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  1. #1
    My home has significant problem with the balance of the A/C system. Blower is installed in attic and has 2 duct runs coming off. One run is a 14" line going to the front rooms and another is a 12" line going to the back. All the registers connected to the 14" line (total of 6 I think)blow very strong. Feels and sounds like a blizzard coming out. The registers being fed by the 12" line (4) are extremely weak. You can feel the cold air coming out if you put your hand directly under them, but it feels more like it's trickling out rather than being pushed out under pressure. I suspected there was a kink in the 12" line (all flex duct) and wanted to resolve.

    I've had 2 service techs out to look at it with the following results:
    #1) Inserted plugs made of galvanized sheet metal into all the registers on the 14" line. This did increase the flow to the registers on the 12" line but only marginally. The registers on the 14" line gave out very little air after this. He told me this was a short term measure until he could get a balancing team out.
    #2) Now that all the vents are putting out very little air I have another tech from teh same company to address the root cause (which I still believe to be a kink in the 12" line). Tech holds his hand under the register and says he can feel air coming out. I explain what the first guy did and that I want to find the kink, fix it, and remove the plugs in the front line. He says there's no problem, that's normal, but maybe I should change my filter.

    So, I've started looking myself and this is what I find:
    SIZING: The main trunk sizes (14" and 12") seem correctly sized. 14" line goes to larger and more rooms than the 12".

    KINKS: I've now cut several holes in the walls to inspect and find there are no kinks in the 12" line or any of the 4 6" lines that feed off it.

    DISTRIBUTION: Something strange. The 12" line terminates at a galvanized steel box/plenum. Input to this box is not a 12" circle but a 7"x15" oval. Estimated cross sectional area of the oval to be about 70% of the 12" circle. The duct enters on the top of the box and splits the flow into 3 seperate runs. Two 6" runs and one 8" run which splits into two 6" runs later. All the connections to this plenum are on one side. Input is on top right and outputs are on right side. The left side of the box is dead space.

    My question: I believe the fact that the 12" circular duct is getting squeezed into a smaller oval is bad and may account for some of the poor performance of the 12" duct run. But it doesn't seem enough to account fully. Could the design of this distribution box between the 12" main line and the feeder lines be the real killer? My limited understanding of air flow through ducts makes me believe the box may be creating a lot of turbulence as air flows through.

    Thoughts?





  2. #2
    Join Date
    Jul 2006
    Location
    N.Y., N.J.
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    129
    Flex can collapse internally and look fine on the outside, splitting branch ducts is bad business and almost never works properly, and sheet metal duct is lined internally if its not insulated on the out side, inside liner also can come undone and create big air flow problems, your duct sizes may be fine but the distribution is terrible, i suspect a thorogh inspection should uncover the problem.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    chad,

    What you found is certainly part of the problem.The box the 12" goes into should be 12X12 and long enough to take the branch runs out of,don't go larger then 12X12.

    Now how and where the 14 and 12 come off the plenum at the equipment could be part of the problem too.

    Consider installing dampers on the 14 and the 12 to balance ,when you resolve the problem.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    I agree with the others; from what you describe, that box is a real airflow killer. It's likely full of turbulence, killing velocity.

    Flex duct and pistons...the bottom feeders of the HVAC industry.

    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Jul 2004
    Posts
    322
    Do both the trunk ducts come off the air handler perpendicular to airflow, or is one parallel and the other perpendicular? Length of ducts, whether they are pulled tight, and number and radius of turns also affect airflow.

  6. #6
    Wow, thanks everyone for your responses. I think I'm getting somewhere now. My answers to your questions:

    FINEST: Flex can collapse internally and look fine on the outside, splitting branch ducts is bad business and almost never works properly, and sheet metal duct is lined internally if its not insulated on the out side, inside liner also can come undone and create big air flow problems, your duct sizes may be fine but the distribution is terrible, i suspect a thorogh inspection should uncover the problem.
    RESPONSE: Interesting that flex can collapes on the inside and not be detectable from the outside. I hadn't considered that option before. I think that's not the issue though and here's why: When I discovered the distribution box I disconnected the 12" line from it and turned on the A/C to see if I got any airflow. The 12" line was blowing like crazy. This told me the problem was downstream of the point I disconnected. Since all four runs coming off the box have low flow I'd have to have a kink in each individual line, which I considered unlikely. Is that reasoning good?

