Originally Posted by
1.86EPR
i.e. is there some special reason air destined for the cockpit avoids the mixing manifold?
Almost certainly its a result of the design requirements of ...
§ 25.831 Ventilation.
(a) Under normal operating conditions and in the event of any probable failure conditions of any system which would adversely affect the ventilating air, the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crewmembers to perform their duties without undue discomfort or fatigue and to provide reasonable passenger comfort. For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.
....
(d) If accumulation of hazardous quantities of smoke in the cockpit area is reasonably probable, smoke evacuation must be readily accomplished, starting with full pressurization and without depressurizing beyond safe limits.
(e) Except as provided in paragraph (f) of this section, means must be provided to enable the occupants of the following compartments and areas to control the temperature and quantity of ventilating air supplied to their compartment or area independently of the temperature and quantity of air supplied to other compartments and areas:
(1) The flight crew compartment.
...
You'll note the subtle difference in paragraph (a) between the crewmembers and passengers, the requirement for smoke evacuation in (d) and the requirement for independent control in (e)(1). Combined, these requirements probably drive that the cockpit supply be made as independent as possible of the cabin supply. (Fire in cabin being one case that comes to mind as being a factor)