Just a point of order here. TC's with a basement typically have a dedicated heating duct to the basement, this for cold weather camping. Also its a good idea to make sure that the dump valves are contained within the heated basement and not outside the heated area.
My basement duct was a very restrictive accordion 2" which (per remote thermometer measurements) did almost nothing to prevent freezing. I kept it in place but ran a separate new 4" duct with a shutoff valve...after taking temp measurements the basement now stays within 5-10deg of interior temps...