I have a 1900's house that the previous owner upgraded (from gravity?) to a 90% 130k BTU furnace. It was tied into some existing in-wall ducts to supply heat to the 2nd and 3rd floors, and floor vents are used to heat the 1st floor. The only cold air returns in the house are three rather large floor registers on the first floor, all more or less in the same room. All heat supplies are in the middle of the house or near middle walls.
I have an ecotec motor in the furnace and it is set to run continuously on low speed to give some balance to the house. All walls and the attic are well insulated, but the windows and cold air return placement make the upper floors colder than the main floor much faster than the thermostat knows... hence the need for a continuous low running fan to distribute heat a bit better.
The house has in-wall mini-ducts which connect to baseboard medallions that are not currently hooked up to anything. They are located opposite to all of the heat supplies on the 2nd floor (none on the 3rd floor). When I say mini duct, most measure at 2.5" and one at 3.5". They run inside exterior walls and are not plugged. I hooked a 5hp vacuum cleaned up to them and got good flow down from them down to the basement where the furnace is. Being that I have no 2nd floor returns in any rooms, would it be worthwhile to add additional ductwork and tie them into the cold air return of the system? I am hoping they could be used as returns to get a bit better overall air circulation in the house. The rooms they are located in are not that cold, but if any doors to the rooms are closed, they do get warm due to limited circulation air flow. I realize they are small and will provide minimal air flow, but I thought hey, better than nothing.
At worst, they could be plugged with insulation to provide a very slight improvement in exterior R value insulation, but I think there would be a bigger bang for the buck comfort-wise by making them returns.