How to Install Round Sheet Metal Duct

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This project shows how to install round sheet metal duct. The poorly installed flexible duct is replaced with sheet metal duct from the air handler supply plenum to the distribution trunk ducts for more efficient home heating and cooling.

Project Index

Flexible Duct vs Sheet Metal Duct

Flexible air duct is used extensively in residential HVAC systems because it’s quick and easy to install. The problems with flexible duct are:

  • the internal wire helix results in a series of ribs that creates significant friction, reducing the air flow
  • flexible duct is often poorly installed with twists, tight bends and insufficient support straps that choke the air flow
  • In my opinion, flexible duct is best for branch ducts to individual rooms.
Flexible Air Duct - Wire Helix Ribs Increase Air Friction

Flexible Air Duct – Wire Helix Ribs Increase Air Friction

The advantages of round sheet metal compared to flex duct are:

  • very smooth duct with minimal air friction
  • more durable compared to flexible duct

Replace Flexible Duct with Round Sheet Metal Duct

Sheet metal duct is more expensive and takes longer to install than flexible duct. Therefore, I choose to replace the main runs of flex duct from the supply air plenum to the duct board distribution trunks with rigid round sheet metal duct. This photo montage illustrates the flexible duct replacement with round sheet metal duct installation phases:

Flexible Air Duct Replacement with Round Sheet Metal Duct

Flexible Air Duct Replacement with Round Sheet Metal Duct

Referring to the numbered photo above:

  1. The poorly installed 12 inch flexible duct is pulling away from the supply air plenum start collar, sagging and pinched by the nylon support straps.
  2. Round rigid sheet metal 12 inch duct is installed before sealing with mastic.
    Wow! What a difference compared to the crappy flexible duct.
  3. Sheet metal duct with R-8 insulation sleeve.

Flexible Duct Installation Errors

The supply air plenum ductwork is probably the most important section of duct in the home because it’s at the air handler – the source of heating and cooling – where air pressure, air flow and energy losses will be the greatest. I noticed several problems with the flexible duct when I was installing the UV light treatment system:

  • Sagging, pinched and separated flex duct:
Pinched and Unsupported Flexible Air Duct

Pinched and Unsupported Flexible Air Duct

  • Flexible duct is incorrectly installed, leaking and pulling away from the start collar:
Flexible Duct Improper Installation

Flexible Duct Improper Installation

I decided to replace all the flexible trunk duct at the supply air plenum with rigid round sheet metal duct.

How to Install Round Sheet Metal Duct

Round Sheet Metal Duct Installation Planning

Because I’m replacing existing ductwork, a building permit and HVAC load calculation is not required – but you should check with your Building Department or Authority Having Jurisdiction (AHJ) for specific requirements in your area.

I estimated the sheet metal ductwork materials by making a hand drawing of my attic ductwork with measurements, then created a ductwork diagram in Visio:

Residential HVAC: Air Handler Sheet Metal Air Duct Diagram

Residential HVAC: Air Handler Sheet Metal Air Duct Diagram

I’ll install two 45 degree sheet metal adjustable elbows for greater efficiency instead of a single 90 degree elbow. 90 degree elbows should be avoided if possible because a 90 degree turn creates a lot of air friction because the air has to stop its forward motion for an abrupt change in direction. Adjustable sheet metal elbows are fantastic products that allow for perfect alignment with turns.


I’ve since had the 17 year old Heil central AC system replaced with a high efficiency Bryant Evolution system. The AC dealers said there wasn’t enough return air capacity and also recommended replacing the rigid duct board distribution boxes (bottom left & right sides in the above diagram) with round sheet metal because it has less air friction versus duct board. Duct board has fallen out of favor and seldom used in new installations.

The new 12 inch round sheet metal distribution trunk is the section lying on the attic joists with the flex duct connections:

New Round Sheet Metal Trunk Duct and Flex Duct Branch Lines

New Round Sheet Metal Trunk Duct and Flex Duct Branch Lines

Sheet Metal Duct Installation Materials

My sheet metal duct materials list and cost are itemized in the following table (prices do not include sales tax):

Round Sheet Metal Duct Materials List

Round Sheet Metal Duct Materials List

The 12 inch diameter sheet metal duct materials cost was slightly under $10 per foot. Because I did the job myself, labor was free.

You probably won’t find many of the sheet metal ductwork materials at home improvement stores. I purchased the materials at a local HVAC specialty supply store where the pros shop. Just walk in, ask for what you need and tell them you don’t have an account. Pay via cash or credit card. The salesman was very helpful and even gave me a tour of the warehouse to look at different duct parts and insulation materials.

