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Rectangular ductwork hung vertically

Discussion in 'Residential' started by kena, Sep 13, 2017.

  1. kena

    kena New Member

    Messages:
    762
    I know how to size ducting and have done so successfully. The biggest hurdle I have is our plumbing manager went to one of our suppliers and had them design the system. I'm 100% responsible for the job, but he ordered everything and set the budget. I like the company I'm with... but that is just f'd up! I've been doing everything I can to pull the HVAC part of the job from his hands... but, he just isn't letting go. He has a boiler and all the hydro air runs to do, and finish the rough plumbing. To add a twist, they decided to hang the duct vertical, which honestly I don't care. I just want to make sure the supply/return are sized properly so we actually have a pressurized system. I just want to walk away from the job at this point since I have allot of jobs lined up, and don't need the headache. I know one thing... I'll fight till my last breath before taking responsibility for anything on this job.
    '
     
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  2. otto

    otto Bakery Chef

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    19,840
    Location:
    in a van down by the river
    But you still need a new tire
     
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  3. MOFO the psychic gorilla

    MOFO the psychic gorilla sitting on two achers

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    1,851
    So is 4" enough?
    I put 4" to my smallest bathrooms and it was enough.
    What was their reason for using 4"?
    Price? It fit in a wall? Is there time to fix this?
     
  4. Catpower

    Catpower The Crowd Pleaser

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    22,249
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    in the Cabana
    When they first came out, I was at an engineers house and he was telling me how great they were even though his coil started freezing up after he installed them

    I installed a new one and got the magnihelic out and it was right under 1" drop just across the fliter
     
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  5. jamesyarbrough

    jamesyarbrough Amateur Gynecologist

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  6. Daryl_Dixon

    Daryl_Dixon New Member

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    3,780
    The local utility was our "competitor" for a while when I was doing residential. They were going into peoples' homes and running a campaign (with printed literature) advising customers to close all of their basement (and first floor, if a two-story) registers in the summertime. No shit. I wish I was kidding. Can you imagine how people responded when I said that was wrong? BUT IT'S THE UTILITY!!!
     
  7. Catpower

    Catpower The Crowd Pleaser

    Messages:
    22,249
    Location:
    in the Cabana

    Their are places it is against the code to have a return in a basement if the heating equipment is a fossil fuel, their thoughts are somebody might close all the supplies in the basement and cause a negative draft and pull the flue gasses into the basement
     
  8. doc havoc

    doc havoc A cunning linguist

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    Same issue when fire department tells the HO the furnace is the likely culprit of the CO alarm.
    HO "But they're the fire department."
    Yes, but I'm a heating guy and you have a heat pump with electric heat.
     
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  9. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    Wasn't my desire or intent to insult your intelligence...;) And I'm still not sure I'm visualizing what the layout is...single supply plenum with "spider" branch ducts?

    But it shouldn't much matter if the trunk is horizontal or vertical, the end result still goes back to available static and adequate duct sizing to maintain it. If the supplier undersized it (and 4" take-offs sound undersized to me), be prepared to give it back to the plumbing manager...:D
     
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  10. kena

    kena New Member

    Messages:
    762
    The plumbing manager finally handed over the job to me... I am less than happy about it. I have a 4 ton air handler with coil. I need at least 1400 cfm across the coil. Even thought he supposedly sized everything. The supply duct work is like this. He used 8" x 24" rectangular smooth duct for about the first 12', than transitioned to 8" x 16" for the last 12'. I have two 4 inch take offs, and 4 6" takeoffs coming off this ductwork. The plenum isn't finished yet, an I don't have a clue what he ordered. But, that being said he started telling me he ordered the return ductwork, and he said he designed it so we'd match the cfm needed for the takeoffs on the supply, which he said was around 700 cfm. My brain is about to explode. I told him we needed 1400 cfm for the ac coil, so the supply ductwork should have been designed so we had enough size to accommodate that, and a return that would also accommodate that. I asked him if he even did any duct calculations and he said he got the plans from the supplier. I haven't measured any of the rooms yet, but the 4" go to 2 very small areas. The 4 - 6" go to the main area which has a cathedral ceiling. A very large one. I was always told you could quickly guestimate that if you have say a 4 ton system, you can just multiply 48 x 6 (rule of thumb), and that gives you pretty closely what you would need for the supply duct. In this case 288 sq. in. He has 192 sq. in. for the first 12 feet, then 128 sq. in. for the last 12 feet. I can only imagine the static pressure this will give me at this point. I have to calculate it out. There's lots that goes into sizing duct... but does anyone have an easy rule of thumb for me to do a quick guestimate of the size of the supply and return? Currently 2 - 4" takeoffs, 4 - 6" takeoffs. All smooth metal ductwork. I know I still need to measure the sq. ft.
     
