What was found
(CAN) AFTER DEPARTURE THE DUCT OVERHEAT WARNING CAME ON.EMERGENCY CHECKLIST CARRIED OUT. SMOKE BEGAN TO EMIT FROM FLOOR INTO CABIN AND COCKPIT. AIRCRAFT RETURNED AND SHUTDOWN AS PER FIRE CHECKLIST. AIRCRAFT EVACUATED.UPON INVESTIGATION THE AIR CONDITIONING BYPASS VALVE WAS FOUND FAILED IN THE OPEN POSITION. INSPECTION OF DUCTS AND MANIFOLDS FOUND NO FAULTS. BYPASS VALVE REPLACED AND AIRCRAFT RELEASED.