Another heat problem
 

I did a search and found some helpful tips, but none seem to answer my question. I have a '99 Silverado with 5.3. I can have the fan on high and the temp dial to cool and it blows alot of air out. If I turn the temp dial to hot, the airflow cuts down to almost a trickle, but I can hear the fan running full blast yet. I'm not getting much heat at all and I actually took the cabin air filter out to see if that was an issue and it does the same thing without a cabin air filter. I may have a blocked heater core which is why I'm not getting much hot air, but why does the airflow almost stop when turning the dial to hot? Any help would be appreciated.