28 vs 30 Gauge Round Sheet Metal Duct

Residential building codes are based on the International Code Council’s (ICC) Mechanical Code. In 2009 the ICC changed the requirement for round sheet metal duct up to 14 inches in diameter from 30 gauge to 28 gauge minimum thickness. The Mechanical Code change from 30 gauge to 28 gauge sheet metal was adopted by the ICC, IRC and several States with little awareness by the HVAC industry. It has since been recognized as a wasteful and expensive mistake and is in the process of being reverted to the original 30 gauge requirements. Some States have adopted temporary rules to allow 30 gauge round sheet metal duct. The State of Georgia where I live has not adopted the 28 gauge duct requirement, so I used 30 gauge round sheet metal duct.

Sheet Metal Duct Installation Tools

I used the following tools to install the sheet metal duct. The Malco C5R crimper and tin snips are a bit unique to most home workshops; as it were I didn’t need the crimper. A magnetic spirit level is useful for hanging the duct level. The plumb bob was used to locate the center of the duct from the high roof rafters to attach the steel duct strap.

Round Sheet Metal Duct Installation Tools

Round Sheet Metal Duct Installation Tools

A pair of leather work gloves is recommended because the sheet metal edges are sharp. I have a pair but never put them on.

Flexible Duct Removal

I cut the outer nylon zip tie and slipped the outer insulation jacket off the start collar. What a crappy installation! The flex duct liner was not sealed with HVAC foil tape and held with another zip tie. The installer didn’t have a long zip tie and joined three short ties together.

Air Handler Supply Plenum Flexible Duct and Start Collar

Air Handler Supply Plenum Flexible Duct and Start Collar

After cutting the 2nd nylon tie, the duct fell off the start collar:

Flexible Duct Disconnected from the Duct Board Plenum

Flexible Duct Disconnected from the Duct Board Plenum

I disconnected the rest of the flex duct by removing the black nylon support straps and disconnecting the far start collar.

Air Handler Supply Plenum and Evaporator Coils

I’ve always wanted to get a really good look inside the supply air plenum – is it dirty or moldy? The plenum looks very clean. Also, a good view of the how the start collar “fingers” grasp the duct board at the far side flex duct.

Air Handler Duct Board Supply Plenum and Start Collar

Air Handler Duct Board Supply Plenum and Start Collar

Looking down on the A-frame evaporator coils and the Honeywell UV bio-treatment lamp in the next photo. Some rust on the sheet metal, the condensate drip pan and coils are very clean and the splotchy mold on the foil insulation is bleached and crusty. The UV lamp appears to be a good job!

Honeywell UV Light Treatment and AC Evaporator Coils

Honeywell UV Light Treatment and AC Evaporator Coils

This project is continued in How to Install Round Sheet Metal Duct – Part 2.

Take care,

Bob Jackson

Copyright © 2019   Reproduction strictly prohibited.


  1. emat November 14, 2013 at 10:34 am - Reply

    i’ve read that the lighter weight metal creates noise issues and you should go with at min 26g. how is it working out for your installation? i am having to do this not only in the attic but the 1st floor ceiling because the lowest priced contractors just threw the ducts and everthing else in. at least they didnt homerun all your runs from the supply and return plenums.

    • BobJackson November 14, 2013 at 1:45 pm - Reply

      There’s nothing wrong with 26 gauge sheet metal. The 30 gauge is working fine in my home.

      Nighttime temperatures have been in the mid-20’s the past several nights and there’s no noise from sheet metal duct with the furnace running… it’s totally quiet.

      > i am having to do this not only in the attic but the 1st floor ceiling
      Ducts located in the attic (outside the building thermal envelope) must be insulated with R8 insulation. Ducts located between the 1st and 2nd floors do not require insulation, but if you’re concerned with noise then insulation would dampen the sound. Insulation also cushions the duct so there’s no metal-to-wood framing or duct-to-hanger strap contact that could make noise.

      The improvement that I’ve noticed with the sheet metal duct is stronger air flow. I now hear a louder whooshing sound from certain louvered vents that are mostly closed in the winter because I prefer sleeping in a cool room.

      Moby Dick by Herman Melville, Chapter 11 – Nightgown:

      We felt very nice and snug, the more so since it was so chilly out of doors; indeed out of bed-clothes too, seeing that there was no fire in the room. The more so, I say, because truly to enjoy bodily warmth, some small part of you must be cold, for there is no quality in this world that is not what it is merely by contrast.

  2. emat November 14, 2013 at 10:35 am - Reply

    also thanks Bob for this and the other how to’s. always excellent info here!!!

