Furnace turns on during summer. 90 deg outside, no ac, thermostat set@72. Furnace turning on. What is the problem?

You need a technician not an exorcist.

Heat coming on or just air handler fan? If unit is in the attic, the fan ON switch may be tripped by ambient heat. 90* ambient with solar load could easily hit 125 or more in an attic and turn the fan on.