Residential HVAC Questions, Please help? 8

[elfman]

I am trying to learn more about residential HVAC design, load calculation guidelines, duct run design, etc. Is there one good resource for information?

For instance, which is better, one larger duct feeding several registers that are somewhat distant from the plenum, or three smaller diameter ducts that run all the way from the plenum to the registers? Intuition says that to minimize losses both to flow and duct wall conductivity minimize surface area by using a larger duct to feed the registers. Is this the case, or do other factors weigh more heavily?

 

[MarauderX]

http://www.hvac-talk.com

there are several resources for design and installation there.

John Peterson

http://www.wba-arch.com

 

[SnTMan]

Hi all, I am new to this site, but found this thread on Google. I am also interested in residential HVAC design, namely the following:

I would like to replace the ductwork in my house which is in poor shape and is located in a very constricted crawlspace. I am trying to roughly size a trunk and branch layout to replace my "spiderweb" layout. I would expect to replace the gas furnace at that time, but would like to retain the existing return duct in the attic and the condensing unit.

At this point, I am not concerned with heating/cooling loads and so forth, as I have existing equipment which seems to perform well enough, furnace is 106,000 input BTU, AC is 2 or 2 1/2 tons (I think) and the current diffuser layout would be retained.

The calculations are straightforward but, not being a professional in this field I do not have a feel for what are "good" numbers.

Finally! My specific questions:

1) What is a "good" value for volume flowrate, i.e. CFM flow per ton cooling?

2) What is a "good" value for system total pressure loss and what will an "average" air handler produce.

3) When balancing duct runs, how much difference in total pressure loss would be considered negligable?

4) I am also interested in further guidance on velocities in the various parts of the system, particularly "exit" velocities where the flow exits into the room space.

I have found a several good sources on this subject, but they seem to be oriented toward large commercial systems and don't contain some of the more practical aspects of small systems.

Thanks in advance for any input.