When I'm driving and the engine bay has warmed up, hot air blasts out of my vents, under the dash, and out of the defrost vents into the car cabin. All the heater slides are off and it only happens when moving so I'm thinking it has to be the vent system. It's pretty uncomfortable out here in Phx when it gets warm to have all this hot air blowing on you.

Any suggestions on how to stop the airflow? I know next to nothing about the heater mechanical system. Non a/c car btw.

Thanks.