When most people think about upgrading or replacing their HVAC system, they focus on the equipment: the furnace, air handler, or air conditioner. But one critical component often gets overlooked — the ductwork. Your duct system is the delivery mechanism for your heating and cooling, and its design directly affects comfort, efficiency, indoor air quality, and system performance.

In this article, we’ll explore the different types of ductwork used in the U.S., regional usage trends, why “spider duct” systems are problematic, and why you shouldn't just keep the old ducts when installing a new HVAC equipment.

Common Types of Ductwork in the U.S.

1. Sheet Metal Ductwork

• Material: Galvanized steel or aluminum

• Shape: Rectangular or round

• Pros:

  • Durable and long-lasting

  • Smooth interior surface minimizes friction and airflow resistance

  • Easier to clean and resistant to mold

• Cons:

  • More expensive to install

  • Requires professional fabrication and sealing
    

Where It’s Common:

• Northeast and Midwest: Often found in basements and older homes where metal ductwork was standard for decades.
  
  

2. Flexible Ductwork (Flex Ducts)

• Material: Wire coil covered with fiberglass insulation and a plastic inner liner

• Shape: Round, flexible tubing

• Pros:

  • Inexpensive and easy to install

  • Ideal for tight spaces and angled runs

• Cons:

  • Can sag, kink, or be poorly supported, reducing airflow

  • Inner liner is prone to damage

  • Difficult to clean

Where It’s Common:

• Southern U.S. (Florida, Texas, Arizona): Frequently used in attics due to ease of installation and lower material costs.
  
  

3. Fiberglass Ductboard

• Material: Fiberglass board with a foil exterior

• Shape: Rectangular panels cut and assembled into ducts

• Pros:

  • Good insulation properties

  • Quiet due to sound-absorbing fiberglass

• Cons:

  • Can degrade and shed particles over time

  • Difficult to clean; may harbor mold if moisture is present

  • Not as durable as metal ductwork

Where It’s Common:

• Southeast U.S.: Popular in tract housing developments built from the 1980s to 2000s.
  
  

The Problem with “Spider Duct” Systems

“Spider duct” is an informal term used to describe a layout where all flexible duct runs branch out from a central plenum or air handler — often with little thought to length, sizing, or airflow balancing.

Why Spider Ducts Are a Bad Idea:

• Lack of engineering: These systems often ignore Manual D (duct design standards), leading to uneven airflow.

• No balancing: Without properly sized trunks and branches, air doesn’t distribute evenly.

• Long, uninsulated runs: Air loses temperature over long distances — especially in attics.

• Kinks and sags: Improper installation of flex ducts causes static pressure to increase and reduces airflow.
  

Spider systems are commonly found in attics in southern states (like Texas and Florida), especially in mass-produced homes where cost and speed took precedence over design.

Why You Can’t Keep Old Ductwork with New HVAC Equipment

It’s common for homeowners to ask:

“Can’t we just install a new furnace or AC and leave the ductwork alone?”

The short answer is: you shouldn’t — and here’s why.

1. Old Ducts Are Inefficient or Damaged

• Ducts degrade over time. Joints leak, insulation deteriorates, and contaminants build up.

• Studies show that in many homes, up to 30% of conditioned air is lost through leaky ducts.
  

2. Duct Sizing Matters

• New HVAC systems (especially variable-speed or high-efficiency units) require correct airflow (CFM) to function as designed.

• Old ducts often don’t match the airflow needs of modern equipment, leading to short cycling, poor dehumidification, and system strain.
  

3. Code Compliance and Modern Standards

• Older systems rarely meet current building codes or ACCA Manual D standards.

• Today’s systems require balanced, pressure-tested duct designs to ensure comfort and efficiency.
  

4. Duct Design Must Match Manual J and Manual S

• If you’re using Manual J to calculate heating/cooling loads and Manual S to select equipment (as you should), you must also use Manual D to design the ductwork.

• Keeping old ductwork defeats the purpose — it’s like putting a new engine in a car but keeping the old, cracked fuel lines.
  

5. Air Quality and Health

• Ducts collect dust, mold, and allergens over time.

• If ducts are more than 15–20 years old, replacing them improves indoor air quality and reduces respiratory health risks.
  

Final Thoughts: A System, Not Just a Unit

When upgrading HVAC equipment, think of your home as a system — not a collection of parts. Your ductwork is just as important as your furnace, air conditioner, or heat pump.

Cutting corners by reusing old ducts or sticking with outdated designs like spider systems leads to:

• Higher utility bills

• Uncomfortable rooms

• Poor air quality

• Shorter system lifespan
  
  

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Understanding Ductwork

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