Tesy heater shows E12; anti-legionella or boost heating program fails to complete.

Safety first: Turn off power and allow the heater to cool before opening covers. Hot water at anti-legionella temperatures can cause severe burns.

Steps:

• Check for limescale: If the heater is in a hard water area and has not been descaled, heavy scale on the element can reduce heating efficiency. Drain the tank and inspect the element for scale buildup.
• Descale element and tank: Remove the Tesy heating element flange, mechanically remove scale, and/or soak in descaling solution suitable for stainless steel/enamel tanks. Rinse thoroughly.
• Verify NTC sensors: Check both main and outlet NTC sensors for correct resistance. Replace any faulty sensors.
• Check safety thermostat: Ensure the manual reset safety thermostat (if present) is not tripping prematurely. Replace if it trips below its rated temperature.
• Reassemble and test: Refill the tank, bleed air, restore power, and manually start an anti-legionella cycle. Confirm that E12 does not recur.

Note: If all hardware is good but E12 persists, a control board firmware or logic fault may be present; contact Tesy service.