Ask some questions,maybe we can help.
I am trying to understand ductwork sizing as it relates to residential AC. I have read Manual D cover to cover but sections 3 and 4 make no sense to me, I just don’t get it, call me stupid but those sections are very hard to understand the way it’s is written.
Are there any suggestions for other written material that explains those two areas? Thanks
[Edited by jrc2905 on 02-20-2005 at 09:55 AM]
The obvious is obvious
Ask some questions,maybe we can help.
I have one..Question.Lets say you go to a house that has three story.one unit and only one distribution system.
You head to the third floor to see if you are getting any air.You have lots of noise and what appear to be good air coming out of the vents.
So knowing that the duct are to small,because you have the jet noise in the house,you decide to change the duct to a
bigger size to reduce the noise.
But you can only change the ducts in the first floor,which
you do so.
Now you have sucessfully reduce noise,but you have created another problem on the third floor because not only is there
no noise,theres no air as well.
So what happen to the air?
Did you replace the trunk line and supply runs to the first floor, or just the supplies.
Air follows the path of least resistance,which after the change ,is to the first floor.
What if I only increase the size of the duct main trunk,supply and return just before it ties into the riser
on first floor.Total of 30 feet for both supply and return
coming off the package unit.
No change on the sizes of the outlets and only thing added was a 12inch return.Which added another 25 feet.
Increasing the size of any of the ducts ,to the first floor,reduces the restistance(static) of that section of ducts,air that went to floors above,now has a less resistive path to follow.
The blower ,only "knows", air in(return) equals,air out supply,other than leakage,it will follow the least resistive path,from and back to the blower.
[Edited by dash on 02-20-2005 at 02:37 PM]
You should put balancing dampers in the first floor supplies.
Thats about the only way you'll get the air flow back to the third floor again.
What is confusing how to figure the fan operating point?
The obvious is obvious
Originally posted by jrc2905
What is confusing how to figure the fan operating point?
Manual D,shows fan curves,most all residential "fan Data" from mfrs. is a chart,not a curve as shown in Man.D.
Look at your mfrs fan data ,if it's a chart ,convert the chart to a curve for one speed,then the other speeds.This should help you understand.Just use a sample chart and plot the points on the fan chart,then connect the dots into a "curve".
When you get to the actual Friction Loss worsheets,to determine the FR,you see that all you need is the cfm at what ESP,from the fan chart.
My guess is that mfrs Fan data ,was in the form of fan curves ,years ago.Today the only fan curves a see is for Variable speed fans.
Men,
as a HO, I'm very impressed with the knowledge you have about your craft. You definitely deserve the rates you charge! Dash can you install my system for me? Just kidding, I'm sure the person I have is just as qualified.