because electric resistance heating has nowhere else to put the energy.
- 35 °F rise — southern summer — 5.6 GPM
- 50 °F rise — mid-Atlantic — 3.9 GPM
- 65 °F rise — northern winter — 3.0 GPM, the listing figure
- 70 °F rise — hard northern winter — 2.8 GPM
Against a 2.0 GPM shower head, the winter figure is one comfortable shower with something else running, or two restricted ones. That is the honest ceiling of whole-home electric tankless in a cold climate, from the largest unit in the class. Anything promising more is quoting a summer number.
Our our tankless sizing calculator runs this against regional groundwater data, and our tankless sizing guide explains why household size is the wrong first question.
120 amps is the real gatekeeper
Three 50 amp double-pole breakers. A 120 amp continuous draw at 240 volts.
To put that in context: a typical electric range is 40 to 50 amps, an electric dryer 30, a heat pump air handler 60. This heater alone is larger than all three combined, and unlike them it is treated as a continuous load, which requires conductors and overcurrent protection sized at 125% of the running current.
What that means by starting condition:
- 200 amp service, modern panel, spare spaces. Straightforward. Three breakers, appropriately sized feeders, possibly a subpanel. $500 to $1,100.
- 200 amp service, full panel. Subpanel and feeder. $1,000 to $2,000.
- 150 amp service. A load calculation will usually fail. Upgrade first.
- 100 amp service. Not viable. Do not buy this unit.
breaker and wire sizing for tankless heaters walks through the load calculation so you can get a realistic answer before an electrician is standing in your basement.
This is where the Tempra 29 Plus loses most of its potential buyers, and it loses them before price is ever discussed. A $695 heater that requires a $4,000 service upgrade is a $4,695 heater.
Where it sits against the alternatives
Against the EcoSmart ECO 24 — $695 against $420, 28.8 kW against 24 kW. The output difference is 20%; the price difference is 65%. You are not buying kilowatts, you are buying Advanced Flow Control and a manufacturer that quotes real numbers. In a warm climate that is a poor trade and the EcoSmart ECO 24 review: the heater is the cheap part covers the better-value option. In a cold one it is the right call.
Against the Rheem RTEX-18 — not really a comparison. 18 kW is 1.8 GPM at a 70 °F rise, which is not whole-home anywhere cold. The Rheem RTEX-18 review: priced against a bigger heater explains where that unit does fit.
Against gas — the honest answer is that if you have a gas line, gas wins in a cold climate and it is not close. A Rinnai RE180iN review: the flow rate is real, the venting is the bill delivers 4.2 GPM at a 70 °F rise where this unit manages 2.8, and it does it without touching your electrical panel. The Rinnai costs more up front and adds a venting bill, but it does not risk a service upgrade. the gas-versus-electric tankless comparison works through gas versus electric properly.
Build and warranty
The Tempra Plus is assembled to a noticeably higher standard than the rest of this group, and the warranty reflects it: 7 years on the heating element and exchanger, 3 years parts and labour.
The detail worth noting is what is absent. EcoSmart's lifetime cover is conditional on professional installation, original ownership and the original address, and a claim can fail on any of the three. Stiebel attaches no professional-installation condition to the core cover. Given that this unit needs 120 amps and will be professionally installed regardless, that is a smaller advantage in practice than it looks — but it means the warranty is harder to lose by accident.
Element replacement, when it eventually comes, is also simpler here than on most of the category. The elements are accessible from the front without disturbing the plumbing connections, which turns a job that is otherwise a partial de-install into an hour's work.
Who should not buy this
- Anyone on 150 amp service or less. The load calculation decides this, not the budget.
- Anyone in a warm climate. You will never reach the ceiling where Advanced Flow Control earns its money. Buy the EcoSmart and keep $275.
- Anyone wanting a point-of-use heater. This is a 120 amp whole-home appliance. The Rheem RTEX-13 review: excellent at the job it is not sold for is the right shape for a single fixture.
- Anyone with gas at the property. Run the the ten-year cost comparison against a tank numbers first.
What works
- Advanced Flow Control. When you exceed capacity it throttles flow to hold temperature. No other unit here does this, and it is the single most valuable feature in the category.
- The listing quotes 3 GPM, which is its output at a 65 °F rise. Stiebel publishes a cold-climate number where competitors publish a best case. That honesty is worth something before you have even opened the box.
- 28.8 kW — the highest electric output in this comparison, 20% above the EcoSmart ECO 24.
- 7 years on the element and exchanger, 3 years parts and labour, with no professional-installation condition attached to the core cover.
- Consistent temperature at low flow. Electronic modulation is continuous rather than staged, so a hand wash does not trigger a cold slug.
What doesn't
- 120 amps at 240 volts across three 50 amp double-pole breakers. This is the largest single load in most houses that have one, and it disqualifies any 100 amp service outright.
- $695 against $267 for a Rheem RTEX-13. You are paying 2.6× for roughly 2.2× the output plus one very good feature.
- Advanced Flow Control is not free. Exceed capacity and your shower pressure visibly drops. Some people find that more annoying than a temperature wobble, and it surprises them because nothing warns you.
- No built-in Wi-Fi or app. At this price, competitors at half the money include connectivity.
- 450 Amazon ratings against 1,404 for the RTEX-13 and 2,191 for the RTEX-18. A thinner body of owner reports to read before buying.
The verdict
The Tempra 29 Plus is the best electric tankless water heater here, and most people reading this should not buy it.
That is not a contradiction. It is the best because it handles its own limits better than anything else in the category, and because Stiebel Eltron is the only manufacturer in this group that describes those limits accurately before you have paid. Advanced Flow Control is a genuine engineering answer to the defining failure mode of electric tankless heating.
But 120 amps is an enormous ask, and the feature that justifies the price only pays off in a climate cold enough to keep the unit at its ceiling. If you are north, on 200 amp service, and you have been let down by an undersized electric unit before, this is the one to buy and the score reflects that. If any of those three is not true, you are paying a premium for a feature you will not use.
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What breaker does the Stiebel Eltron Tempra 29 Plus need?
Three 50 amp double-pole breakers, carrying a total 120 amp draw at 240 volts. That is the largest continuous load most houses will ever have, and it means a 200 amp service with real headroom is the entry requirement. On a 150 amp service a load calculation will usually fail once a range, a dryer and an air handler are counted. Get an electrician to run the calculation before ordering.
What does Advanced Flow Control actually do?
It restricts water flow when demand would otherwise exceed the unit's heating capacity. Every other electric tankless heater responds to being overwhelmed by delivering the same volume of water at a lower temperature — the cold shower problem. The Tempra Plus instead delivers less water at the temperature you asked for. In a cold climate, where capacity is routinely exceeded, that is the difference between an appliance that behaves predictably and one that does not.
Why does the listing say 3 GPM when the unit is 28.8 kW?
Because Stiebel quotes flow at a realistic temperature rise rather than a flattering one. 28.8 kW is 98,270 BTU per hour, which is 3.0 GPM at a 65 °F rise — roughly a northern winter. At a 35 °F summer rise the same unit delivers 5.6 GPM. Competitors quote the summer figure and let you discover the winter one. Stiebel prints the winter figure and looks worse on the shelf for it.
Tempra 29 Plus or Tempra 24 Plus?
Climate decides it. The 24 Plus draws 100 amps to the 29's 120, and delivers about 2.3 GPM at a 65 °F rise against 3.0. In the southern half of the country the 24 is enough and saves both money and panel capacity. North of roughly the 40th parallel the 29 is the one that keeps two fixtures running in January. If your panel can take 120 amps and you are anywhere cold, buy the 29.
Is Stiebel Eltron better than EcoSmart?
For this application, yes, and it costs accordingly. The hardware quality is comparable — both use copper exchangers and genuine modulation. The difference is Advanced Flow Control, which EcoSmart does not offer at any price, and it matters most exactly when an electric tankless heater is under stress. If your climate means the unit will regularly be pushed to its limit, that feature is what you are buying. If you live somewhere warm enough that it never gets pushed, the EcoSmart ECO 24 does the same job for $275 less.
Can the Tempra 29 Plus run two showers at once?
In a warm climate, comfortably — 5.6 GPM at a 35 °F rise covers two 2.5 GPM shower heads with margin. In a northern winter at a 65 °F rise it delivers 3.0 GPM, which is two low-flow heads at 1.5 GPM each and nothing else, or one comfortable shower. Advanced Flow Control means that if you try for more, both showers weaken rather than both going cold.