Origins of Low Velocity Friction Rates - Why the difference?

[BravoCompany]

Typically I've seen the HVAC industry's most common rule of thumb for sizing low velocity duct to be:

(.08-.1 " wc/100 ft) for supply duct.
(.05" " wc/100 ft) for return duct.

Does anyone know if these values are published and if so where exactly?

Why the discrepancy between values?

I was taught to use these values some time ago and have since heard several different lines of reasoning.

My assumption has always been that the lower value on return side offers more equal pressure equalization between spaces-- no doors slamming shut etc. The realized advantage being that no balance dampers are required on the return air side of the system. This assumption is only speculation.

Looking at the 2005-ASHRAE Fundamentals Chapter 35 Fig 9 along with the explanation under Equal Friction Sizing Method. One questions the .05 " wc/100 ft.

I would be interested to hear what anyone else may know or wish to discuss regarding these values.

Would anyone like to comment?

 

[BronYrAur]

I have only speculation myself, but I suppose someone along the way determined that a "typical" low pressure system sized at 0.08" (or even 0.1") per 100' is all that a rooftop unit or small fan coil can handle. Those units typically don't have much external static capability (.5" or so) and your cfm/sqft will use up all the air before your duct run gets too long to cause a static loss issue.

One thing to note about your question, this is just duct friction loss, not room pressurization which would cause doors to slam, etc.

 

[NCDesign]

My experience goes along with BronYrAur's speculation. At my previous place of employ we found that if we kept our supply and return static between 0.05"-0.08" per 100' we never had any airflow issues with smaller residential and light commercial split and rooftop units (20 Tons and below) regardless of make or model. If you look at an airflow vs. pressure drop table for a 5 ton gas furnace you will see that a lot of the newer 90%+ AFUE units can barely push 1900 CFM at 0.4" ESP on high speed. Once you factor in the coil and a dirty filter, you only have about 0.1-0.15 ESP to actually move the air through the duct, out the diffusers, and back through the return. The fee on jobs this size didn't warrant taking 2 or 3 hours to lay-out and size the duct work so you could use the rule of thumb and lay it out and size it in matter of minutes. (This assumes that you do a lot of that work and calculations by hand. If you are using an AutoCAD plug-in, etc it can take care of that for you as well.) It all boils down to the fact that your fan & motor have X amount of pressure and hp available and you want to be able to size your ductwork quickly with some idea that it will actually work.

I never really explored the differential in return and supply sizing but always assumed it was due more to economics than anything else. On a lot of jobs (light commercial, cheap) you have a larger amount of supply duct than return. So, if you use a higher friction number, 0.08"-0.1" per 100', you have smaller supply duct (less $$), but it would use a bigger portion of your available static. You make this static 'deficiency' up by enlarging the return duct a little by using 0.05" per 100'. The assumption being that the return would be substantially shorter so you would realize some cost savings.

A side effect of this is that, generally, when you size using those rules of thumb you will not have any noise issues with your ductwork.

 

[mechanicaldude]

I've been a HVAC consulting engineer 16 years and have always used .08"/100' for BOTH low pressure supply and return as a standard. Now those number may drop for specified purposes such as transfer duct sizing or maybe other job specific applications, but in general i always use the .08" as my standard for sizing. For most applications these days (new or existing) there is generally very limited space above the ceiling and getting the ductwork as small as possible is necessary to fit it within the limited space.

 

[willard3]

Using rules of thumb to design ductwork will get the Engineer in trouble when there is too little duct space, the aspect ratio of the duct/duct space is wrong and etc.

I have had to clean up waaaayy to many duct systems designed by rules of thumb. Best way to design duct systems is to know in detail duct velocities and pressures. Air noise in the duct is also common with air system complaints.

Careful analysis of duct systems shows the problem with cheap low-pressure fans having less than 0.5" wc pressure capacity. Free air supply and return with a dirty filter will exceed this pressure........

 

[BravoCompany]

I appreciate all of your responses. I would however comment myself that the discussion has veered off track slightly here.

1.Do the values have a specific known published origin?

2.Why the descrepancy?

BronAraur thanks for your thoughts as they spoke directly to what was asked.

A couple of points to consider here:

1. The primary reasoning I believe is sound. There is less noise and pressure loss associated with pushing air out of a small duct to a largeer open space rather than to draw air from a large open space into a small opening; I believe this could very well be the reasoning behind the descrepancy at least in part. A Return air grille is can be notably louder than the same grille used at the same cfm for supply.

2. No one has suggested using rules of thumb to soley design by. On the contrary, we are looking to establish a foundation for values that must be rooted in some definite reasoning or they wouldn't be so prevalent and widely accepted. Think of it as revese engineering here. If you have a fan with no static capabilities then you must oversize duct work to compensate; I think that one goes without mention; careful analysis really isn't needed to state what the manufacturer is explicityly saying-- this fan can simply not move air against any significant resistnace.

3. The issue of negative relative pressurization between spaces causing doors to slam shut is direclty related to friction losses in the return if no balance device has been placed to compensate for pressure differential. If two rooms side by side are served by return air duct that is over restrictive, the air will seek a path of least resistance and begin to travel between the spaces(doorways) rather than the duct until pressure is equalized. This inturn can lead to the a door cracked slightly closing unexpecedly-- I've seen this more than once.

4. ASRAE does recommend low velocity duct work design betweeen .10 and .08 " per 100 ft I believe as I don't have the figure here in front of me. The .05"/100 I have not found any where.