HVAC A/C duct minimum size


  #1  
Old 11-01-12, 03:08 PM
J
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
Cool HVAC A/C duct minimum size

I read on EHow or something like that that an A/C duct minimum size is 8". I'm wondering where this code reference is from.

Does that mean you can't run an 8" duct and branch it into two 6" ducts for a room?
 
  #2  
Old 11-01-12, 03:25 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
I haven't the slightest idea where that comes from. Duct size is determined by the number of CFM (cubic feet per minute) of air that needs to be transported through the duct, the purpose of the air (heating, cooling or ventilation), the temperature of the air and the desired temperature of the room, the dimensions of the room, the number of other ducts also supplying the room, the allowable noise factor and maximum velocity of the air in the duct.

All that stated, the cross-sectional area of a six-inch round duct is roughly 28-1/4 square inches. The cross-sectional area of an eight-inch round duct is roughly 50-1/4 square inches. So the eight-inch duct would be just slightly small if you needed maximum airflow from the two six-inch ducts. Not knowing a lot more about your proposed application I would state that it will probably be okay.
 
  #3  
Old 11-01-12, 07:17 PM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
Min code for duct wrap is R8! Are you sure this isn't what you were looking at
 
  #4  
Old 11-02-12, 04:34 AM
J
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
Here is the link.

How to Install a Tubular Duct | eHow.com

Insulation is required only if the duct would have a potential heat or cooling loss from being in an unconditioned space per uniform mechanical code 2012.

And although a 8" round duct is 50 sq in and 6" 28 a 8" round duct is rated for over 200 CFM compared to a 6" of 100 or so CFM from what I remember offhand. I think it's because the percentage of friction area is lower as the pipe size goes up. Charts on these are available on alpinehomeair and the net.
 
  #5  
Old 11-02-12, 11:20 AM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
Like most of the E-how articles this one gives less than ideal information. Almost anytime you see a blanket statement you can bet it is wrong for certain situations. They are correct when they INFER that cooling takes a larger airflow than heating BECAUSE the temperature differentials are usually closer for cooling than they are for heating. Typical cooling differential is about 20 degrees cooler supplied air than desired room temperature whereas for heating the differential is more often 40 or 50 degrees with supply air at 110 to 120 degrees. Smaller differential means a greater air flow is required.

Smaller ducts will require significantly higher velocities to maintain a specified air flow and high velocities mean higher noise levels. There also becomes a point where the velocity is so high that the flow ceases to be laminar and becomes turbulent. Once you hit that point the air flow will actually decrease and the noise level rises dramatically.

It has been far too many years since I have done any duct design so I can't offer you any concrete figures but the basics I have outlined still hold true.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: