Stiebel Eltron Tempra 36 Plus vs Rheem RTEX-36: Which Electric Tankless Wins?

Two 36kW electric tankless water heaters. One costs nearly double the other. Is German engineering worth the premium, or does Rheem's American value proposition win?

If you're comparing the Stiebel Eltron Tempra 36 Plus against the Rheem RTEX-36, you're choosing between two fundamentally different philosophies: precision German manufacturing with advanced flow control versus straightforward American reliability at half the price.

This comparison breaks down real-world performance differences, warranty coverage, installation requirements, and long-term value. By the end, you'll know which model fits your home, budget, and expectations.

Quick Specs Comparison

The Key Difference: Flow Control Technology

Stiebel Eltron's Advanced Flow Control is the primary reason for the price gap. When demand exceeds capacity (cold inlet water, multiple fixtures running), the Tempra 36 Plus automatically throttles flow to maintain set temperature. You get consistent 120°F water at 5 GPM instead of inconsistent 95°F water at 7 GPM.

The Rheem RTEX-36 does not throttle. If you ask for more hot water than the unit can deliver, outlet temperature drops. In northern climates with 40°F inlet water, running three showers simultaneously will produce lukewarm water, not reduced flow at the target temperature.

This matters most in:

• Cold climate installations (inlet temps below 50°F)
• Homes with simultaneous high-demand use (multiple showers, dishwasher, washing machine)
• Applications where temperature consistency is critical (commercial kitchens, medical facilities)

If you live in a warm climate (inlet water above 60°F) and use one fixture at a time, flow control provides minimal benefit. The RTEX-36 will maintain temperature just fine under those conditions.

Temperature Consistency in Real Use

Stiebel Eltron's microprocessor-controlled heating elements deliver ±1°F temperature stability. Reddit users and professional plumber reviews consistently report zero temperature fluctuation during single-fixture use and minimal variation when multiple fixtures activate.

The Rheem RTEX-36 shows ±3-5°F swings when flow rate changes (someone turns on a sink while you're showering). This is standard for non-modulating electric tankless units. Not a dealbreaker for most residential users, but noticeable if you're sensitive to temperature shifts.

Cold Weather Performance

In regions where inlet water drops to 40°F (northern states, mountain areas), the Tempra 36 Plus maintains 120°F output at approximately 4.5 GPM. The RTEX-36 maintains the same temperature at roughly 4.2 GPM. The difference is small in absolute terms but meaningful when the calculation determines whether you can run two showers simultaneously.

Both units handle freeze protection identically: install indoors, insulate pipes, or use heat tape on outdoor runs. Neither is designed for outdoor installation in sub-freezing climates.

Warranty Comparison: What You're Actually Covered For

Stiebel Eltron's 7-year heating element warranty includes 3 years of labor coverage. This is rare in the electric tankless market. If the unit fails in year two, Stiebel covers parts and the plumber's service call. Rheem covers parts only (1 year for non-element components).

Heating chamber failures are the most common tankless issue after year five. Stiebel's extended coverage directly addresses the highest-risk component. Rheem's 5-year heating chamber warranty is industry standard - not bad, but shorter than competitors like EcoSmart (lifetime warranty on elements, though with more restrictive conditions).

Practical implication: Over a 10-year lifespan, the Tempra 36 Plus will likely cost $200-400 less in out-of-warranty repairs. This narrows the upfront price gap but doesn't eliminate it.

Installation Requirements: Identical Electrical Demands

Both units require:

• 240V electrical service
• 150-amp circuit breaker (dedicated circuit)
• Three 8-gauge copper wire runs
• Professional electrician installation (not DIY-friendly for most homeowners)

Installation cost will be the same for either model: $400-800 depending on distance from breaker panel and local labor rates. If your home currently has 100-amp service, upgrading to 200-amp service adds $1,500-3,000. This is a sunk cost regardless of which tankless model you choose.

Neither unit is significantly easier to mount or plumb. Both weigh around 30 lbs and use standard 3/4-inch NPT connections.

Long-Term Reliability: Owner Feedback

Stiebel Eltron Tempra units have a strong track record on HVAC-Talk and r/Plumbing. Common praise: silent operation (literally zero noise), bulletproof German components, and excellent customer service when issues arise. Complaints center on price and the fact that flow control can be overly aggressive (throttles flow when you'd prefer slightly cooler water at higher volume).

Rheem RTEX-36 owners report solid performance for the price. Units tend to run for 8-12 years without major issues if installed correctly and maintained annually (flushing, descaling in hard water areas). The most common failure mode is a blown heating element around year 6-8, which is a $150-300 repair if done outside warranty.

Neither unit is known for catastrophic failures or safety issues. Both are UL-listed and meet North American electrical codes.

Energy Efficiency: Negligible Difference

Electric resistance heating is 99%+ efficient by definition. The electricity you pay for turns into heat with minimal loss. Stiebel Eltron's "self-modulating" technology doesn't save energy - it improves comfort by adjusting power output to match demand, but you're still paying for the same kilowatt-hours to heat the same gallons of water.

Annual operating cost for either unit in a family of four with average usage: $450-600 depending on local electricity rates. The difference between the two models over a year is under $10.

If energy cost is your primary concern, you should be comparing electric tankless to gas tankless or heat pump water heaters, not splitting hairs between two identically efficient electric models.

Who Should Buy the Stiebel Eltron Tempra 36 Plus

• You live in a cold climate where inlet water drops below 50°F in winter
• You regularly run multiple hot water fixtures simultaneously
• Temperature consistency matters to you (you notice and hate temperature swings)
• You value longer warranty coverage and are willing to pay for it
• You plan to stay in your home 10+ years and want the most durable option

The Tempra 36 Plus is the better engineered product. It costs twice as much because it solves problems the RTEX-36 doesn't address. If those problems don't apply to your situation, you're paying for features you won't use.

Who Should Buy the Rheem RTEX-36

• You live in a warm climate (inlet water above 60°F year-round)
• You primarily use one fixture at a time (low simultaneous demand)
• Your budget is tight and you need a capable 36kW unit under $800
• You don't mind minor temperature fluctuations during use
• You want a straightforward, no-frills electric tankless that works

The RTEX-36 delivers the same core function - endless 36kW hot water - at half the price. For many homeowners, especially those in the southern half of the US, it's the smarter buy.

Final Verdict

If you can afford the Tempra 36 Plus and you live in a climate or usage scenario where its advanced features matter, buy it. The flow control technology and extended warranty justify the premium in cold climates and high-demand households.

If you're in a warm climate with moderate usage, the Rheem RTEX-36 is the better value. You'll get 8-10 years of reliable service, save $700+ upfront, and likely never notice the features you're not paying for.

Both are excellent units. Your choice comes down to climate, usage patterns, and whether you value engineering refinement over cost efficiency.

Shop Now:

• Stiebel Eltron Tempra 36 Plus on Amazon
• Rheem RTEX-36 on Amazon