Rinnai HE Series How To Use Manual page 169

Over Temperature Warning (Safety shutdown because unit is too hot)
1. Check for low flow in circulation system causing short cycling.
2. Check for foreign materials in combustion chamber and/or exhaust piping.
3. Check for blockage in the heat exchanger fins.
4. Clean out heat exchanger fins or replace heat exchanger.
Code 16 Troubleshooting Procedure
17