The house my wife and I recently bought has two furnaces, both natural gas forced air. The 'front furnace' supplied the original house, the 'back furnace' supplies the addition and second floor. It appears as though somewhere along the way, the front one died, and instead of fixing or replacing it, the previous owners decide to cap off the supply and return connections to the 'front furnace' and installed 32' of insulated 8" flex duct between the main supply ducts.
The vents being fed from the 'back furnace' system are fine, and those rooms are heated well. The rooms that were supplied by the 'front furnace' have little to no air flow, and are between 5-12 degrees cooler than the rest of the house. The 'front' duct work is quite warm to the touch, so it seems like hot air is getting into the main 'front' supply duct, but it's not moving into the rooms. We closed all the 'back' vents in the house as a test, and it marginally improved the 'front' vent air flow.
The 'back' furnace is 100,000 BTU, and the house is 4100 sq ft.
I built a quick vent booster out of some old cpu fans and some scrap lumber to test with. I put it in one of the 'front' rooms and after 2 cycles of the 'back' furnace, that room was 3-4 degrees warmer.
Rather than make a vent booster for each room, I was wondering about the effectiveness of putting a duct booster fan at the end of the flex duct as it feeds into the main 'front' supply duct. We live in Michigan, so it's going to get colder before it gets warmer and something needs to be done soon. If I can get away with a quick fix for this winter and have it fixed proper in the spring, that would be ideal as money is tight having just closed and moved in.
Any suggestions, comments or opinions would be greatly appreciated.