Our new to us timber frame home is about 10 years old. I knew when we bough the house there were heating problems. The master bedroom is over the garage next to the loft. You could hang meat in there in the winter. I know part of the problem is the insulation in the garage celing. These rooms are notorious for air infiltration. There are also duct problems in the house. Bad design and probably bad connections. My plan is to rip out the drywall in the garage ceiling and replace the fiberglass insulation with a good quality spray foam. In the basement I will remove the ceiling and redo the ductwork. The furnace is a mid efficiency and still works well but does not push enough air to the upstairs. I doubt that the furnace will ever be able to overcome the length of the pipe runs and all the elbows. I would like to replace the furnace with a HE Variable speed DC unit. The house has a 21 ft. ceiling to the peak and a fan that runs all the time will help some. The main floor and the basement have in floor heat but not enough to heat the house without a furnace. I am wondering if maybe putting another furnace in the garage would help. This would heat that bedroom and the loft ( about 600 sq. ft. At the same time I would install AC in both furnaces. The duct work to the upper area would be very efficient with a minimum of bends. No long runs and properly sealed. There is already a large closet in the garage that would work for the furnace. Being HE no chimney would be needed. There is even a sub panel in that area and the existing gas line is in the garage. I realize this is an additional expense but I want this home to be "right" and comfortable The house is 1900 sq. ft( Including upstairs). plus a 1300 sq. ft. basement and is located in Manitoba ( read cold winters). The replacement of the existing 80% furnace would trigger a small rebate. Would having 2 furnaces be efficient? I bet they both would be on low fire a lot. Any ideas or criticisms appreciated.
If you can cap off all of the duct work going to the second floor. You will be able to use a much smaller furnace for the first floor.
Your air flow problem is from undersized duct. The length is the problem per say. Its the size they used, doesn't allow for the length it is(kinda like having a car that gets 50 miles to the gallon, but only has a 2 gallon gas tank).
The adding a second furnace for the second floor is a good idea.