What are the Different Methods for Duct Design?

Duct design is crucial for efficient heating, ventilation, and air conditioning (HVAC) systems. Methods include the Equal Friction technique for consistent airflow, the Static Regain approach optimizing for larger systems, and the Velocity Reduction method for cost-effectiveness. Each has unique benefits tailored to specific needs. Curious about which duct design best suits your space? Let's explore the details together.
Patrick Roland
Patrick Roland

Duct design is a critical part of passing warm or cool air throughout a building. There are primarily four major types of ductwork that distribute and intake air in a Heating Venting and Air Conditioning (HVAC) system. The trunk and branch method, the radial method, the spider method and the perimeter loop all perform this task in different ways and usually correspond to a particular building type.

All duct design revolves around the furnace unit. Air conditioning, while its unit is located outside of a building, also passes through this unit in order to incorporate the same ducts. A furnace is normally located in the basement of a home or building and its size depends on the building. Homes frequently have a furnace about the size of a refrigerator, but large buildings can have multiple furnaces that can take up entire room's worth of space. Regardless of size, all ductwork revolves around this spot in a building because it is the source of all air being ventilated.

A vent cover leading to ductwork.
A vent cover leading to ductwork.

The trunk and branch method of duct design is one of the most common. Normally, this type of duct installation consists of two large, main ducts that exit the furnace in opposite directions, running directly below the middle of a building, much like a tree trunk. Several branch-like smaller ducts shoot off from this main duct. These smaller branches lead to individual rooms and provide heating and air conditioning.

All ductwork revolves around the location of the furnace because it's the source of all the air being ventilated.
All ductwork revolves around the location of the furnace because it's the source of all the air being ventilated.

Another popular method for duct design is the radial configuration. In this formation, several branches of ductwork stem off directly from the furnace and individually reach every room. This method works for smaller homes and buildings, but can be inefficient for larger buildings because of the amount of ductwork needed.

The spider style of duct design is similar to the radial configuration, but slightly more efficient. In this configuration a few main ducts stem off from the furnace, but once these ducts near the rooms, they split off in multiple directions to reach the appropriate vent. The configuration often looks like the legs of a spider.

The perimeter loop is probably used least by HVAC duct professionals. This configuration takes the radial method's use of individual ducts leading to every room, but adds an additional element. Each individual duct is connected around the perimeter of the building by added ducts, making a loop that passes excess hot or cool air to the next vent.

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Discussion Comments


How does HVAC duct design work on a multi-story house or business? Does the furnace usually have a duct that goes up to the other stories and then follow the same pattern as the first floor?

What is the highest number of floors you can have before you have to start having furnaces on higher floors? Surely, once it got to a certain point, it would be too inefficient for a duct fan to have to push air up.


@kentuckycat - I have the same problem in my house. I wonder about the overall efficiency of the different duct designs.

I've never been under the house to look at my duct design, but the furnace is at the back of the house, so I would guess I have the trunk and branch method. I wonder how much extra energy it takes to push that air all the way to the other end of the house, which is where the living room and main bedrooms are.

Has anyone here ever had an energy audit done on their home? Do they look at these things and tell you what you might be able to do better? I was also wondering if residential duct designs are usually insulated at all to keep the heat or AC inside the duct. I think the metal would dissipate a lot of the air you are trying to circulate.


Is any one of these designs necessarily more efficient than another just based on the amount of eat that is lost in the system?

I know that in my house, smaller rooms like the bathroom always get way too hot, and our living room with high ceilings is never quite as warm as the rest of the house.

At least from this article, it seems like the perimeter method would be best, since you could close off a vent, and the heat would keep moving. I guess whether to use that system would come down to the amount of extra duct work needed or how much energy you might save using that method.

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    • A vent cover leading to ductwork.
      By: vlorzor
      A vent cover leading to ductwork.
    • All ductwork revolves around the location of the furnace because it's the source of all the air being ventilated.
      By: scaliger
      All ductwork revolves around the location of the furnace because it's the source of all the air being ventilated.