A/c Airflow Issue

[robertwill]

1999 GMC C2500 Sierra (Old Body) Crew Cab 220,000mi

 

Before I go tearing into the interior a/c components I was wondering if anyone has delt with the following issues:

 

1. A/C fan motor is working, air flow speed is controllable.

2. Temperature is adjustable and working.

3. Mode actuator doors are working.

 

Now, when the system is set for cold air there is good airflow, but when set for heat the airflow starts out strong then gets less and less until nothing is coming out of the vents. The airflow completely stops within 30 min. The fan motor is running and you can hear the air trying to push. It sounds and feels like the air is trapped underneath the dash inside the ductwork.

 

Any advice or recommendations would be much appreciated! Thanks