How to Fix / Troubleshooting

Safety: Turn off power and isolate water supply before removing any wet components.

Steps:

Verify pump operation: Ensure the circulation pump is running and valves are open.
Locate flow switch/sensor: On models with an internal flow device, it is typically mounted in the water manifold.
Inspect for debris: Remove the flow switch or sensor per the manual. Check for scale, plastic shavings, or debris preventing movement. Clean gently and reinstall with new gaskets if needed.
Check wiring: Confirm the harness from the flow device to the control board is intact and firmly connected.

If E20 remains, the flow switch/sensor may require replacement.

Heater shows E20; may indicate internal flow sensor or flow switch error on certain Lochinvar models.

Possible Causes