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  1. #1
    Join Date
    Jan 2003
    Location
    New Jersey
    Posts
    71
    Hi to all,

    We had some remodeling done a couple of months ago and needed three additional ducts for a new room. There is about 10 inches clearance above the main supply duct, where I thought the new 6" ducts would be connected using one elbow.

    However, the HVAC subcontractor thought it would be too difficult to cut a hole in the top of the supply duct (with only 10" above the supply) so he brought the new 6" duct over the supply duct and connected it to the far side using two elbows. This was done for all 3 new ducts and there is very little air coming out of the registers. I think that removing the extra elbow might give us enough air in the new room.

    I would appreciate it if anyone could tell me:

    1) Is there enough room to connect a 6" elbow to the top of the supply with 10" between the top of the supply and the ceiling? Is there a type of fitting that could be attached to the top of the supply that transitions to a 6" duct but requires less vertical space than a standard 6" elbow?

    2) Does 10" provide enough room to work to cut the hole in the top of the supply? If not, can the hole in the top of the supply be made by cutting the bottom of the supply, then cutting the hole in the top, then patching the bottom of the supply.

    Thanks in advance,
    D





  2. #2
    Join Date
    Aug 2002
    Posts
    1,651
    If the airflow is not correct or enough, then the ductwork was not designed correctly or a load calculation was not done or done properly..

    I take it that they added these supplies to a existing supply trunk line??
    If they did, the likely hood the the supply trunk is sized large enough for additional runs is very unlikely..
    Most of the time additions need to have the runs run back to the plenum for correct air distribution...Only someone that is knowledgable in duct design will be able to make the assesment..

    You need to find a hvac contractor that is familiar with:
    Load calculation(Manual J)
    Duct design(Manual D)

    Goodluck
    J

  3. #3
    Join Date
    Jan 2003
    Location
    New Jersey
    Posts
    71
    J,

    Thanks.

    I believe the existing supply trunk is large enough, based on the airflow we're getting at the other registers. In fact, some have had to be dampered.

    I'm trying to determine the effect of having two 90 degree elbows on each duct as opposed to one and if removing one elbow on each of the 3 ducts should help.

    Thanks again,
    Mike

  4. #4
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    It will not help if the existing duct system and HVAC were not designed to handle the load of the new addition.

    You sir were HACKED!

  5. #5
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    Yes, 10" is enough room to fit a 6" Top Takeoff, however it is a real PITA to cut it in. In a joist space right? Probaly just took the easy way out with the two 90's.
    Where are you? Are you done yet? I got ONE more call for you.....

  6. #6
    Join Date
    Jan 2006
    Posts
    214
    thats plenty of room to cut a 6 inch take off.the subcontractor must not be much of an installer.i agree you got hacked.

  7. #7
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,516
    i have cut oulets in tighter spaces then that. more times then i care to think about
    he was lazy and took the easy way out
    if you dont have enough air you may want to balance the system a little. there may also be the chance the new outlets reduced the external static pressure to a point
    have the blower check for fan speed and see if it can be kicked up to increase the static pressure a little more to make up for the 300 or cfms needed for the new outlets

  8. #8
    Join Date
    Jan 2003
    Location
    New Jersey
    Posts
    71
    Thanks, guys. I knew I would get some good advice here... but I didn't think I would get so much and so soon. This is a great forum!

    Regarding my problem... I'm still on really good terms with the GC, who usually does very high quality work, and this was definitely not a bargain job. So his HVAC subcontractor will probably agree to replace all the double 90 degree elbows with one out the top without too much arguing or complaining. Do you think that will help or are the extra elbows not a factor in the weak performance.

    Again, heating is more than adequate in the rest of the zone and the airflow at the other registers seems just about right, so I'd rather not increase the blower unless it's absolutely necessary.

    Thank you all so much!

  9. #9
    Join Date
    Dec 2005
    Posts
    894
    I have never or have ever heard of anyone that knows air dist. to cut into the top of a plenum. Maybe that isn't what your talking about, maybe a couple pics would help.

  10. #10
    Join Date
    Jan 2003
    Location
    New Jersey
    Posts
    71
    Ikey,
    Sorry if I wasn't clear. I was asking about taking the new 6" ducts off the top of the supply trunk line, which would require 1 elbow -- as opposed to taking them out the side of the supply trunk line, which is easier to cut into but requires 2 elbows per duct.

  11. #11
    Join Date
    Jul 2004
    Location
    Kent, WA
    Posts
    129
    I'm not so sure even Manual D provides the answer. Adding elbows adds an equivalent length to that run. I think an elbow is something like 50' of length. The register boot is usually the worst at around 80' of equivalent length, then you have the length of the duct itself. Finally, the number of takeoffs down stream adds another equivalent length to the branch takeoff. If the elbows, duct length, and end boot doesn't total up to more that what was your longest equivalent duct run, then the system friction rate has not been affected.

    The larger question is, for the friction rate that your system is operating under, what flow will you get with a 6" pipe of that equivalent length. Manual D will tell you your flow limit, but it won't tell you what you'll get in a specific duct run. That is why you have to use balancing dampers, because most people don't know enough to size each run to the perfect length. The flow obviously isn't enough. Removing one elbow will reduce the length and increase flow. But I'm not so sure it will be enough. The length is probably going to be reduced about 25% and perhaps less. Is a 25% increase in flow enough?

    You could also try replacing the two 6" elbows with two 7" elbows and then reduce down to 6". How much this would help I can't say.

    Duct work is more art than science for most people.

  12. #12
    Join Date
    Aug 2002
    Posts
    1,651
    The point I was making about manual D and manual J is that someone that know manual d and duct design and manual J would know if the current trunk is sized large enough and if three six inch ducts will provide enough cfm based on the load calc.. It is obvious that if his contractor did not ensure that the room was getting the correct cfm it is unlikely the he did the required steps to insure that the runs were properly sized..

    Give us some more information like how big is the room?
    How many sides are exposed to the elements?
    Is the room built over a crawl space or a basment?
    How many windows?
    How long is each six inch run?
    How long is the trunk line before each run is branched off of it?
    How many elbows are in each run total?
    Is each run insulated?
    Is each run metal or flex duct?
    Is there a return in the room?
    If not, is there a path to an existing return?
    What was the required cfm for each run determined by the load calc?
    Did the contractor take measurements as to what cfm each run is currently delivering with a balometer or "transverse of the duct"?
    Any many more evaluations needed by someone that know duct design would need to look at to determine what needs to be done!!

    Goodluck
    J

  13. #13
    Join Date
    Jan 2001
    Posts
    8,234
    That additional elbow isnt cutting your airflow.......you either need to rebalance the system or look for a restriction in those lines.....maybe they are crimped on top of the duct or choked off in some other way....possibly the dampers in them are closing off the duct....Ive seen that quite a bit around here....takeoff out of side of duct then 2 elbows to do a 180 right back over the top of the trunkline.....probably not as efficient as one elbow but its not going to make a dramatic change in airflow.....

    look for choked down flex...loose connections....I see a lot of flex where the inner liner isnt connected...they just attach the vapor barrier.....check dampers......rebalance system

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