Frost was on the outside, on top of the shingles above the attic without furnaces. Attic space with furnaces, no frost on the shingles. My ducts are reasonably tight, lots of mastic, rebuilt 2 plenums when I moved one for a remodel last year. Again its warm here, my January gas usage was high at 86 therms compared to December's 52 therms. Saving 2% on $30 worth of gas won't do much for me, its more of an experiment for a cheap improvement. Maybe a feather in my cap. Both heaters are power vented with their own dedicated runs. I don't think there will be much of a restriction for the combustion air, 2" pipe is all 50kbtu furnaces need. The ruud installation manuals always show this as an installation option in tight or restricted spaces. The factory concentric vent kits for the 90%+ furnaces use a similar idea, flue gas in a center pipe while outside combustion air flows around it in an outer jacket. And the combustion air pvc pipes are also 2" on the 90% 50k and 70 kbtu units. Because my heaters are in the open attic cold air draft from combustion make up aren't really a problem.
t.
