Not exceptable either
My name is not David but here is my take on this.
Condensate drain lines should not be connected to the sewage drainage system under any circumstances unless the Furnace is a high efficiency furnace which is exceptable because it is a continious flow trap and does not go dry in the heat season.
We basically have two types of A-coil designs postive pressure and negative pressure. Lets look at both types and discuss when and where cross connection between air stream of the furnace A-coil and the drainage system can occure.
Postive pressure coils will have a cross connection in the heating season during the times when the blower is not operating and the P-trap is dry.
Negative pressure coils will have a cross connection during the heat season when the P-trap is dry and when ever the blower is in operation and when it is not in operation.
So what I am basically stating the primary drain should never be connected to the sewage drainage system on a 80% furnace with a direct drain line. The drain has to be an indirect drain (air gap) required
To David Valley in the Pic the condensate drain line was insulated because it was in the attic and will sweat see them both ways is not required one way or the other.
It is prefectly legal to discharge the condensate exterior to the home as long as it does not create a problem to someone else.