  3. Tom March 12, 2014 at 5:51 am - Reply

    Thank you! This is very helpful material, as I’m looking at doing some flex to round metal duct replacment soon. Just chatting now, I did a re-do of a 1980’s or 90’s installed duct system in a 1940’s house in Dallas. It was a tiny 850 sq feet bungalow. The HVAC was a mess, so I had quite a shocking learning curve on my first home. The hole in the floor where freon line ran under crawlspace into the air handler in a little closet was not sealed. That crawlspace had flooded in the past, and the moisture went straight up into the whole system and contaminate it. The original ducts were round metal. Installer did not properly seal the insulation around them, and when I removed, there were long lines of black mold at every junction – so SEALING INSULATION is very important or condensation mold will result. I went with the easy flex duct installation after seeing all that metal go to waste. Now at my newer house (1998 house), I’m looking at taking out some older flex duct and I’m thinking well done round metal will be better than flex. There might be a mix, keep some of the flex, and metal replace a certain supply line that has a 360 degree turn in it (and it’s been making us itchy in those rooms).

  4. Amy February 18, 2015 at 5:21 pm - Reply

    Hi Bob,

    We are remodeling a house and putting in all new flooring. Most of the rooms in the
    house have floor vents that are 6 inches wide by 10 inches long. I would like to change
    these openings to 4 inches wide because I do not like the larger vent openings and
    covers. Is it alright to change the boots out to the smaller size?

    • Bob Jackson February 18, 2015 at 9:20 pm - Reply

      Converting from a 6×10 inch to a 4×10 inch vent boot will be OK if the boot collar diameter is the same as before. E.g.:
      Before: 6×10 in with 6 inch duct collar for 6 inch duct
      After: 4×10 in with 6 inch duct collar for 6 inch duct

      I’m assuming you have 6 inch ducts in the above example. The new vent boot must match the existing duct diameter.

      You will lose some airflow and/or notice the air is blowing faster & louder from the 4×10 vent boot because the vent is 33% smaller compared to the 6×10 vent.

      Have you considered keeping the 6×10 vent boots and installing decorative floor vent covers/grates? This would maintain the airflow efficiency and hopefully be more appealing.

  5. Trev April 20, 2017 at 5:08 pm - Reply

    Thanks for all the detailed information. I’ve learned A LOT.

    Recently I paid to have some HVAC work done, but was still having issues. After reading your site and other articles online, I found that the company did a sloppy rushed job.

    The duct board plenum is connected to the air handler incorrectly.
    The start collars were not sealed at all. There was cold air leaking at both collars.
    The holes created for the collar are mishapen so the collar is bent.

    It’s a shame you have to look far and wide for a contractor that does a good job and takes pride in his craftsmanship.

    Makes me wonder how many other people are suffering from sloppy A/C work…including health wise.

    Thanks again for all your work

  6. Bob Faitel November 27, 2017 at 10:21 am - Reply

    I have a condo that has a very high ceiling in the living room with 2 heat ventS, our dining room is attached and has 1 heat vent in front of our door wall, all 3 vents are on a common outside wall. The room is always cool even though we have our temp. set at 72 deg’s.
    Would you advise adding an additional heat vent on the opposite side of the living room? The living room is approx. 15’x22′ and the ceiling is 2 stories high. Please let me know if you require some additional information.


    • Bob Jackson November 27, 2017 at 5:15 pm - Reply

      See How to Add an Air Duct to a Room for several branch duct installation and sizing tutorials. Especially my comment dated “November 25, 2012 at 1:00 pm”.

      Are all 3 vents connected to a common branch duct? If so, the branch duct have to be large enough to support a new 4th vent to make a difference. The ResDuct™ – Residential Air Duct Calculator can help determine if the branch duct has sufficient capacity for a new vent. Usage is explained in my 11/25/2012 comment in the other project referenced above.

      Is there a ceiling fan in the 2 story living room? Ceiling fans are great for mixing down warm air.

  7. Larry Wasson November 16, 2018 at 12:22 pm - Reply

    Rerouting some of my HVAC round metal 26 gauge duct work to conform to my Manual D schematic. Is the minimum distance between a TEE and WYE two feet? Can a WYE be mounted to the Plenum takeoff?

    • Bob Jackson November 17, 2018 at 10:54 am - Reply

      > Is the minimum distance between a TEE and WYE two feet?
      A Rule of Thumb is to “Space the takeoffs at least 6 inches apart and 12 inches from the end cap.” per Section 12.2 #8 in HVAC – How to Size and Design Ducts Course No: M0 6-032 by CED Engineering. This source says to “Always stagger take-off ducts by 12″ to maintain pressure.”

      > Can a WYE be mounted to the Plenum takeoff?
      A Wye plenum takeoff would be unconventional and not recommended. Follow the best practice duct designs (Section 8.0 in the above link) and install a branch duct takeoff in the trunk duct. Duct System Design Considerations by RSES is a good resource.

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