The DHW flow sensor sits in the cold-water inlet. When water flows past it, it signals the control board to fire the burner. Failure = no burner fire when you open a hot tap.
Symptoms
- Open hot tap; nothing happens; close and re-open; works sometimes
- Code F5 (Bosch flow sensor specific)
- LC (low water flow) message with adequate flow
Diagnose
Inspect paddle wheel for sediment. Check sensor wiring continuity. Verify inlet water filter is clean.
Replacement
DIY-replaceable on most Bosch models:
- Close inlet isolation valve
- Disconnect sensor harness
- Unscrew housing
- Inspect paddle wheel
- Install new sensor with new O-ring
- Reinstall
Cost
OEM: $40-90. Labor (professional): $150-250.
Bottom line
For intermittent hot-water issues on a Bosch with no other code, flow sensor is the prime suspect. Cleaning the paddle wheel often resolves; replacement covers the rest.