HVAC Pro Blog
Rules of Duct Design

I went many years as a Service Tech and never had a clue as to these rules. As a matter of fact, I went years as a Service Manager, and quite a while as an Instructor, without seeing these all in one place! I had this problem with learning the hard way - who knew you could pick up a book (or read a blog!) and learn this so easy? If you haven't been able to tell, I am all for giving this type of information to those that need it; so please share with your service managers, techs, sales, and even the greenest of tin knockers.


This is not new information, taken directly from ACCA Manual D (Residential Duct Design), and is very basic in order to distribute the conditioned air to the conditioned space. We all have a story as to what happens when one of these rules is not followed. For the most part, if someone missed on one it turns into a checklist. Unfortunately, many service techs put blinders on when walking into a basement or attic, like a horse with a feed bag. The thought is that the unit must be malfunctioning, after all - most tech schools and higher learning institutions do not teach duct layout - so is it really the techs fault? This list just may open their eyes, and find the real problem. Please: do not increase the airflow, install a larger horse power motor, or find yourself installing a duct booster to overcome these shortfalls - that becomes a waste of energy!


Rules of Duct Design (ACCA Manual D)

  • Up to 24' of trunk length: use one size
  • Over 24': reduce trunk duct every 15' to 20'
  • Use tapered reducer for trunk reduction on capped trunk
  • Standard trunk is 8" high
  • Trunk width not to exceed trunk height by more than 4 times
  • Use offset take-offs rather than straight take-offs
  • Stagger the branch take-offs
  • Damper each run as close to the trunk as possible
  • Do not branch off any closer than 12" to the end nor off the end of a trunk
  • No take-off 4' after a reduction or 1.5 times the greater dimension of the duct
  • Never take-off a reduction or increase the mains any closer than the diameter of the branch duct
  • On supply and return, when the trunk is wider than the plenum, a transition fitting must be used

What all of these rules boil down to is proper distribution of airflow. Without a check on all of these statements, you will experience a loss of static pressure, velocity, and comfort. It is amazing how many Carpenters (no offense, I know many great Carpenters!) think that since they can buy a box of flex at Home Depot, this makes them a Sheet Metal Worker! As much as I hated sending my $250 to the state last year for my Master Sheet Metal License, I can certainly see the need for it.


Embrace these time-tested rules, it will make your tech's life much easier and just maybe increase your 'non-profit' margin!




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