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.

Update:

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

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