Installing Rectangular Ducts

[imported_ssabin]

While I didn't originally plan on moving the existing heat ducts in my basement, after reading about the importance of getting the ducts near the floor, I have decided to move my existing heat ducts in my basement ceiling to be near the floor on outside walls. In all cases, this appears easy to do. I'll take off the round pipe-to-register box conversion piece, extend the round pipe to the outside wall, then convert it to rectangular ducting which will fit within my 2x4 wall studs.

However, since I've never done this before, I am not sure the proper way to install the rectangular duct sections. I bought the sheet metal pieces (each being half of the rectangle) at Home Depot, 3.25"x10". The pipe serving the existing register is 6". Do I assemble the rectangular duct, then install into the wall, or do I attach a section to a stud then assemble the other section to the first? If the duct is assembled first, does it need to be attached to the studs somehow? Do I need a special tool to crimpt the two pieces together? What is the proper way to connect two sections of duct together?

I have 3 ducts to move. I also have an issue in that one of the wall stud spaces is just about exactly 10" between studs. Will the rectangular duct fit into this space?

Thanks in advance for the help.

[Wgoodrich]

The two pieces you align with each other to make the box duct. Then you pop them together. When the click they are joined as one duct enclosing that duct as one duct.

To join two ducts together there is a slide piece that you buy. You align the ends of the two ducts end to end then slide the slide piece over the turned up ends making to short duct to now be two long ducts.

You normally would build the duct as one piece to fit the length you need then slide the duct into the stud space. Then there is flat bars that look much like the coupler you used to join the two ducts end to end. You nail this flat bar across the face edge of hte stud. One or two should do you fine per stud space. Then you use sheet metal screws to secure the duct to these flat bars that are attached to the studs.

You will need a rectangular to round adapter to join the two type ducts together again using sheet metal screws.

Hope this helps

Wg

[imported_ssabin]

Thanks, Wg, that was extremely helpful.

This didn't seem like a job that was terribly difficult, but there is sparse info on the internet to help someone learn these simple things. My particular Home Depot didn't have any available knowledgable staff to help me out last time I was there either. I bought the duct sections already, as well as the round-to-rectangular adapter. Looks like I'll need the rectangular slide piece as well.

I plan on removing the top plate between the two studs for the wall section (partition wall) where I am running this rectangular duct. I have reinforced the attachment of this wall to the "ceiling" (i.e., floor joists) on both sides of the duct cavity, so I think I'm safe in removing the top plate. Is this the normal way of doing it?

I will follow-up with results after I complete the job, hopefully in a couple of days.

[Wgoodrich]

YOu are required to install a metal plate across the opening of the top plate with three screws on each side of that top plate. You may use the same metal straps that you use to secure the duct in the wall to tie the plate back together.

Hope this helps and good luck

Wg

[imported_ssabin]

Great. Thanks again!

[dg122]

I did this in my basement. The place where I bought the duct said that I should cut the vent into the duct at 6" from the bottom of the duct. So that is what I did. When the furnace is on there seems to be very little air coming from the vent. Can I improve the flow out of the vent in any way?

Thanks,
DG

[suemarkp]

You did what? Need to know some specifics. What size and shape duct did you attach to your main trunk, or did you just cut a hole and put a register grill on it? Where did you attach it, and is it at the beginning, end, or middle of the trunk? Is there a fitting to make a bend (e.g. round elbow, rectangular take off), or does this new duct just go out straight? How does the duct go after the trunk take off -- straight, or make bends? If bends, how many?

[dg122]

I put a round to rectangular adapter on an existing duct and ran a stack duct down the wall. The stack duct 10"x3.25" is capped at the end (bottom). At 6" from the bottom of the duct I cut a 10"x3" hole for the register. There are several bends in the original builders duct. I shortened the run but the same number of bends are still in the round duct. Would it make a difference if I put some kind of deflector inside the 10x3" hole to force the flow out?

Thanks,
DG

[suemarkp]

Most round to rectangular transitions have a large pressure drop (the only thing worse is butting a 6" round directly into a round hole in that stack duct). Rectangular duct is also more lossy than 6" round. You could get a bit more flow with a deflector (the momentum of the air would help it out the duct), but I think you've just lost too much pressure by the time the air hits that opening.

To get more air out of it, you're going to have to restrict all the other output vents or reduce the restriction in this run. You can only increase restriction so much before you exceed the static pressure pumping abilities of your fan. If any room heats too fast, consider reducing the flow to that room first.

A 7" round take off would put a bit more air into the duct, and eliminating bends or making them larger diameter would help. But if this run has 4 or 5 total 90 degree bends and most others only have 2 or 3, this run is going to be starved unless you have dampers or other mechanisms to reduce the flow in the less restricted ducts.

Also, check your joints and make sure none are leaking (seal with duct mastic not duct tape). That's an easy way to get a bit more flow if they leak.

[dg122]

There are some vents I can restrict. I will put a deflector in the stack. You are correct there are about 5 - 90° bends. I think I will add another return air duct close by.

Thanks for your help Mark.

DG