I want to add a heat register to the next room across from the current register How do I go through the wall and extend the furnace duct work?
Cut open a vent sized hole.
Install blocking above the hole and seal it with caulking..
Cut the duct off so the turn into the other room is removed.
Fill in the gaps around the duct going down.
Install the vent louvers.
The hot air will fill the cavity and be pushed out into the 2 rooms.
Not an elegant solution, but one that will work.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
B. Franklin 1759