I don’t think we are on the same page here the original question was is PVC alright for use. High efficiency only and some furnaces do not have the second PVC pipe. Yes in my area outside combustion air is required usually taken from an attic with the high low metal pipe. But can be taken from a crawl space. Is that my choice no but is not wrong in this area.
I answered the question about the PVC flue (which really is basic, and I am would be a little concerned if he is charging for home inspections, but I applaud him asking anyway)
My point was, if the furnace has an input for combustion air, and if it is not already plumbed to the outside then I recommend that it be done due to the corrosion factor etc. If the furnace does not have an input, as stated above then so be it, there is nothing wrong with that.
Same page Charley, and I think we agree, just saying it in different ways 