Water temperature is too low even when thermostat or setpoint is at maximum.

Safety first: Turn off power before accessing internal components. Avoid setting temperatures above safe anti-scald limits without a mixing valve.

Steps:

• Measure actual temperature: Draw hot water and measure with a thermometer. Compare to the setpoint.
• Adjust mixing valve: If a mixing valve is installed, increase its setting slightly and recheck temperature.
• Test mechanical thermostat: For non-digital models, turn the knob from min to max and listen for a click. If no click or poor control, replace the Tesy mechanical thermostat.
• Check NTC sensor: On digital models, measure NTC resistance and compare to spec. Replace if out of range.
• Control board: If sensors and thermostat are good but temperature remains low, the control board may be limiting power and should be replaced.