Hot Topics: Replacing Old Duct Work

Ductwork patched with duct tape.

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Since the second story of this DIYer's house isn't heating well, he wants to replace some old ductwork in an effort to get the warm air flowing correctly. The question is, what exactly does he replace?

Original Post: Replacing some old ductwork

Spork Schivago Member

We recently got a new Bryant natural gas furnace and they replaced most of the duct work in the basement, however, they tied into the old stuff where it started going upstairs. I want to replace some of that stuff, but I'm not sure what parts to get.

Some of the bedrooms upstairs aren't getting warm enough. The baby's bedroom gets really warm and everything's fine there, however, the master bedroom which is right next door has hardly any air coming out—and I think I know why.

In this picture, you can see there's some 90-degree bends. I'm thinking I can redo the duct work in such a way where I can remove one of those elbows, and I think that will help. If I can run it under that gas line, I shouldn't need one of the elbows.

90 degree bends in ductwork.

This picture below is the biggest problem. The air is being forced downwards into that squarish box thing. I think that's where the main issue is and I want to remove that altogether. I can pull that square piece's not very long at all.

Ductwork box.

This last picture shows how those 6"-round duct works connect to what's in the wall. That rectangle piece that goes up through the floor and into the drywall measures around 3.25" x 10." It's old. I can't tell if it was normally an oval piece and the original owners just made the square piece fit, or what. If I could find a way to replace that piece in the drywall, things would be great, however, I don't think I can do that very easily. So, I was thinking of just leaving that piece in the floor, but removing what's attached to it. You know, trying to find some 3.25" x 10" rectangle to 6" round duct work transition piece or something.

Ductwork into a wall.

Do any of you experts have any suggestions on how I should proceed and what parts I might want to purchase to do this? I prefer buying stuff locally from Home Depot and Lowes, if I can.

Norm201 Member

I'm not an expert, but perhaps these comments might help. Typically, a second story will not get very warm (or cool if A/C is used) by the very nature that it's far from the source and the air cools by the time it gets to the desired room. The exception is if the second floor room is in a direct vertical straight line above the furnace.

Removing those bend and elbows will not help very much. Insulating any bare ducts will help.

Another option is to use a fan at the register of the cool rooms to help boost warm air flow. Some of these are thermostatically controlled and will kick in when the furnace goes on. Or, just use a manual fan at the register when the room is in use.

Another thing to check is the damper position on all the ductwork in the basement. The contractors should've balanced them at time of installation to give the best results for all rooms. But that could change as the seasons change and you may need to adjust periodically.

One more thing: is the furnace properly sized for the size and number of rooms in the home?

Spork Schivago Member

The upstairs bathroom wasn't getting much air at all out of the register. The lever to turn it was missing, so I can see how the installation people might have missed that one.

The furnace is the proper size. They did a j-manual. They also installed a cold air return upstairs. The baby's room's ductwork is almost perpendicular to the master bedroom.

I think that's the main reason the baby's room gets a lot of air coming out, but the master bedroom doesn't get much. The guest bedroom is nice and toasty as well. It's just that master bedroom there. To me the air is being forced downward and just hitting that square ductwork there in the photo. I think that drastically affects the flow. That square piece is nice and hot when the furnace is running, but if I reach my hand upward to where it connects to the piece that goes into the wall, it's not very warm at all.

I can easily remove that piece from the ductwork. I'm sure there's got to be some better option than that piece to transition from round to square. I want to try and find something like this:

It's just that I'm having trouble finding the right size there. From my measurements, it needs to be around 10" x 3.25." The third pic shows the transition piece, but where it connects to the piece that goes into the wall, I don't think the wall pieces were square. I'm thinking they were almost rounded and the previous owners somehow made it fit, if you know what I mean.

Norm201 Member

Have you ever taken that square piece off? Could there be an obstruction in that junction? I also see what looks like duct tape being used to seal connections. That's a no-no. Use metal foil tape if you must.

Truth be told, I'm having trouble seeing the pictures clearly. If it's being forced downward, why would it be piped that way?

Spork Schivago Member

We have the direct venting from the furnace, the direct venting from the hot water tank, and the gas lines. Because of where the furnace is, I cannot get any more zoomed out with our camera. I'm pretty much up against a wall. I've taken more pictures though that hopefully will help you get an idea of the orientation.

If you look at this picture and then look at the others again, you can hopefully see how the air is being forced down into that transition piece, instead of being forced upwards like it is in the other runs.


spott Member

That duct in the wall is called a wall stack duct and is a common piece of sheet metal made to fit between 2 x 4 studs in the wall. They make transitions from wall stack to round. Standard is 3 1/4" deep x whatever width you want (8,10,12 etc.). Next, the elbows do make a difference, especially the way yours are installed. Every elbow is equivalent to five feet of straight pipe.

Those elbows have joints on them that are adjustable for different angles. It looks as if you could maybe remove one. You can also get flex duct, but beware the resistance is a little different between the flex and the hard pipe—but it may save you from using all those elbows.

Depending how the bottom of that wall stack is put together (sometimes it will be with a removable cap) you could put a 31/4 x 10 90 on there and then put a 31/4 x 10 to 6 straight adaptor.

Furd Member

I pretty much agree with what Spott wrote. I think I would try to fit a 3-1/4 x 20 45 degree elbow to bring the stack out straight and then make the transition to the round duct.

OR, try to run rectangular duct all the way back to the plenum. That 3-1/4 x 10 inch duct has a cross section area of about 32 square inches while the 6-inch round duct feeding it has only about 28 square inches cross sectional area. That means you are going from a smaller duct to a larger duct and when you do that the velocity of the air drops. You really need a larger supply duct from the plenum to the stack for best results. A 7-inch diameter round duct would make a significant difference.

Spork Schivago Member

I disconnected the 6" round ductwork from the square piece. I removed the square piece, and then I removed the "elbow" that was up in there to get a better look.

The stuff in the wall has an I.D. of 12" x 4". It's an oval piece of wall stack, but it doesn't look like it's made out of the same type of material. Maybe it's just rusty.

The oval elbow I removed, and that's in bad shape. I measured the angle and it's around 130 degrees. It's very odd and I cannot find one anywhere I look. At this point, I want to replace that 130 degree oval elbow with a new one and find a 12" oval to 6" round transition. Then that blasted square box will be completely out of the picture. All it is, is a square box with a cap on one end, the other end open, and a hole someone cut in the middle of it. I don't think it's supposed to be there.

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