Flow Control is the technology that separates Stiebel Eltron Tempra Plus from every other US-market electric tankless. It\'s the reason Tempra Plus works in cold-climate homes where competitors don\'t.
The problem Flow Control solves
Standard electric tankless (Rheem RTEX, EcoSmart, Eemax HA, Titan) use constant-power modulation. The unit delivers its rated kW; the output temperature is whatever physics dictates given the flow rate and incoming water temperature.
Math example: A 27 kW unit can heat 1 GPM by about 187°F, 2 GPM by 93°F, 3 GPM by 62°F, etc. In winter with 40°F incoming water, opening the shower (2 gpm) plus a sink (1 gpm) = 3 gpm total. Available temperature rise = 62°F. Output: 40 + 62 = 102°F. That\'s a tepid shower.
The standard electric tankless solution is to "just buy a bigger unit." But sizing for worst-case winter peak demand often pushes the unit into territory that exceeds your electrical service capacity.
How Flow Control works
Stiebel Tempra Plus measures incoming water temperature and flow rate continuously. When the unit detects flow that would result in below-setpoint output, the internal flow control valve throttles the flow down until the unit can maintain setpoint at the reduced flow.
Same example: 3 gpm requested, would result in 102°F output. Flow Control throttles flow to 2 gpm, allowing 93°F rise → output of 133°F. Setpoint maintained; you got slightly less water but you got HOT water.
What you feel
- Water pressure at the fixture drops slightly when total demand exceeds capacity
- Output temperature stays at setpoint
- You may notice reduced shower flow when laundry simultaneously runs hot
- Smaller fixtures may "borrow" flow capacity from larger ones
Why this matters for cold-climate buyers
Cold-climate winter sizing for whole-house electric tankless requires either:
- Constant-power unit oversized for worst-case demand — often 36-40+ kW. Requires service upgrade. Cost: $3,000-5,000 install premium
- Flow Control unit sized for typical demand with Flow Control handling peak — Tempra 29 Plus often adequate where constant-power competitor would need 36 kW
Flow Control lets you use a smaller (and cheaper) electrical service and still get reliable winter hot water performance.
Tradeoffs
- You give up some peak simultaneous flow — Flow Control is "soft cap" behavior
- Households running multiple high-flow fixtures simultaneously will notice flow restriction
- Less applicable in warm-climate sizing where the unit is amply sized regardless
Field test
Owner reviews of Tempra Plus repeatedly describe the same scenario: "I was skeptical about electric tankless in our cold climate; the Tempra works beautifully. Shower stays hot even when the dishwasher runs. We don\'t notice the slight flow reduction." This is Flow Control working as designed.
Competitors with similar features
- Rheem RTEX: constant power, no flow modulation
- EcoSmart: constant power, no flow modulation
- Eemax HA: constant power, no flow modulation
- Titan SCR: constant power, no flow modulation
- Bosch Tronic Heater: some models have throttling features (Greentherm gas tankless uses similar concept)
Stiebel Eltron is essentially alone among US-market whole-house electric tankless in offering true Flow Control.
Bottom line
Flow Control is real, meaningful technology — not marketing hyperbole. For cold-climate or moderate-climate whole-house electric tankless, it\'s the single best reason to choose Stiebel Eltron over US-market competitors despite the 50-80% price premium. Warm-climate installs where capacity is plentiful see less Flow Control benefit.