Shown here is the primary reason that our basement is cool in the winter and cold and damp in the summer.
In case you missed it here it is again.
It’s the return air from the basement, it is up at the ceiling rather than near the floor. And in our unfinished basement the supply ducts dump the supply air from the ceiling as well.
Why this makes it cold in the basement.
In winter, the basement only gets heat to the top of the room. This is because the hot air supplied from the furnace does not mix with the cold air in the room. Being hot air it is less dense and wants to go up, and it does, right back into the return air plenum. It gets even colder when an outside door is opened and the cold air runs along the floor and down into the basement. I tried the quick fix of closing all the upstairs vents and opening all the basement vents but it still is 6 degrees Celsius colder in the basement.
In summer the basement seems colder than it is in the winter, and humid. It is also from the return air taking the hottest air from the top of the room. The cold air from the air conditioning (central air) gets into the basement even with all the basement vents closed and the return air only takes the warmest air from the basement. There is little air mixing between the upper floor and the basement.
The solution to this is to put a return air vent in the basement near the floor to draw in the cold air from the floor.
In winter this will draw away the cold air so that the warmer air can get down to the lower parts of the room.
In summer this will allow the cold air in the basement to be circulated through the house, and will allow the humidity levels in the house to equalize.
I will post a follow up on this as I change the return air path at our house.