How to size a spiral duct

Written by william vogel
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How to size a spiral duct
(Jupiterimages/ Images)

Spiral ducts combine light-gauge metal with a spiral lock-seam construction that ensures maximum strength and rigidity. The spiral duct is a versatile product that comes in a number of thicknesses and shapes to meet all types of projects. Sizing a spiral duct is a complicated process that involves figuring out everything from the total length of ductwork needed to the volume of air needed to meet heating and cooling loads and may require consulting a professional.

Skill level:
Moderately Challenging

Things you need

  • Tape measure
  • Pencil
  • Calculator
  • Energy-modelling software (optional)

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  1. 1

    Calculate length of ductwork needed. This can be done on a computerised model or by hand utilising a tape measure and a calculator.

  2. 2

    Calculate heat or cooling load. Depending on whether the duct is distributing hot or cool air, or simply ventilating the air inside the room, the gauge, or thickness of the duct, will need to be sized appropriately. What also needs to be figured into this are the number of occupants and their activity within the room. This may require insulated ducts which add to the thickness of each duct, as well as its diameter.

  3. 3

    Calculate circulated mass of air needed to ventilate a room. Determine the size of the room. The larger the room, the larger the diameter duct. The volume of air able to be moved through the spiral duct system needs to meet or exceed the volume of air held by the room. It is important to check your local municipal building code to establish a minimum rate at which a room must be ventilated. Also distributors of spiral ductwork will have a chart listing the volume of air each gauge will be able to distribute.

  4. 4

    Calculate HVAC mechanical size. Properly sizing the heating, ventilation, and air conditioning unit of the building is important when creating an efficient spiral duct system. Because these calculations factor into many of the calculations concerning air flow, heating and cooling, these calculations must be accurate.

Tips and warnings

  • This project requires technical knowledge and may require consulting a professional.

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