Boiler shows F7, may attempt to start but shuts down quickly

Safety: Isolate electrical power and close the cold water inlet and gas supply before working on the hydraulic components.

Inspection and cleaning:

• Locate the DHW flow sensor assembly on the cold water inlet to the boiler. It typically contains a small turbine and sensor.
• Shut off the cold water inlet and relieve pressure by opening a hot tap.
• Remove the flow sensor housing according to the service manual. Be prepared for residual water.
• Inspect the turbine for limescale, debris, or damage. Clean gently with a soft brush and descaling solution if necessary. Ensure it spins freely.
• Check the inlet filter/strainer and clean it thoroughly.
• Test the sensor (reed or Hall) with a multimeter while manually spinning the turbine to confirm pulse generation. Replace the flow sensor assembly if no pulses are detected.
• Inspect and reconnect the wiring harness to the PCB.

After reassembly: Open the cold water inlet, check for leaks, restore power and gas, and run a hot tap to verify that the boiler detects flow and F7 does not reappear.