Sorry for the thread dredge, but we are (finally) in a position to do something about the heating in this house.
I have had an installer do a heat load calc., and he has given a quote that involves a furnace upgrade for one furnace, provision for A/C when we can afford it. He has also suggested moving the main return to a position over the furnace room door which would allow the elimination of a badly-done boxed plenum in the crawl space, as well as eliminating at least one other return (on the hall wall). In short, he seems to know his business, and spent enough time to think through the problems in this house.
1. With a HE furnace, how do you get the combustion air to the furnace, if the furnace room is marooned in the center of the structure? (What we had in a previous house was PVC taken straight to an outside wall; in this case you could go down below the floor and take it out at foundation level or?) I was unclear what he was planning & wanted to know more since I am also trying to deal with roof issues. I'm assuming exhaust could be taken out through a lined chimney or dedicated roof vent?
2. One thing he commented on was the way in which the house was zoned between the two furnaces. The house is a very long single level with a 'great room' as the center. Both furnaces had their main returns in that great room (although he was going to relocate that for furnace #1), but the supplies in the great room are split based on proximity to the respective furnace. This is a high-ceilinged room with lots of single pane glass.
His suggestion was we might want to consider splitting the supplies so that Furnace #1 would have all the supplies on the east side while Furnace #2 had the west side. I think he said that if the thermostats were put in the correct locations, then if you have say thermal gain from one side of the house, you would then call on the furnace that would do the most good--the one on the cold side.
Does this seem worth doing (it would involve some duct work in the crawl space....