Just a quick note for those of us who fight to get some of that nice hot air in the cellar distributed upstairs where we really want it. Unless there is a cold air return, then point the box fans, the in line fans, the register booster fans, and even the fans on our stoves at the wall. Because in essence without a cold air return that is pretty much what we are doing. I have bought and done it all, but until I finally put in a cold air return nothing worked. I live in a 200 year old farm house. With the stove in the cellar I'm keeping my 2 second story bedrooms at a constant 68 degrees, and this is with a 4 inch duct and this is with the recent sub zero temps. I have burned out in line fans bought booster fans and nothing worked until I hooked up a simple duct line from a floor vent on the first floor to my fan on the back of the stove. Actually I got flexible duct and virtually wrapped it around my 135 cfm fan on the back of the stove and ran the other end to the floor vent. If you stand by the vent you can feel the cool air rushing down and I can feel the hot air at the other end in the bedrooms. My first floor was a steady 72 with this system until I did some other things, now I'm at 76, (my wife's comfort zone), most of the time, but that is another story. Bottom line is unless there is no air returning there will be no air going up. Good luck.