Duct Sizing 1

[Lanzi]

Thank you for your time,

I am converting a garage to living space, and would like to run ducts for heat/cool off my existing main duct. I could use some help in adequately sizing the ductwork. First, I'd like to state that my residential house heat/cool system is fairly standard issue, so if I'm missing some critical information, please assume average values.

The basic question is: What size duct extension (8"x?10, 12, 16") should I use to tap off the main duct to accommodate 4 six inch supply ducts??? The question boils down to what size (8x?) supply duct is appropriate for 4 six inch ducts. This question is really independent of any other factors, but I will try to provide as much data as I can:

Room: 20x25 with 10'ceilings = 5000 ft3 (several windows).
Average Distance from main duct = 28'
Main duct = 8x32 with 1" internal duct board. Currently has 17 six inch taps. 0.10 wg average line loss...etc...
Point of tap = The main duct tap would be right next to the furnace, before all the other house supply ducts. It would have to be at 90 degrees, and require one additional 90 degree bend.
Average blower/furnace data: For the purposes of the misc details, please assume average everything (or I will not be able to get an answer). Basically, the 2440 ft3 house is perfectly heated/cooled with the current set up (furnace/AC Boyertown LB145 (145000 btu), blower (1900 cfm), ducts (17 six inch off of main duct-8x32, etc. are all running fine).

I'd like to tap off the main duct with a 8x?? duct running 28 ft to the middle of the new room crawl space. then, tap off this extended duct with 4 six inch ducts to the perimeter of the room. The question boils down to what size the extension duct should be to handle 4 six inch ducts??? 8x10 seems to small. 8x20 seems too big, and may take away to much air from the rest of the house??? If I had to do it immediately, I'd go with an 8x12, if I read the charts correctly??

A recommendation would be appreciated. Is an 8x12 OK?? or 8x10??? or 8x??? Let me know if you need some more specific information where just "average" doesn't do it, and I will get back to you.

Thanks,

 

[imok2]

In a residential unit a 6" duct carries about 110 CFM at 560 FPM, so you have 4 x 110 CFM = 440 CFM total which comes to an 8" x 12" duct at 600 FPM with external insulation.
However you will be taking this total amount from the existing system...can you live with that and still maintain comfort conditions. I think I would consider a ductless mini split system for about $600.00

http://www.ac-world.com/?gclid=COa82OzQ-5ACFRY0sgodsQOJ1w

 

[Wareagl487]

Lanzi-

Knowing your local would help but you provided plenty of information to for a rough estimate of your cooling/heating needs.

I'm with imok2 with some concern in tapping off your existing system based on the total area presently served. By the total CFM it appears you have a 5-ton unit. That equates to 500 sq. ft./ton - I would expect this for a home with numerous windows.

I would lean toward a separate system @ 1-ton. I think you would be happier as you would have independent temperature control in your new living space.

Andy W.

 

[imok2]

Question? if you have an 8" x 32" external duct and 1" insulation internally doesn't that leave you with an internal area of 6" x 30" and at 1900 CFM the velocity comes to 1530 FPM and the static close to 0.5/100ft,WOW I'm baffeled!

 

[Lanzi]

Thanks for confirming the 8x12.

Yes, I've always thought the 6x30 was way undersized for a main duct, too. I actually hired a large local HVAC company to inspect the design a few years ago, and they said that everything was OK.

Someday, I may install AZEL dampers thoughout, and totally zone the house. I could then finely costomize the system.

Thanks ALL!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[willard3]

I would worry more about fan pressure than system cooling capacity for a start.

Residential fans are very limited in their pressure capacity and original contractors make the ducts the size that maxes the fan pressure capacity. Not much left over.