4 inch supply ducts??
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  1. #1
    Join Date
    Jun 2008
    Posts
    41

    4 inch supply ducts??

    Last wk we looked at a job that another contractor was in the middle of before the GC bounced him out. They asked us to verify his work up to this point and submit any recommendations or changes. There were several things that stood out such as the PVC venting that was run in 2inch that needed to be 3, return drops, etc. In 3 locations there were 4inch supply ducts(round) run..two to the underneath kitchen cabinets and the other to register in front of an outside sliding door...Question..I thought minimum supply ducts were to be 6", am I mistaken. If not where is the reference that points to this, IMC, SMANCA, etc..thank you for your time..Jim

  2. #2
    I recently moved into a house with 4-inch supply ducts throughout (connected to the plennum "octopus style"). Being a Tinner of twelve years, I nearly died laughing at the sight. I mentioned this to the owner of the rental house and was told the you lose too much heat through big ducts(6")...and that her father had installed systems this way his entire 45 year career. The gas was turned on this morning and much to my surprise, the system heated quite well...however, I'm curious to see whether or not the furnace will go out on high limit. I guess it all comes down to the correlation between volume and velocity. I can't ever see myself installing something like this...for the reason of pride, if nothing else.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,620
    No min 6" duct that I know of.
    They may have used 2-4" ducts instead of one 6" to get better spread/air mixture in the kitchen.
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  4. #4
    Join Date
    Jun 2004
    Location
    Western MA.
    Posts
    505
    I`ve seen systems like the one you described,almost looks like an early version of a high velocity system,had really unique sidewall registers and if I remember correctly the system didn`t take too kindly to the new furnace we installed.
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  5. #5
    Join Date
    May 2000
    Location
    Urbandale IA. USA
    Posts
    4,881
    I did calculations on a house awhile back, and it came out with 30-40 cfm for ALL supplies. (small house, 2 bedroom)
    For the bathroom, it called for 12cfm and 2" supply.
    All of my suppliers said that the smallest floor boot connection was 4".
    So my co-worker and I put in 4" all the way arround.
    It works well. We had a total of 5 supplies, dont remember the resulting cfm on each supply, but it worked well.
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  6. #6
    Join Date
    Dec 2001
    Location
    Ocean Pines, MD
    Posts
    6,954

    If I get the job.

    I'll post pic's of this duct and equipment I found. System had been maintained for years by the same co. but one of their "maintenance service tech's" found the system, 100kbtu on a 2.5t a/c, CF, tripping on it's limit he called in the calvary. The next guy looked at it, told the lady she needed a new system but they didn't have one to fit. I think they profiled her and walked, er, ran away.
    In the crawl I found a 20"X14" plenum with 8-4" round pipes coming off it. Each floor boot had a separate butterfly felt gasketed damper in it(?).
    Did a load on the house and even not insulated much at all it came in at 40KBtu on a 900 sq ft home.

  7. #7
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Lots of misinformation here.


    Anyone that thinks you should put in 6" minimum ducts has never performed a Manual D calculation before. Most contractors put in 6" because that is the most readily available and economical but doesn't mean it is necessarily the right size. I guess dampered it should be ok.

    But, if you have ever designed duct per Manual D you will inevitably get some ducts that are of odd sizes, 4", 5", 7", 9"...just the nature of the beast. Why waste money on 6" duct for a small bathroom that will be adequately fed with a 4" run? Heck, you can still install a damper on it because it may still get too much air.

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