On Sun, 15 Feb 2009 14:04:51 -0800 (PST), wrote:

I am in the process of expanding a first floor door opening, but there
is ductwork on each side of the door which feeds a second story room.
One is a supply and one is a return, both of which are 3 1/4 x 12".

Can I reduce these to something smaller like a 3 1/4 x 6"?

If I change both the supply and the return will my furnace still
operate correctly?

Any help is appreciated.

Maybe. If they just service one room you can measure the area of the
register openings. Your duct is about 40 sq. inches. If the opening
area of the registers is much less you can probably reduce the duct
size. Generally you want the area of the duct to be greater than
that of the register opening to keep the resistance to air flow low,
but if the register opening is only, say 2x9 then you might get by ok.
But you run the risk of that room not getting adequate heating/cooling
or uneven heating/cooling because of insufficient air flow. Cooling
is usually the issue on second floor rooms.

You also have to consider whether you'll be reducing the air flow of
the whole system enough to affect the furnace, but that's pretty
unlikely in your case.

You'd be better off moving the duct to the next stud bay if you can.
Of course, that's probably much more work. Easier to do it now though,
while the wall is open, than to have to open it back up when the room
doesn't get warm/cool enough.

HTH,

Paul F.