Wondering if I can reduce the duct size in my basement to get a little more headroom? Currently have a forced air and the duct work is running right through my planned playroom/family room. thanks
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Welcome to BT jaycafe.
Quick answer is yes, the bit if's are how.
It would be good to keep the same or larger cross section area or house heat distribution will suffer. For instance, if you now have a 24" wide by 10" deep duct and you want to reduce it to 6", you should install a big flat 6" by 4 foot duct!. Alternately, you can install multiple 6" ducts.
Say an existing 24" x 10" duct feeds 10 different 6" round circular ducts. You can have 2 separate 6" by 24" ducts each feeding 5 of the 6" ducts.
etc...
Flatter and wider is doable. You may sacrifice a little static pressure loss. If you make it smaller (in area) ... you will increase static pressure and/or air velocity (i.e. more noise). Increased static pressure may be detrimental with respect to air flow over the heat exchanger as well. Like everything else ... no free lunch. You need to approach it with a little care (which, by logging on here you've already started to do).
Some of the other experts in ductwork will likely give you more practical details for your answer. What is your current main trunk size? Branch sizes?