Tapping into basement ductwork

BasementVirgin

Apologies if this is a naive question, but why is it always recommended to tap into heating/AC ductwork from the top? I want to tap into rectangular ductwork in my basement so as to get air circulating in a room I am building there. It would be so easy to cut into the side or bottom of the existing rectangular ductwork, yet all posts on the internet I have seen say cut in from the top, without explaining why. They only say it will be difficult due to the proximity of joist etc. and indeed in my case any cutting into the top of the ductwork would be extremely difficult. Is top-tapping simply so as to avoid reducing headroom or is there a crucial technical reason I cannot see why taps are always made from the top?
Thanks for any help.

Plumber101

You can cut into the side or bottom just fine.

Main reason for cutting into the top is that the branch runs or in between the floor joices and can be concealed easy.

__________________
John Wayne once said "Life is tough and it is even tougher if you are stupid"

Tempstar and Trane Dealer

yuri

Heat rises and you get better flow pattern at the top of the duct. You can go off the side and off the bottom. Bottom is not very good and the side okay. In the industry we try to get everyone doing it the same so we don't get ductwork looking like spaghetti and air flow problems later.

newtech

Almost all resi is tapped off the top, and it is between the floor joist.(to save room for a ceiling) On commercial buildings they are almost always tapped off of the sides(because they have more room above). There is no problem cutting into the side of the duct for a run. Just remember if cutting into the side you will lose ceiling height.

BasementVirgin

Thank you Plumber101, Yuri and Newtech! Much appreciate your responses. So basically top tapping is indeed to conserve headroom. I note it can also improve air movement due to hot rising convection aiding flow, but on the other hand I presume bottom tapping would aid A/C i.e. cold flow - however basements usually need heating rather than cooling.
The main thing is that it won't be the end of the world when I have to cut into the side of the existing ducts!
Thanks again.

beenthere

Better off running a new truck just for the basement. Your curretn trunk line is most likely undersized. So tapping into it for the basement can have a large effect on your main and or second floor heating and cooling.

Running a new small trunk for the basement will allow you to install a damper to shut it off in the summer, when you don't need cooling in the basement. Or atleast regulate the air flow to the basement better.

newtech

I think he said one room, not the whole basement.

beenthere

Then it can be a small trunk line. LOL