A Package Terminal Air Conditioner (PTAC) or other room Heating, Ventilation, and Air Conditioning (HVAC) appliance may present a temporary authentication code to a user staying in a room in which the room HVAC appliance is installed to enable the user to authenticate his or her mobile device and control the room HVAC appliance during their stay. The temporary authentication code is valid for a limited duration, such that a user previously authenticated to the room HVAC appliance loses authentication, and is required to re-authenticate with a new temporary authentication code when the prior temporary authentication code has expired. As such, a user lacking physical access to the room HVAC appliance, such as after the user's stay has ended, is no longer able to access the room HVAC appliance.