Water heater produces no hot water, but heating (if present) works normally.

Safety: Isolate power and close water valves before working on internal water components.

Steps:

• Check if burner fires on hot water demand: Open a hot tap and observe the heater. If the burner does not ignite, suspect the flow sensor or diverter valve.
• Inspect DHW flow sensor: Clean or replace the flow sensor/turbine as it may not be detecting flow.
• Check diverter valve movement: On combi-type Biasi water heaters, the diverter valve should move to DHW position when a tap is opened. If it sticks, it may need cleaning or replacement.
• Descale plate heat exchanger: If the burner fires but water remains cold or lukewarm, the plate heat exchanger may be blocked with scale.

Note: Diverter valve replacement is moderately complex and often best left to a technician.