Hi.
I have a 2002 Blazer 4x4 and need help with a problem with the heating. It pushes hot air out of the foot heaters. It only does this tho at speeds of about 30 mph or above. anything below that and no air comes out. Everything works as it should and the problem goes away when the ac is on.

I hope I didn't confuse you too much and thanks for the help!

 

2 things come to mind - vacuum lines are pooched &/or the actuator door in the cab.
I doubt things are working as they should because it sounds like a vacuum line has a leak & your controls aren't capable of contoling the HVAC options.
You should have a vacuum box inside the DS fenderwell & a line leading to it.
Check all the vacuum lines from the manifold to the brake booster & anything they're connected to.
As for the door actuator in the HVAC search around in here for ideas.