HELLO, I HAVE RECENTLY ADDED AN ADDITION TO MY HOME. I NEED TO HOOK MY ADDITION INTO MY CENTRAL HEAT AND AIR. I DO NOT HAVE ANY IDEA OF WHAT SIZE DUCT TO RUN AND WHEN TO REDUCE DOWN. ANY SUGGESTIONS? THE TOTAL LENGTH FROM MY DELIVERY TRUNK IS PRETTY CLOSE TO 50' LONG. MY ORIGINAL DELIVERY TRUNK IS A 24" X 10" RECTANGLE. FEEDS 6, 6" FLEX DUCTS TO MY MAIN HOUSE. RETURN 24"X 10" ALSO. I WILL BE RUNNING TO 6 REGISTERS TO MY ADDITION.
2" X 12" VENTS
===============================DEL 5 TON
===============================RET
______________THIS WALL IS 17' FROM DELIVERY TRUNK______
| D?START |
| |
|V V| 11' 1/2"
| GAME ROOM |
| |
________________________________________________ ____
| | xRETURN |
| CLOSET |
| | |
|___________________V____| BEDROOM | 19'
| | V|
| | |
|V BATHROOM |
| | D? FINISH? |
|________________________|______________V_________ ______|_
26'

The further you get out, the less pressure you are going to have. My guess is, you might go up a size, 7''-8''. Small changes in diameter make a large difference in flow. Even if your system is adequate to handle the increased load, you may want to look at resetting the blower speed for both heating and cooling. You want the temperature change to be in a certain range, but I can't tell you what it is.