HVAC Duct System Installation – Part 1


  • Duct systems are the distribution network for conditioned air to be moved throughout a building.
  • Understanding duct systems and airflow is necessary to be able to troubleshoot and maintain HVACR systems.
  • The most common materials used for duct construction include:
    • Galvanized steel
    • Fiberglass ductboard
    • Wire-helix flexible duct
  • A correctly installed duct system:
    • Should last as long as the house.
    • Should not leak.
    • Should be insulated well enough to prevent duct loss and gain to unconditioned space

Air-Distribution System Components

Forced-air systems are used to distribute conditioned air in residential and small commercial buildings.

The basic components of a forced-air system are the blower, return-air duct work carrying air to the blower, and supply-air duct work carrying air from the blower to the building.

Most systems have large boxes on both the return and supply ends of the blower called plenums to distribute air to the duct work.

Air-Distribution System Components

  • The duct design can be compared to a tree.
    • The main ducts leaving the plenum are called trunk ducts.
    • The individual ducts running to each room are called branch ducts.
    • The point where a branch duct comes off of a trunk duct is called a takeoff.
  • The return-air openings are covered by return-air grilles.
  • The supply-air openings are covered by supply-air registers.
  • Between the blower and the diffuser, the airstream must change direction and shape.
    • Turns are made with elbows, often called ells.
    • A Y fitting is used to split one large duct into two smaller ducts.
    • A change in the size of a rectangular duct is accomplished with a transition.
    • A change in the size of a round duct is done with a reducer.
    • When the register at the end of a round branch duct is rectangular, a fitting called a boot is used to allow the round duct to connect to the rectangular register.

Air-Distribution System Components

  • The blower provides the pressure difference to move the air through the ductwork.
    • The amount of air the blower can move and the amount of energy needed to move the air is controlled by the resistance to airflow from the ductwork and all of the system components in the airstream.
    • The duct offers resistance to airflow, creating a pressure drop as the air travels through it.
    • Besides the ducts, pressure drop is added by every component that the air travels through including:
      • Filters
      • Humidifiers
      • Heat exchangers
      • Coils
      • Registers
      • Grilles
    • The amount of pressure left for moving air through the duct work is the difference between the amount of pressure the fan can produce and the amount of pressure drop from all system components.

Duct Location

  • The three most common areas to locate the duct system are:
    • In the attic
    • In the space between the joists in the ceiling or between floors
    • Under the building in either the basement or crawlspace
  • The structure and layout of the building determine the available location for the system components.
    • The building’s design affects the air handler location, which then controls the duct system layout.
    • The duct location will determine the amount of duct insulation required.
  • For maximum system efficiency, the duct system should be located so that it is within the building’s insulation envelope.
    • In a residence with a full basement, the basement is an excellent location for the trunk ducts and air-conditioning equipment.
    • Equipment can also be located in a closet space or utility room.
  • All enclosures must meet local fire and safety codes.
  • It is not always possible to locate the ducts within the building’s envelope.
    • For slab construction, ducts and even equipment can be placed in the attic.
    • Ducts located in unconditioned areas must be properly insulated, and an allowance must be made for heat loss included in the load calculation.
    • Energy-efficiency guidelines require that ducts located outside of the building’s envelope have R8 insulation value.
    • Ducts for perimeter (heating only) systems can be located in the slab.
  • Before beginning the duct system layout and design for any building, check with the local building department, county, or state code and/or regulatory agencies to determine the specific insulation requirements for the building’s type and location.

Equipment Types

  • The design of the duct system is affected by the type of equipment selected.
  • Forced-air equipment is typically either:
    • Packaged equipment
    • A split system
  • With packaged equipment, the entire system is located outside the structure and the ductwork is typically located in the basement or crawlspace.
  • Split systems consist of:
    • An inside blower
    • An outside condensing unit
  • Split systems offer more flexibility in duct location because of the many possible blower locations.
  • The blowers can be:
    • Upflow
    • Downflow
    • Horizontal

Common Split-System Duct Configurations

  • Upflow blower in the basement with all ducts in the basement
  • Upflow blower located in the house with supply ducts located in the attic and return in the house
  • Downflow blower in the house with supply ducts in the crawlspace and return in the house
  • Horizontal blower in the crawlspace with all ducts located in the crawlspace
  • Horizontal blower located in the attic with all ducts located in the attic

Duct System Types

  • The four most common duct configurations are:
    • Radial:
      • Most frequently installed in attics
      • Can also be installed in crawlspaces and basements
      • Commonly used in small houses built on concrete slabs
    • Reducing radial
    • Extended plenum:
      • Sometimes referred to as trunk duct systems
      • Extended plenum systems can be located in:
        • A crawlspace
        • An attic
        • A basement
    • Reducing extended plenum:
  • Two less common types of duct configurations are:
    • The perimeter loop system
    • The central plenum system

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