Duct Sizing. Two Words and A Bit of Fun…

Duct sizing exercise in air conditioner sizing, is a vital step in making sure that cooled air at the evaporator, reaches every room, at the specified comfort.

The process is not a single step one. It requires corrections and iterations, before you get the practical duct size for your home or office.

Corrections and iterations are needed, as formulas are inter-related. We have to, usually make a good guess, and proceed further.

Shall we explore more?

Before that, a little introduction about ducts. Duct, is a dedicated passage, mainly for gases. The main function is to deliver the gas, from one point, to another, by means of forced, or natural flow.

Ducts in air conditioning systems are fitted with fans and/or blowers. Hence, air movement within the system is from the forced pressure difference at the fans and/or blowers.

This exercise is especially useful for central air conditioners.

Techniques for duct sizing:

There are two famous ways of duct sizing, the

  • equal friction technique, and

  • the static regain technique

We’ll go deeper into the methods of duct sizing on this page.

Equal friction and static regain methods vary in the approach, but these two share some similarities.

Similarities between these methods are, we need initial data, iterations, and we need duct sizing charts, prepared by ASHRAE.

Preparation before duct sizing:

Provided, is the suggested preparation checklist for you, before embarking into the sizing exercise,

  • know your maximum clearance for duct placement, between your attic’s floor, and the roof, as well as floor space between lower level, and upper level.

    This will set your maximum duct height to be placed within the building

    Duct sizing involves knowing the maximum clearance between floors and attic

  • locate, the most suitable space to place your air handling unit, and get the lengths of ducts for the system.

    Sketch a rough plan for the supply and return duct passage. Adjust proposed air handling unit’s location, to make sure duct lengths are minimal. That will minimise heat loss or gain, along the duct. It will also minimise insulation cost, if any.

  • identify sections within the duct system.

    Main duct leading to branch/es, is a section. A branch take off, to air register is a section. A section, is a continuous run, without any branch.

    Mark sections of the duct with suitable numbering system “a, b, c” or “1, 2, 3”.

    Duct sizing involves sketching the routes of your ducts

  • get approximate number of elbows and t-joints, and the equivalent straight lengths.

    Elbows and junctions pose extra resistance within the duct system. Hence the equivalent straight length of these fixtures need to be determined.

    The database is available within “Manual D”.

    What is “Manual D”?

    Similar to “Manual J”, only twice thicker, and oh yes, it has been created by ACCA, specifically for duct sizing

  • get values for heat gain and heat loss from the building using “Manual J”, and determine the total, and individual design flow rate of air, using psychrometric chart

    Then, mark the appropriate mass flow rate of air, at each section.

  • this one is important. Get a set of two duct sizing charts, prepared by ASHRAE.

    One will give you,

    • the circular duct diameter, for a given flow rate and velocity,

      Duct sizing chart for circular ducts

    • and another will give you the equivalent four-sided duct, for a given circular duct diameter

      Equivalent duct sizing chart for rectangular ducts

  • use the comfort factors as presented in comfort zone page.

    Please bear in mind that the maximum air velocity for comfort of 1 m/s, is not the air velocity in duct. It is rather the draft onto the occupants.

    Properly located air registers will solve the draft issue.

We are prepared. Now, let’s begin duct sizing!

Technique 1: Equal friction duct sizing:

Characteristics of this technique are,

  • easiest method to use,

  • quick results,

  • very suitable for low speed residential applications,

  • require dampers, as this method is more difficult to be balanced, compared to the static regain method

There are pros and cons.

Steps to take,

  1. find the longest route in feet from the fan/blower to the register.

    NOTE: The longest route includes equivalent lengths of bends, and junctions

  2. use the value of 0.1 inch water gauge (29 Nm-2 or 29 Pascals) of total resistance within the duct.

    This value of resistance is being widely used by certified contractors as the total resistance within the duct system

  3. divide (2), with (1), and multiply with 100.

    We have to do this multiplication, as the duct sizing chart’s resistance values, are formulated using resistance per 100 feet of duct.

    So, if the total required resistance is 0.1 I.W.G, and longest duct run is 200 feet, then the resistance per 100 feet will be 0.05 I.W.G

  4. value in (3), will be the design duct resistance, per duct length

  5. use value in (3), and circular duct sizing chart for this step.

