Installing start collar to duct of the same size

[Stephen Tashiro]

There a plenty of online videos about HVAC duct work that show installing a "start collar" to the side of a duct that is wider than the collar. (e.g.https://www.youtube.com/watch?v=ZUlRYer9-3Y ) Are there any that show the case where the duct is exactly the same dimension as the collar? (e.g. 8 inch start collar to the 8 inch side of an 8x24 rectangular duct) Or is there a special fitting that is used in such cases - something different than a start collar?

[Jamie Buxton]

At any HVAC source, you can buy a crimper tool. The crimping process makes the end of the duct smaller in diameter.

[Stan Coryell]

Hi Stephen,
What kind of start collars do you have? I'd guess dovetail. You won't be able to use that. For residential, take the joint of duct, cut it square (cut off the drive ears), notch the corners 1" deep , and fold the short sides out 1". Now you can screw it on to the trunk line. To do it extra nice use "s" slip for the lap on the long side instead of screwing it.

On commercial work a good pick off guy would catch that and make the start collar as I described. We haven't made dovetail collars in 15 years. 1" flg. O/4 (1" out on 4 sides) 12" long would be standard. The 12" is measured to the drive ear. So, in effect the fitting would be 12 1/2" long (1/2" in the slip.

Tip: cut a piece of slip to fit in the small side, apply two pairs of vice grips, and bend the 1" flange. Or, you could buy a "bar folder".

Hope this helps,
Stan

[Stephen Tashiro]

Hi Stephen,
What kind of start collars do you have?


I don't have any materials yet. In a house, I have an empty cavity above a hallway ceiling and below the main ceiling joists (the bottoms of roof trusses). The cross section of the cavity is 37 1/2 wide by 11 7/8 inches tall. The cavity used to hold a 24x8 inch trunk line. From that line, there were rectangular ducts 14x6 branching off to become register boxes on the walls of bedrooms. This was 1958 vintage duct work, rusted through due to the use of an evaporative cooler in the summers.

I'd like to install new ducts while tearing off as little drywall as possible. I've been thinking about a 24x8 trunk line and 14x6x8 register boxes. What's the best way to make the connection between the register boxes and the trunk line?

[Jason Roehl]

I don't have any materials yet. In a house, I have an empty cavity above a hallway ceiling and below the main ceiling joists (the bottoms of roof trusses). The cross section of the cavity is 37 1/2 wide by 11 7/8 inches tall. The cavity used to hold a 24x8 inch trunk line. From that line, there were rectangular ducts 14x6 branching off to become register boxes on the walls of bedrooms. This was 1958 vintage duct work, rusted through due to the use of an evaporative cooler in the summers.

I'd like to install new ducts while tearing off as little drywall as possible. I've been thinking about a 24x8 trunk line and 14x6x8 register boxes. What's the best way to make the connection between the register boxes and the trunk line?

Insulated flex duct.

[Stephen Tashiro]

Insulated flex duct. Even if I ran two 10 inch flex duct lines parallel to each other, I wouldn't get airflow equivalent to a 24x8 sheet metal duct. The largest flex duct that would fit in the cavity without mashing it would be 10 inch.

[Stan Coryell]

Flex would be the last option.
24×8@.08 friction = 1000 cfm
12 round = 500 cfm
12+1 1/2 for insulation = 15
We limit flex to 6' on branch lines due to static loss and noise. Never in trunk lines regardless of static.

[Bruce Wrenn]

Get largest register boot that will fit into duct, and use it as a starter collar. It ain't rocket science

[Stephen Tashiro]

Get largest register boot that will fit into duct, and use it as a starter collar. It ain't rocket science

I don't understand what you are suggesting. In looking for register ducting, the largest standard register boot that will fit into the openings for registers is 14x6x8 which ends with an 8 in circular collar. That collar isn't long enough to reach the 24x8 rectangular duct line unless the line is set to one side of the cavity. If the line is set to one side of the cavity, the boots that must connect on the other side won't reach it.

[Jason Roehl]

Even if I ran two 10 inch flex duct lines parallel to each other, I wouldn't get airflow equivalent to a 24x8 sheet metal duct. The largest flex duct that would fit in the cavity without mashing it would be 10 inch.

I was answering the question of what to run from trunk to register, not replace the trunk with the insulated flex.

[Stephen Tashiro]

I was answering the question of what to run from trunk to register, not replace the trunk with the insulated flex.

Returning to the original question, I'd need a 8 inch "start collar" on the trunk line to attach the flex duct to the trunk line, wouldn't I? Or is there some other way to do it?

[Bruce Wrenn]

Use the rectangular part of the register boot to connect to duct, then 8" flex pipe to to connect to desired register boot. An eight inch round flex pipe with a register boot on each end, one of which is used as a starter collar.

[Stan Coryell]

Re: 8 round on 8 square
If using 8 round dovetail collar, crush it oval, say 9x7.
If using 8 round start with flange out, screw collar on trunk and bend flange over duct.
We normally order "HETO" collars. They are sort of a square to round. 8 small end to 10x6 (give or take) with a 1" flange out on 4.
You can use a register boot, but it would have to be 10x6 and you would have to bend the flange out.
If the collars have dampers that are not accessible, remove them (preferable) or screw them permanently open.
FYI : don't try to cut out the longitudinal connection on the side of duct (Pittsburg or snap lock), just make the round hole flat to follow the seam.