Water is only lukewarm even when the temperature is set to maximum and flow is moderate.

Safety: Turn off power before opening the unit or testing elements.

• Check supply voltage: Measure line voltage at the heater terminals and confirm it matches the nameplate rating.
• Verify power setting: Some Stiebel Eltron models have internal dip switches or installer settings to limit maximum power. Confirm they are set for full rated power if the circuit allows.
• Test heating elements: With power off, measure resistance of each element. A failed element will reduce total heating capacity and cause lukewarm water.
• Descale heat exchanger: If elements and power are correct, perform a descaling to remove limescale that insulates the heating surfaces.
• Professional diagnosis: If under-heating persists, the power control electronics may be limiting output and should be checked by a technician.