    Read the design I.W.G value, and find intersection point at the mass flow rate axis.

    You will also get the appropriate circular diameter, on one diagonal line, and the design air velocity within the duct, on another diagonal line

  6. use data of your maximum height clearance within attic, and floor space to select the duct height.

    Give space provision for insulation (if required), installation, and maintenance

  7. now, we can utilise the second duct sizing chart.

    We know the maximum duct height, and the equivalent circular diameter, hence we can find the equivalent four-sided duct

  8. repeat steps (5) to (7) for all other sections. Iterate to get the best design

Why iterate? Hang in there; let us just finish the next most popular method, before the need for iteration is justified.

Technique 2: Static regain duct sizing:

Hi there, welcome to technique 2 of duct design. I hope technique 1 was clearly explained.

Getting back into the business of duct sizing.

Static regain method have these characteristics,

  • more difficult and time consuming than equal friction method,

  • you have to do calculation, from the fan’s/blower’s outlet, and proceed, section by section, in order

  • uses static and dynamic pressures of air flow, to balance pressures within duct system,

  • suitable for high velocity applications,

  • and better pressure balance compared to equal friction method

The steps, after you have done the preparation bit,

  1. start at the beginning of the main duct

  2. fix the maximum velocity within the whole system or the maximum I.W.G per length you want, within the duct.

    If you choose the former, use the total flow rate figure (in cubic feet per minute) from the prepared data, and find the I.W.G per length value.

    If you pick the latter, use the total flow rate figure to find out the maximum air velocity within the whole system

  3. adopt I.W.G per length value chosen in (2)

  4. next, use the equation of,

    section’s total head

    = section’s frictional head + section’s dynamic pressure head,

    as suggested by Massey in "Mechanics of Fluids" book.

    The units should be in I.W.G, as the charts are formulated in that manner.

    Dynamic pressure head is contributed by the velocity of the fluid within the section, whereas

    Duct frictional head is contributed by sum of,

    • value in (2), multiplied by the length of the section,

    • and duct dynamic pressure head, multiplied by the sum of loss coefficients from bends and obstructions.

      Value of loss coefficients are obtained through empirical data. Therefore, we need a database for such data. “Manual D” has those.

    Duct sizing formula using static regain method
    Modified from Massey

  5. use relationship in (4), to calculate the total head within the section

  6. now comes the tricky part.

    You have to make a good guess of the following section’s duct diameter.

    It is usually smaller, as the flow rate will be divided into branches.

  7. use the square of ratio, of current duct size, to the previous duct size,


    the ratio of current section’s volume flow rate, to the previous one

  8. then find the relevant loss coefficient

  9. use the current duct size, and section’s flow rate, to determine the I.W.G per length.

    Of course, we have to use the duct sizing chart

  10. repeat step (4) for this section

  11. subtract (10), with dynamic pressure head, in (5)

  12. iterate with different section size, until you get value in (11), closest to 0.

    You have to repeat steps (6) to (10)

    Choose the closest round figure

  13. repeat steps (6) to (11), until you have covered all sections. You’ll have your duct sized just nicely.

    Just don’t forget to use the duct sizing chart, to find the equivalent rectangular shaped duct

A little about loss coefficients in duct sizing:

Losses in ducts occur due to,

  • obstructions,

  • converging and diverging sections,

  • and bends, as well as junctions

These values have to be obtained from experimental data. Thanks to researchers of fluid dynamics, we have them stored in database.

One of it being, “Manual D”.

As promised, the reason for iteration in duct sizing:

We have to do iteration due to following reasons,

  • we need to find the closest round number of duct sizes

  • we need to match different sections’ sizes as much as possible, to minimise use of material

Finalising duct sizing:

Hmm. That "kinda” rhymes.

Anyway, properly sized ducts, is just as vital as properly sized air conditioners. We have to get it right or we will have an uncomfortable space, albeit properly sized air conditioners.

The process is not easy, but it is definitely worth the effort.

Good news though. Duct sizing has been made easier by software that will calculate and route duct works within your building.

You can use any method that you like, equal friction, or static regain. You may also use software.

Or, you can hire a contractor, and verify their working through duct sizing topic we were discussing a while ago.

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