The HVAC system draws outside air from the outside except when in recirc mode. The air is drawn in from the top of the cowl (above the firewall). Recirc mode is only available when the a/c is on.
Now your problem is likely a failing or failed air temperature actuator which is an electronic actuator that controls the blend door (or air temperature door). The actuator is located on top of the HVAC housing and accessible by dropping the glove box down and out of the way. The blend door controls air flow over the heater core.
Search should turn up quite a few threads discussing this.
This schematic shows the function of the air temperature actuator: