Hello. This is the first time on this site for me.

My question is about a heat and air unit. I was in the bed and smelled something getting hot. I looked in the furnace closet and there was a very bright red glow coming from inside the furnace. What would cause this?

The furnace may have a pilot relite system called a hot surface igniter, glows almost white hot, it is made of carbon and is located in front of the burner .It is only energised when there is a call for heat , if this is not the case then unplug the furnace until you determine what the glow is .

It was a white hot looking glow. What would make the igniter react if the air is on?

Normally the igniter is only energised for about 45 sec. during a call for heat and go off but may retry to light the burners as long as there is a heat call , some will go into a lock out after three tries . Your problem of igniter staying on when there is no heat call is possibly in the ignition control board or printed circuit board of the furnace, that is to say if the glow was coming from your igniter.
You need to have a qualified service person take a look at it and determind the actual problem.

That's what I have done. Thanks for your help.