I don’t know much about furnaces. The furnace itself is actually in the cold concrete basement. The basement temp probably remains at 45 degrees in the winter. Now, there is a large vent in the living room which I’m guessing is some type of feeder since no air comes out of it when the furnace is on and I think I feel air moving into it.
Re: return air - be sure to keep the filters clean or replaced regular. Follow mfg info on filter for how many months between filter changes
This willhelp efficiency of your furnace.
Well that might be where much of your air gets scarfed up by your propane furnace, if it isn’t an ordinary cold air return.
Also, if the hot air ducts pass through that chilly crawl space, they could lose heat to the crawl space by conduction and leakage — and if the cold air ducts also do, they could be feeding needlessly cold return air to the furnace. Covering them with insulation would fight this.