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; works sometimes
- LC (low water flow) message with adequate flow
Diagnose
Inspect paddle wheel for sediment. Check wiring continuity. Verify inlet water filter is clean.
Replacement (DIY)
- Close inlet isolation valve
- Disconnect sensor harness
- Unscrew housing
- Inspect paddle wheel
- Install new sensor with O-ring
- Reinstall
Cost
OEM: $40-90.
Bottom line
For intermittent hot-water issues on a Takagi with no other code, flow sensor is the prime suspect.