I have a new high-efficiency furnace with two large air returns in the floor on the main level, another air return on the 2nd floor (and also opening to the cathedral ceiling space), and an 8" air return located at floor level in the basement. The heated part of the basement is separate from the furnace room. Supply for the basement is ceiling outlets. The furnace is mounted up about a foot and a half from the basement floor due to the shape of the basement. Thermostat is on the 1st floor.
The problem is (surprise) the basement is cold, the air does not get warm enough. Actually there is a gradient and your head can be quite a bit warmer than your feet. Closing down the upstairs supplies helps somewhat but not enough. The basement supply seems to be adequate as it is comparable in amount to the upstairs supply and in addition you can feel warm air coming out. It seems to me that the air intakes on the first floor are drawing well but that the air intake in the basement is not drawing very much. There are no dampers in the air return system.
The basement intake goes up about 6 feet vertically to reach the main return duct in the basement ceiling. Incidently the basement intake connects to the main return duct not far from where the first floor intakes are.
It seems to me that the heavy cold air is sitting in the basement and not making it up the 6 feet to the main return duct. Consequently I'm wondering would help to put a booster fan on the basement air intake. This would help lift the cold air into the furnace (if the furnace is running) or to the upstairs where it would hit the thermostat (if the furnace is off).
Being a total newbie, my question is: is this a totally wrongheaded idea? What other approach could help? My HVAC guy is busy with family right now and I'm trying to think through what approaches might help.