On a 31C / 88F hot day, tried the 'Econ' ventilation instead of air conditioner.

The air coming out of the dash vents is noticeable hotter than the one coming in by the open windows from ambient. The 'recirculate' flow is not selected. Fan speed is medium-high.

Where is that heat coming from ? My heater core has no flow trough it; it is disabled with a very closed shutoff valve.

 

Pop your hood and look for a black, open celled foam pad about 1" hight, below your windshield. That's your heat source.