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  1. #1

    booster fans in 6" metal ducts

    Does anyone have any actual experience trying to increase air flow in a 6" hard metal duct using booster fans? My dining room gets air from a 25' long 6" duct, but it only as two bends, one at the start above the master duct and the other coming up through the wall. The actual gas blower is 4 tons and should be enough, but the force at the end is weak. Were you able to get a better air flow?

  2. #2
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54
    Duct design is critically important - On your plenum (if a spider duct system) - is there a supply duct at the opposite end of the furnace output? By this I mean if there is a supply duct at the direct opposite end of the output of the furnace (whether it be a spider system or supply trunk line) - move it to the side of the ductwork and cap the end of the duct work. You could be losing a significant amount of static pressure that would be critical loss in the longer runs of a duct system (in a properly sized duct system). If you eliminate the obvious problems first, as a last resort, add the fan.
    Last edited by DavidInAustin; 12-02-2007 at 01:15 PM.

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,189
    We've done it, to no success.

  4. #4
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    They don't work. Expierenced the same prob. Turned out I had a bryant frnace that could be equipped with a 1/4, 1/3, or a 1/2 horse power motor. Blower)

    I have 2500 sq ft and when I removed the the duct booster I happen to check my blower. It wa on a 1/3 so I bumped it too a 1/2. Got better airflow but still not as good as rest of house.

  5. #5
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Quote Originally Posted by rkthomas13 View Post
    Does anyone have any actual experience trying to increase air flow in a 6" hard metal duct using booster fans? My dining room gets air from a 25' long 6" duct, but it only as two bends, one at the start above the master duct and the other coming up through the wall. The actual gas blower is 4 tons and should be enough, but the force at the end is weak. Were you able to get a better air flow?
    The propeller fans are nearly useless. The backward inclined booster fans work well.

    But it sounds like you have a bad duct design.

    If the ducts are accessible enlarging the trunk and branch duct will improve the air flow.
    Remember, Air Conditioning begins with AIR.

  6. #6
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,436
    I installed a propeller type booster fan in my house about 6 years ago and it worked great. this was for a small bathroom that had about 20 CFM coming out of the duct. fan boosted it to around 50. still awful, but the bathroom is plenty warm now. i think the duct separated in the wall, but there was no way to find or repair it.

    i don't know how much of a difference it would make to a room like your dining room that has a bigger load

  7. #7
    Join Date
    Apr 2004
    Posts
    7,680
    20-50 is certainly possible if the duct's problem was low supply static or the take off was in an eddy of sorts. Increasing one with a propellar type fan that actually blocks as much volume will hardly get you from say 70 cfm to 100.

  8. #8

    willing to look at my duct diagram and suggest?

    Would anyone be willing to look at a fax of the diagram of my ductwork and suggest where I might try to add booster fans, maybe spider cage fans or whatever to get more airflow on the other side of my house? I just can not find any hvac tech here who is willing to take on this kind of trial and error work. I can add the fans, but I need a pro to tell me what size and type to add and where.

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Sure 352.795.6601

  10. #10

    diagram of ductwork

    Thanks. I shall post the diagram, (which I did myself, so not very pro), today. I also have digital pictures of the ductwork itself as well as the furnace, the steel supports, the joists, etc., which I could email to you so you have a better picture of my situation.
    I have tried to find a pro here with real experience of using dampers, booster fans, etc. to augment air flow and even out the heat, but so far have no really found anyone. I found plenty of guys who want to put in a new heat pump in the attic, but no one who has real experience making the existing system work better.

  11. #11
    Join Date
    Mar 2002
    Posts
    327
    If you call tech support for the blower manufacturer they will tell you where to put the fan for best performance. I did it and got 50% more air when I placed it where they suggested ( 2/3 of the way down the line) compared to where I placed it the first time (1/4 down the line).

  12. #12
    Join Date
    Dec 2006
    Posts
    82
    Make sure you get a real duct fan if you go that route. Here's two good ones:

    www.fantech.net/fkd.htm
    www.atmosphere.com

  13. #13

    what choice do I have?

    Thanks for the help. I shall call the tech support of both fan makers tomorrow. Why would their fans be better than the propeller type sold at HD?
    Can you suggest any other path? Local pros have recommended a new 2 ton heat pump in the attic with flex ducts to each room on the top floor, cap off the existing ducts and then experiment with the existing ducts for the basement and lst floor. This seems like a lot of duplication. And they do not want to work on the existing 150 btu system until the new one is in. But I blocked off the top floor vents and it only improved the air flow to the other side by a little. I still have rooms that are 12x14x8 serviced by one register that has to travel 36 ft. plus 3 right angle bends (or more than 66 ft. with each bend equal to 10 ft.).
    I want to try dampers and booster ducts first unless someone has a better idea. Any suggestion would be much appreciated.

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