How much duct work fo I need

Discussion in 'HVAC Heating & Cooling' started by droemer, Sep 14, 2008.

  1. droemer

    droemer New Member

    Messages:
    10
    Location:
    Bradenton, Fl
    After having an authorized Lexxon dealer install a new airhandler, we are now being told we need more return air duct.

    Is there a calculation that needs to be performed based on house sq. ft, etc?

    Does anyone know?

    Thanks,
    Doug:confused:
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The A/C contractor has the manuals ( and today, software) to do load calcations, figure CFM, etc. The installation manual for your equipment will also have information. I don't know of any quick "plug-and-play" source that will dial in the information for you.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,360
    Location:
    New England
    To keep things in balance, and get the heating/cooling required, there must be a proper airflow over the heating/cooling coils. If you have too much restriction in either the supply or return ducts, it will unbalance things, create more wear and require more power to run the fans, and decrease performance, costing you money and longevity. It could also prevent you from achieving the desired performance. If you have confidence in the pro you hired, listen to him! If not, get a second opinion. It's really hard to tell without being there and having all of the variables (of which there are many) on hand to perform the calculation.
  4. Mikebarone

    Mikebarone DIY Senior Member

    Messages:
    206
    Location:
    Phoenix, AZ
    Just some thoughts...

    The problem of not having enough return air supply is a very common. Last night I just completed the addition of a new 14” air return, to an existing unit. The suction pressure went down from a 0.8 inch water column, to around .025 in water column. (“Water column” is a way to test the suction pressure in the system). In increasing the air return it should also increase the air coming out of your cold air delivery vents. The one vent I took a sample on, (before and after) went from 500 feet per minute, to 600 feet per minute, (air delivery speed).
    Some fan motors in the air handlers have an adjustment that you can turn up or down the speed of the fan. I don’t know if this will help in your situation or not. As it’s been said before, there are sooooo many factors to consider. I’ve found, (because I’m not an a/c pro….just a “been around handyman”) that the water column test is a good visual test. One thing that I had to take into consideration, (before I added this 14” air return) is if there was enough cold air volume of duct work. Just adding more return volume, could shift the restriction to the delivery area.
    You mentioned in your post that you got a new air handler. Was this the same size, as your old unit?


    Mike
    Last edited: Sep 15, 2008
  5. droemer

    droemer New Member

    Messages:
    10
    Location:
    Bradenton, Fl
    How much duct work do I need

    Mike,

    Thank you for your reply. The Lennox dealer installed the same size airhandler, but it has a variable speed motor. I was told that this also make a difference with regards to air flow.

    We'll see what happens when they finish today with the duct work. I was told by the General Manager that they use a "ductalator"(spelling) to determine the amount of duct work. They told me that we have a 5 ton unit which requires 2000 cfm's.

    Thanks,
    Doug
  6. Mikebarone

    Mikebarone DIY Senior Member

    Messages:
    206
    Location:
    Phoenix, AZ
    Hey Doug,

    The ductalator (in my opinion) is a fine tool in starting the calculations for duct work. Along with the ductalator, I thing some good old faction common sense, and experience is what makes a well balanced system. Like I said before, I’ve seen a lot of systems that have been installed by well known HVAC contractors here in Phoenix, and my faith of knowing what they really know what they are doing is diminished. The water column test, after and before work is done, (I think) tells almost the entire story of weather everything is working in harmony or not. I think it should be mandatory, for all systems to me tested after installation, or after improvements are made to an existing system. There is not a factory hook up, on the air handlers to perform this test so, I had to drill my own hole to do the test, (and that I also think is wrong).
    Another test I think that should be performed is a leak test, to all new installations, and all improved systems. It would probably be mind boggling on how much power savings this would be state wide if these two tests were mandatory. Not only that, but if these two tests were mandatory, it would force the contractors to do a lot better job, (again in my opinion) then what they are doing now. I did I leak test in my friends house using smoke bombs, and it looked like his attic was on fire, with all of the duct work leaks that were up there.

    Here is a chart, (from Carrier) that I found that is based on performance of using that Water Column tester. Notice that anything more then a 0.5 is off of the chart. On the unit that I just added that 14" air return to, her's was up to 0.80 and now is down around a 0.25, (and still has room for improvement at that 0.25).

    Blower Performance Chart
    Air Handler CFM @ ESP. – in. W. C.
    Model Blower Speed

    in W.C. .10 .20 .30 .40 .50

    18 Low (Red) 754 716 674 626 569
    High* (Black) 1020 958 886 800 689

    24 Low (Red) 998 938 868 786 681
    High* (Black) 1042 978 905 817 704

    30 Low (Red) 1146 1091 1029 959 877
    High* (Black) 1349 1289 1224 1150 1066

    36 Low (Red) 1407 1341 1268 1186 1090
    High* (Black) 1561 1481 1392 1290 1169

    42 Low (Red) 1785 1705 1616 1516 1402
    High* (Black) 1938 1846 1744 1629 1495

    48 Low (Red) 1870 1789 1699 1599 1484
    High* (Black) 1943 1858 1765 1660 1540

    60 Low (Red) 2077 1986 1886 1774 1646
    High* (Black) 2161 2066 1962 1845 1710


    Good luck, for sure,

    Mike
    Last edited: Sep 15, 2008
  7. Mikebarone

    Mikebarone DIY Senior Member

    Messages:
    206
    Location:
    Phoenix, AZ

    You were told, (after they installed the new air handler) that now they need to modify your existing duct work to accommodate the new unit. Before they started the job, (and gave you a bid for the work) they also assumed that your existing duct work was sufficient for the new unit. There again, if a water column/ CFM test was performed, prior to the initial bid, they could have told you that your existing duct work was insufficient for the new unit, and there maybe an extra cost to improve your existing duct work.
    I know hindsight is twenty twenty, but I am just a little frustrated, (based on what I’ve had to repair so far) of the HVAC industry standards and workmanship in general.

    Mike
    Last edited: Sep 15, 2008
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