Rectangular Duct Work Question

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sure, but that room may be much cooler. It is not any different than closing the damper on the register to cut flow by about half.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 15 Feb 2009 14:04:51 -0800 (PST), snipped-for-privacy@hotmail.com wrote:

You can do anything you want but obviously you are choaking both of them down by 50%. Not the greatest idea but It might increase air noise just a bit?? How many other home owners in your home may have done something like what you propose? You can: 1) try it and see what happens 2) Have a manual D done and see what it comes up to Bubba
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 15 Feb 2009 14:04:51 -0800 (PST), snipped-for-privacy@hotmail.com wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

they might try taping over 1/2 of each register and see how things work.
a second floor bedroom in a air conditioned home might not coool properly in summer but be warm enough in winter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.