  11. Rather B Riding

    Rather B Riding Behind bars

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    568
    4 ton general rule is 1600 CFM but everything changes so much these days if the equipment install book says 1400 roll with it. Doing the math 4-6" runs and 2-4" runs it's going to whistle like a bird all the way till it freezes up. If you you make the 4" runs 6" and make the 6" runs 8" you are closer but you still need a couple more runs.
    As far as return air. The general rule is 8x24 RA drop with 16x25 filter for 3 and under 10x24 with 20x25 filter for 4&5 ton systems.
    I hope they let you take full control over what gets done on the job. Sounds like your hardest task is going to be dealing with the plumbing manager. Good luck.
     
  12. Septua

    Septua New Member

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    55
    Location:
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    If I was going to go with thumb rules, it would be selecting a friction rate of 0.1.

    The initial 8 X 24 ain't too bad for the trunk size, but I can't imagine how anyone could come up with 4", or for that matter, 6" for the branch runs that are going to be attempting to carry ±250 CFM. Of course friction rates are per 100'. If all the runouts are short, it could work...

    I see comments all the time relating specific duct sizes to some volume of airflow. I don't know how they come up with it, since length has to be included in the formula.
     
  13. Septua

    Septua New Member

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    55
    Location:
    Southeast
    That's what you see a lot, but it ain't enough...:D Recommended max filter velocities are 300 ft/min, which equates to about 200 sq in per ton (400 cfm). The bug catcher variety filter pressure drops aren't bad at 300 or 500 f/m, but most HO's are buying the pleated filters which needless to say, are substantially more restrictive.
     
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  14. kena

    kena New Member

    Messages:
    762
    All of the runs are about 10' from the duct. Not to long. But the small take offs only allow so much air to flow. The 24" x 8" duct for only 12' and then dumping down to 16" x 8". Yes, I need from 1,400 - 1,600 cfm for the coil. Based on the supply duct sizing, don't you think my static pressure will be to high and my cfm to low? I do want .1 static pressure on the supply. That is what I would design it for if I had designed it.

    And for the return, based on .05 static pressure, I should double the cfm, right?
     
  15. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    ..which will also "build up" the trunk static...

    Nothing wrong with downsizing the trunk, if some of the branch ducts were connected to the 8 X 24 section.

    Again with no desire to insult your intelligence or knowledge, your choice of terminology confuses me. When you say "based on .05 static pressure", I assume you're referring to .05 friction loss? The friction loss value chosen doesn't indicate (and my terminology may be lacking here) actual static pressure that will develop in the system.

    If a designer is lucky enough to accurately calculate the available static pressure, the total duct system "equivalent length" and pick the correct friction loss value, the blower wheel will end up "seeing" a total external static pressure equal to whatever blower speed/static pressure performance curve was chosen.

    If the total external static design value chosen was around 0.5, then the measured supply and return static pressures taken close to the air handler will total up to ±0.5.

    If you size the return for .05 friction rate, the round pipe diameter would be 18" give or take, for 1500 cfm. And if it's short, about the only static generated will probably be due to the filter. If the filter is pleated, and undersized with a 500-6oo face velocity, the static will probably be in the 0.2 inWC range.

    Getting back to the earlier question...yes, I'd say the system has a high probability of exceeding the blower's static limits and not delivering 1400 cfm. :D If you could manage to salvage the train wreck with grossly oversized return duct and filter area, then return losses would be minimal leaving more available fan static for the supply side ducting.

    As an aside, I've got a couple of pic charts showing some filter loss test results I did some years ago...I'll get them posted up here.
     
  16. kena

    kena New Member

    Messages:
    762
    I feel so insulted :eek:
    I don't always say what I mean or explain things well. I designed the duct system and got the air handlers working. My static pressures are well within the reasonable range. If I had made no changes to the original design I'm not sure I would have had a working air system. The coil most certainly would have frozen up. The return air would have been killed by the static pressure in the supply.
     
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  17. Septua

    Septua New Member

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    55
    Location:
    Southeast
    What did you change?
     
  18. kena

    kena New Member

    Messages:
    762
    I opened up the 4" to 6" and the 6" to 8" on the supply trunk, and added a couple of 6" ducts to another room.
    The return was sized differently so I had plenty of air coming back.
     
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  19. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    :cool:
     
  20. Zman

    Zman Hanging by a Thread

    Messages:
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    Location:
    The "I know a Guy" State
    Refreshing to see some People care. Some so called Techs around here could care less,and would leave it for the next Guy!