Results 1 to 13 of 13

Thread: How To Do Duct Air Flow Measurements??

  1. #1
    Join Date
    Apr 2022
    Location
    Springfield, MO
    Posts
    148
    Post Likes

    How To Do Duct Air Flow Measurements??

    I'm fighting with a 10 ton heat pump in another thread and after some repairs I'm finding a 27 degree Delta T across the coil, too low air flow. In checking out some of the supply ducts I found one with no discernable air flow and two with minimal air flow. So now I'm looking for how to measure each duct's air flow to resolve this issue before I can properly diagnosis the rest of the system. I'm thinking a hot wire anemometer. I'm not really finding much in the way of measuring individual duct air flow. I could use any experienced help with this. I'm dealing with primarily flex duct in the suspended ceiling, ouch! Can't spend much on this item. Found a TPI unit for about $160 and a Testo405i for about $130.

  2. #2
    Join Date
    Jan 2012
    Location
    Missouri, USA
    Posts
    3,174
    Post Likes
    I just got a testo 420 flow hood, I like it. Every hvac company should have one.

  3. Likes kdean1 liked this post.
  4. #3
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    9,881
    Post Likes
    I own the same one. I like it.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  5. #4
    Join Date
    Apr 2022
    Location
    Springfield, MO
    Posts
    148
    Post Likes
    Thread Starter
    Only about 10 times what I can afford. Not that I can even really afford that either.

  6. #5
    Join Date
    Oct 2016
    Posts
    2,903
    Post Likes
    With that much temp drop across the coil you know it’s not moving enough air. Look at the duct layout. Is the transition off the unit good and smooth? Size okay? Enough grilles for 4000 cfm? Any bad fittings?

    That’s what I would be looking for instead of measuring flow. You already know it’s low.

  7. #6
    Join Date
    Apr 2020
    Location
    va
    Posts
    5,451
    Post Likes
    Use a utube manometer and unit static air flow tables. Gives you tje total air flow, at hardly any cost.
    I’ve even used a piece of plastic tubing & ruler. Good manometers are under $100.

  8. Likes slctech liked this post.
  9. #7
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,563
    Post Likes
    I have the Alnor LoFlo for supply grille measurements, and the Testo 420 for return air. The return air is very accurate with the 420, but not so much with the supply air. I found the LoFlo to be pretty decent accuracy on supply vents, and you can input several preset k-factors for different types of grilles for improved accuracy.

    Here's one on eBay, used. I bought mine on eBay for the same price, but it was pretty much brand new.
    "The only true wisdom is in knowing you know nothing" Socrates

  10. Likes kdean1 liked this post.
  11. #8
    Join Date
    Jan 2012
    Location
    Broomall, PA
    Posts
    4,121
    Post Likes
    Maybe you can borrow/rent one.
    You mentioned flex duct. Maybe a strap came lose, someone pinched one, or a critter chewed open a hole.
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  12. Likes R600a liked this post.
  13. #9
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    12,021
    Post Likes
    The lo tech way would be to shutdown a couple of good runs and see if the poor runs improve.
    If it does there is likely a duct restriction or the unit can't overcome the static.
    Then test to see if it's supply or return. A return can usually be tested by removing the blower door.
    You will find out w/o spending money. At least not now.

    Static pressure can be tested with an inexpensive magnahelic. I think <$75. is another way to test the system.
    With a mag you can evaluate both ducts.
    No guaranty of accurate numbers but you might not care.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  14. #10
    Join Date
    Apr 2022
    Location
    Springfield, MO
    Posts
    148
    Post Likes
    Thread Starter
    There is only a box below the air handler with two 24x24 filter grills. No problem with return air. I think there are only a few 8" or 10" trunks off the plenum above the air handler. I suspect I need to add some trunks to the system. As for straps coming loose, the ducts are laying on the suspended ceiling tiles. A FLIR would be nice to look at the ductwork to quickly see if there are any leaks. Wishful thinking right now. Maybe next year.

  15. #11
    Join Date
    Oct 2016
    Posts
    2,903
    Post Likes
    I don’t know….that might be a little tight for the return. One more filter grille would be better.

    You need enough run outs for 4000 cfm. Figure an 8” is good for 200 and a 10” for 350. So you need 15 or 20ish.

  16. #12
    Join Date
    Apr 2022
    Location
    Springfield, MO
    Posts
    148
    Post Likes
    Thread Starter
    There is no room for any more returns. However, there is a 10" flex over to the side. It might be a 12. No idea where that leads to. I might be able to add a small return into the room on the side of the return plenum.

    I seriously doubt the supply system was properly set up. There are possibly 20 8" runs off the plenum, but only a max of 4 directly attached to the plenum. Definitely needs more. First step is finding out why some aren't blowing at all before adding to the confusion. One problem at a time.

  17. #13
    Join Date
    May 2021
    Location
    Virginia
    Posts
    10
    Post Likes
    Is it possible the flex is all crunched up in that tight ceiling? I see it all the time...they'll install it between the beams and smoosh it to run it where it needs to go. Furthermore, dampers? The small residential types can be hard to see because the idiot insulator put a conduit strap over it and covered it with flex...happens all the TIME

  18. Likes Scoobie liked this post.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •