Duct Design, Practical Aspects

[SnTMan]

OK I read that FAQ, so maybe I better start this over!

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.

 

[imok2]

1) What is a "good" value for volume flowrate, i.e. CFM flow per ton cooling?
ANS.Usually about 350/400 cfm/TON

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

ANS. Most are rated at about 0.5 inches of static pressure

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

Ans. I like to keep total pressure at about 0.7 inches other wise you deminish the amount of air too much to do proper job.

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.

Ans. The main should be about 1000 to 1200 fpm and the branches about 700 to 800 fpm. Now the proper selection diffusers to deliver a design quantity of air for a given throw is a matter of consulting manufacturers'tables and charts but for now some examples are:
100 cfm with a 6" duct and a throw of 5ft.and 330 fpm at the throat of the diffuser
200 cfm with a duct of 8" and a throw of 7ft and 950 fpmat the throat.

 

[rogzog]

Hi,
A useful tool for sizing your duct is a duct calculator. It's a little cardboard or plastic slide chart that helps to easily calculate relationships to size, volume, velocities and friction losses in ducts. Here is a link to where you might be able to get one.

http://www.slidecharts.com/airduct_calculators.htm

For your residence talk to some equipment suppliers (several) and get some submittal data sheets for the equipment you might consider. When sizing your ductwork keep in mind the total CFM and total static pressure capacities of the equipment you might use.

 

[SnTMan]

imok2, rogzog thanks, very helpful.

imok2, back to my question #3: For example: Duct run # 1 has , say .5 IN WG loss, Duct run # 2 has .45 IN WG loss in total pressure. Close enough, or balance further? I would like to avoid dampers because I think they would complicate future duct cleaning.

#4, I don't know if data is available on residential diffusers you get at the lumberyard, but your values will do for now. I may do some research. Thanks.

rogzog, I have set up a spreadsheet to do the dirty work, I just have to input data. One problem, if velocities, or flowrates go too low, you kind of "fall off" the low end of the charts for loss/100 ft. Makes accuracy kind of a problem. May look into one of your calculators.

At this point, I am just doing calulations to see if rectangular trunks of a reasonable size appear feasible. I would not hire a contractor to build "my" ducts without someone else taking a look. For sure.

Very helpful, thanks again.