    DASH: What you found is certainly part of the problem.The box the 12" goes into should be 12X12 and long enough to take the branch runs out of,don't go larger then 12X12.
    SHOPHOUND: I agree with the others; from what you describe, that box is a real airflow killer. It's likely full of turbulence, killing velocity.
    RESPONSE: Would putting some solid object in the distribution box to take up dead space be a reasonable solution? (Ripping out the box and replacing would require a HUGE effort due to how and wehre it's installed)
    Also, How important is the undersized oval connection between the box and the 12" duct?


    FAT BOB: Do both the trunk ducts come off the air handler perpendicular to airflow, or is one parallel and the other perpendicular? Length of ducts, whether they are pulled tight, and number and radius of turns also affect airflow.
    RESPONSE: Both come off perpendicular. Difference is the 14" duct has a long straight run before any bends. The 12" run has an immediate 90 degree bend so it's running perpendicular to the flow in the handler but the opposite direction. Is this a big problem?
    I don't see a difference in the number or tightness of bends in the two trunks other than this.

    Again, I really appreciate your taking the time to respond. In addition can anyone recommend someone in Phoenix who's capable of fixing this issue? Up til now I'd kindof given up on hiring someone becuase the two guys I already had come didn't seem interested in solving the real problem. Don't mind spending a little cash to fix the issue, but I got tired of paying guys to come out and not get satisfactory results.

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Also, How important is the undersized oval connection between the box and the 12" duct?
    I did hypothetical numbers at 800 CFM. AT 800 CFM, a 12" duct, at .78 square feet, will flow (hypothetically with no friction loss) 1025 feet per minute (fpm). The 7" x 15" ellipse (oval), at 800 CFM, and at .57 square feet, will flow 1403 fpm. To check, take area in square feet x velocity pressure in fpm yields CFM. Both shapes check out at about 799 CFM. Anyone here is free to check the math and tell me I'm off base if I crunched something wrong. I won't wilt.

    That being said, there has to be a transition from a 12" round duct to a 7" x 15" duct. Anytime there's a transition in ductwork (increase or decrease) there will be a change in velocity pressure. In your case you'll probably see a small drop in velocity pressure before the air reaches the ellipse (oval).

    However, I still hold the real problem is in that stupid box on the other side of the ellipse transition. It's an air trap. Dash is right on in that the box should not be wider than the 12" duct feeding it, or at least it should echo dimensions formed by the ellipse. By what you describe, I can't see why they transitioned a 12" round duct into a 7 x 15 ellipse, THEN put that into a box and punched all the outgoing ducts on just half the box's surface.

    I'm also curious...is this ductwork concealed in a wall chase coming down from the attic or something, hence the accessibility problems? I've seen new homes under construction where this is done...a large flex duct dangling down from the attic with some goofball transition box that will be buried in the wall once the sheetrockers are done. DUMB.

    I'd say if you're going to go through the trouble of having someone alter the box you have, go all the way and have a proper box made up. You now know where the trouble source of your airflow problem is...you just gotta follow through with it.

    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    May 2006
    Location
    Raleigh,NC.
    Posts
    357
    been a long day,so may have missed something. what are the temps in the rooms in question v/s the rest of the house?
    remember, with electronics; when its brown,its cooking and when its black, its done!!!

  9. #9
    Shophound: Your question about the ductwork being concealed in a wall is right on. The air handler is in the attic of the 2 story house and the ducts go to the first floor. (I have a seperate system for the 2nd floor). The duct is concealed in a pillar that connects the first floor with the attic. The pillar is about 24" square. The footprint of the distribution box is about the same (24" square) but it's not directly below the pillar. It's offset so only the right half of the box is under the pillar. Considering the thickness of insulation around the 12" flex, 7" was the widest the port could be made and still fit inside the pillar.

    The box is worse than concealed behind sheetrock. It's under the plywood floor (which is concealed by sheetrock). Maybe I can get better access from the first floor cieling.

    I know next to nothing about construction, but as soon as I saw how the box was designed my internal shoddy-workmanship-o-meter went off. It's clear nobody competent would design something this way and it's so common for workers to cut corners in these cookie cutter developments. I knew it was screwy, but wasn't sure it would really degrade performance until reading the comments everyone has left.

    Also, I get your math reagrding cross section of the different duct shapes and air velocity through those shapes but I don't think I follow where you're taking it. The air would have to flow faster through the oval due to it's smaller cross section. I assume that's a bad thing. For example, to take an extreme case, if the duct had to narrow down to a 1" circle at the box opening you could calculate the speed air would have to flow to provide, say 800 CFM. But realistically there's some limit to how fast air will flow under given static pressure isn't there? So I'm jsut trying to understand if your calculation say the oval entry is a bad thing or no big deal. And if it's bad, how bad. In my simplistic view I assume since the oval port is ~75% the size of the round duct it will reduce airflow by 75%. Is that a reasonable way of looking at it or am I flying in the night?

