Water from Ariston heater is not hot enough or runs lukewarm, but unit appears to be operating.

Safety first: Turn off power and verify with a meter before touching wiring.

• Check power supply: Confirm voltage at the heater matches the rating plate and that all phases (if three-phase) are present.
• Test elements: Disconnect wires and measure resistance of each element. Replace any open or shorted elements with Ariston-compatible parts.
• Check thermostats: Verify continuity through each stage thermostat when calling for heat. Replace faulty thermostats.
• Descale element: If elements are heavily scaled, replace them or descale the tank to restore efficiency.