The flow sensor on Tempra Plus and electronic DHC models detects water flow and signals the control to energize heating elements. An impeller spins as water flows through; a hall-effect sensor reads the rotation and reports RPM to the control board.
Symptoms of failure
- E1 error on Tempra Plus despite verified water flow at the unit
- Intermittent heating — turns on and off randomly during use
- Fails to activate at low flows that previously worked
- Delayed activation — heating starts several seconds after flow begins
Diagnostic before replacement
- Verify actual flow at the unit inlet — bucket test confirms flow is present
- Clean inlet filter first — clogged filter mimics flow sensor failure
- Test sensor with multimeter on the hall signal lead during flow — should show pulsing signal
- Inspect impeller for debris or stuck condition — pull sensor housing and check
Common cause: stuck impeller
Debris from new construction or hard water can lodge in the impeller chamber, preventing rotation. Cleaning resolves the issue without replacement:
- Power off; close inlet shutoff
- Open hot tap to relieve pressure
- Disconnect flow sensor housing (typically threaded fitting)
- Remove impeller
- Clean impeller and chamber with soft brush; flush debris
- Reinstall; restore water and power; test
Replacement
- Power off; close inlet shutoff; relieve pressure
- Disconnect sensor leads at control board
- Remove sensor housing
- Install new sensor with new gasket/o-ring
- Reconnect leads; verify orientation
- Restore water and verify no leaks
- Restore power; test flow activation
Cost
- Flow sensor (OEM): $80-150
- Contractor labor: $150-250
- Warranty replacement under 3 years parts
Bottom line
Most "flow sensor failures" are actually clogged impellers. Clean first, replace second. Stiebel tech support can guide diagnosis over the phone — call before ordering parts.