I live in north east PA in an old 3 story building built early last century. I am installing a Keystoker furnace in the basement and need to use some duct work to get the heat to the rooms on the first and second floor. I will be using 6" flexible ducts. My question is how do I determine the proper size of the floor registers to use to properly heat the rooms? and also the best way to secure the duct work under the floor? Any help would be appreciated!