  10. #10
    Originally posted by micdundee
    been a long day,so may have missed something. what are the temps in the rooms in question v/s the rest of the house?
    Not exactly sure. One problem is that the vents in the front (on the 14" duct) are still all plugged up from the first guys efforts to balance the system. So the temperature is now equalized better, but now airflow from all vents is pretty pathetic. I estimate the difference is less than 5 degrees between the two areas today. Before the plugs were installed in the front I estimate 8-10 degrees difference. To the point it was uncomfortably hot in the back room and verging on uncomfortably cold in the front.

    Would it help to have exact measurements?

  11. #11
    Originally posted by chaddawson


    In my simplistic view I assume since the oval port is ~75% the size of the round duct it will reduce airflow by 75%. Is that a reasonable way of looking at it or am I flying in the night?
    Sorry, meant to say :I assume since the oval port is ~25% smaller than round duct it will reduce airflow by 25%.

  12. #12
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Chad,

    Sorry I wasn't more clear where I was headed with the math. I *think* (it's late in the day so take that for what it's worth ) what I was trying to say is that, side by side, the 12" round duct at its given square footage (duct calcs use sq. ft. vs. sq. inch) is on par with the 7 x 15 ellipse for flowing 800 CFM. The difference is in feet per minute, being the ellipse will have a higher fpm than the round duct.

    Now I may be all wet. I'm still learning about duct calcs and the flow characteristics of ductwork. Air distribution is probably the weakest area of knowledge for most service technicians, installers, and contractors in the HVAC field...I'm no master of it for certain, but I'm working on improving. I thought if I aired it out here we all might learn something if I was off track.

    That being said, I do know that, along the length of a run of duct, starting from source of air to to end, static pressure decreases. Changes in duct size affect velocity. I'm gonna hit the books more on this as your scenario has given me a real world application to learn by.

    In the meantime I'm sorry you got stuck with the shoddy duct design that you have. I recently read that the original intent behind flex duct was to act as a short run takeoff from a main solid trunk line, in order to speed installation and reduce blower and airflow noise at the registers. Then someone got the bright idea of sticking a box on the end of an air handler and then poking holes in it to run flex duct in six million directions, often quite lengthy, from the plenum, friction loss be damned. More often than not the flex is just laid on the attic floor rather than properly suspended and strapped, it is bent around corners instead of using solid elbows, often kinking the duct in the process, and it is jammed into a supply register boot, also often without an elbow, and also often with a kink. It's madness!

    When I first started in this trade years ago, and before I knew squat one about airflow, I worked for a company that ran flex like this. Air handler in attic, short supply box plenum (at least they used a transition between air handler to coil), and then that plenum poked full of start collars and ductwork snaked out in all directions, none of it strapped. At that time I thought it was the latest and greatest way to do things, compared to the old solid duct runs I was used to seeing as a kid. Knowing what I know now, which is still an ongoing process, those systems we put in years ago...it's a wonder they worked at all. At least the contractor was using the old GE condensers with the Climatuff compressors...hard to kill those.

    Even so I don't think we'd do the box thing. Worst thing we did for distribution was take a larger duct and split it down into smaller sections with a series of wyes. In your case they probably looked at the way the house was built and thought, "Okay, guess we'll have to do it like this so it'll fit". That thought, IMO, probably dominated all else and that's what you got stuck with. So I'd say the house design is partially to blame, as I've walked these things while under construction and I just shake my head when I see the ductwork, and the crazy construction techniques in general.

    Allright I've rambled on enough...beginning to sound like an old fart...and don't know if I really helped you much. Your scenario has given me some stuff to think about and work on, so all's not lost.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Your comments have already been of tremendous help. My whol house is done in nothing but flex duct. Last house was too. Assume this is the common parctice here in Phoenix. The more I hear about it though the more I find out it's a shortcut and a terrible idea in the long run. My flex is suspended from attic rafters, but is sometimes bent at sharp angles and sags between some rafters. I understand I need to support better to avoid the friction loss. But that's niether here no there.


    Anyway, I thought I'd post a drawing of the box in question. It's not exactly like I described (I was going by memory) but I think it's pretty screwy.

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