I’m just a homeowner that’s been looking on line for info. What brought me here was a discussion of combining radiant floor heat with forced air.
  The house is a 25 yr old 2 story square with a garage/family rm section several  steps down to the level of the garage. The family room has a ceiling that vaults from a 10′ wall, with 5.5′ x 7.5′ cluster window, to 16′ that meets up with a bonus room over the garage and an open walkway connecting to the bedrooms in the 2 story. The family room, about 20 x 15, also has two sky lights and is wide open to the kitchen on the connecting wall. Our lot slops from the family rm side down to the formal so there’s a 2′ crawl space under the family rm ending at 6′.
We have a 15 year old gas furnace for the downstairs and the original electric in the attic. There’s just the two of us, retired and really don’t use the formal areas which are on the opposite side of the house. Upstairs we mainly use the room over the garage, which can be also be heated/cooled with a duct run from the lower, and the master at the opposite end. I closed off the other two bedrooms. We only run the upstairs furnace at night when it’s really cold.
In part, a reason for wanting radiant floor is the draft created in the fam rm during the winter. Our zone is 7/8 in SC. The cold air return is in the 1′ piece of wall shared with kitchen, and whenever the furnace kicks in, whoever sits in the chair in front, gets all the cold air coming from the windows, the sky light and the open second story walkway. I don’t know where else the return can be located as it’s directly routed under the kitchen to the furnace.  An option might be to split it into two smaller ones spaced apart braking up the draft but it will still be there.  I thought the radiant heat  in there and the kitchen would be a solution, with the return in the small hall opposite of the steps down to the fam rm.
  I’m researching this now because the AC just went out down stairs, and the upstairs has been out for awhile. I’m wondering what would be the best choice between a heat pump or just replace with regular air. If I think the radiant would work and not be too expensive I’d rather that. The floor coverings all need to be replaced.