The DHW flow sensor sits in the cold-water inlet. When water flows past it (you turned on a hot tap), it signals the control board to fire the burner. If the sensor fails, the unit doesn't know you're drawing hot water — no burner fire, no hot water.
Symptoms
- Open hot tap; nothing happens; close and re-open; works fine sometimes
- Cold water occasionally comes through hot tap because burner didn't fire
- LCO (low water flow) message even with adequate flow
- On NRCB combi: works for heat but not DHW (DHW flow sensor specifically)
Diagnose
The flow sensor sits in the cold inlet, accessible through service access. Checks:
- Inspect the paddle wheel for sediment
- Check sensor wiring continuity
- Test sensor output with multimeter during a known draw
- Verify inlet water filter is clean (clogged filter starves the sensor of flow)
Replacement
DIY-replaceable on most Noritz models:
- Close inlet isolation valve
- Open hot tap to relieve pressure
- Disconnect the sensor harness
- Unscrew the sensor housing
- Inspect paddle wheel — sediment causes most failures
- If sediment-fouled but intact, clean and reinstall
- If electrically failed, install new sensor
- Verify O-ring is seated; reinstall housing
- Open isolation valve; check for leaks
Cost
- OEM Noritz flow sensor: $40-90
- Labor (if professional): $150-250
Warranty
Covered under 5-year parts warranty.
Bottom line
For intermittent hot-water issues on a Noritz with no other error code, the flow sensor is a prime suspect. Cleaning the paddle wheel resolves a meaningful fraction; replacement covers the rest.