Category III Venting HELP!

Greetings,
I am hoping for some enlightenment here. Just investigated a 18 year old propane boiler in a tall crawl space. The owner called me in because she is getting heavy doses of flue gas from a recently replaced side wall vent assembly. The single wall stainless steel replacement vent now routes horizontally across the crawl space exits out the foundation wall and then travels some 6 ft under her deck before termination. It has more than 1/4 inch per ft. grade back to the boiler. BUT a large part of the horizontal vent is now subject to below freezing conditions the majority of the winter. I know Cat III appliances are classified as non condensing. That has got to change when you place the last 6 ft or so of the vent assembly in extremely cold outdoor conditions. Someone besides me might be able to calculate the probability of whether the condensate will drain back to the boiler before it freezes. I just want support for the idea that leaving it outside is not a good idea. Does anyone know of any code language, commentary or other verbiage from ICC Fuel Gas or Mechanical Code or NFPA 54 that will support a prohibition of running single wall stainless cat III vents horizontally in exterior cold temps ? OR are vent assemblies with UL 1738 listing approved to be installed horizontally in outdoor conditions?