As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOT REPORTED AFTER TAKEOFF THAT THE COCKPIT FILLED WITH SMOKE. AFTER TURNING THE ACM SELECTOR SWITCH TO THE OFF AND COLD POSITION, THE HEAT POSITION STAYED ON. THE PILOT RETURNED TO DEPARTURE AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND A DEFECTIVE LEFT ACM MIXING VALVE. THE LEFT AMC MIXING VALVE WAS REPLACED WITH A SERVICEABLE ONE. THE OPERATIONAL CHECK IN AUTO AND MANUAL MODE WERE NORMAL. THE OPERATIONAL FLIGHT CHECK WAS SUCCESSFUL. THE AIRCRAFT WAS RELEASED AND RETURNED TO SERVICE.