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air flow problems. design issue??

a couple years ago before i got involved in ac work and design, my boss sized my in-laws home for air conditioning (my boss is a plumber, not an ac guy). since its been installed the upstairs is unable to cool down before the system shuts down. I went there today to try and shut down some of the downstairs registers and force more air upstairs. Now that I'm somewhat experienced in ac work I have noticed several problems. The system is definately over sized (4 ton system). the duct work wasn't designed for ac. they have furnace heat and their never was ac in the house originally. vents and duct work feeding upstairs are undersized and on the bottom of each room rather than the top. Even after damper adjustments, I dont think I'm getting enough air upstairs to cool the rooms down. my wife wants me to figure something out and give my in-laws a price. the only thing i can think of is that the downstairs system should have been much smaller (probably around 2 to 2 and a half tons) and put a seperate mini-split system in upstairs. I told my wife that the duct work wasnt designed for ac and I'm not going to be able to get the upstairs cool enough. Is their any other ways to solve this problem??


  • Techman
    Techman Member Posts: 2,144
    edited June 2011
    Out -laws

    If you can't straighten this out you might become one!! LOL .How big is the 1st and 2nd floors ?Lot of sun? With an existing warm air duct system several common problems are ,under sized mains-branches-boots-registers.The returns are allway's way undersized for AC. The entire duct system is not insulated,including the risers in the wall's going to the 2nd floor .Mildew and stuff! Can you get one large trunk ,from basement to the attic , for the entire 2nd  floor load and then branch off to each room from up there ? A little bit of relief might be had by putting the t'stat fan switch in the on position.When the ac shuts down downstairs, the cold air  there is still being circulated up there where the in-laws are planning their son-in-laws future.
  • icesailor
    icesailor Member Posts: 7,265
    Air Flow Problems:

    In no way am I an AC expert. I'm a Wet Head and I don't do WA. But these are my observations on what you may be dealing with.

    In MY opinion, WA and AC are diametrically opposed to each other. Hot air rises, cold air falls. A two story house with one zone of heat and the thermostat on the first floor will overheat the second floor up the stairs. An AC house with the same situation, one zone with the thermostat on the first floor will be hot on the second floor and cold on the first floor. The cold air will fall down the stairs and cool the first while the heat from the ceilings on the first will rise to the second floor up the stairway. Closing the second floor doors may help but the return air may be locked in the rooms.

    20 years ago, I plumbed a house for a well known business person. There was an HVAC unit for the second floor and another on the first floor. There was a large stair opening to the second floor. The second floor had cathedral ceilings.  Half the returns on the second floor were on the floor and the other half were about 8'. During the summer and AC operation, the first floor never came on and it was usually 5 degrees colder than the thermostat setting but the second floor never shut off. I have a long list of experience with this problem.

    Many jobs I see have one return for the whole system and multiple zones and supplies to rooms on multi floors. The rooms have a supply but they depend on the doors being open for the air to get back to the return. Mostly, the bottom of the doors are so close to the floor that the door will be sucked shut when the fan comes on.

    Some form of balancing and forcing air to go where it doesn't want to go may help. But it is still a WA system that is trying to do a bad job of AC.

    RDSTEAM Member Posts: 134
    thanks guys.

    for the assistance with this disaster. to answer some of the questions:

    no attic. one stairwell leads to the upstairs. at the top you have a bathroom right in front with a bedroom on either side. entire ceiling is exposed to the sun. no return. no real possible way to get a trunk line up to the second floor. I told my wife that they have two options. mini-split (which she doesnt want because it "ugly") or start tearing down walls so I can install larger duct work up to the second floor and larger registers.
  • Jack
    Jack Member Posts: 1,046
    Mini-split..."it's ugly"...

    I always tell people that the longer you live with them the better looking they become. I can use my wife as an example. She didn't want a ms, but I put one in and within 24 hrs I was a hero. After 34 yrs that happens less and less;) I have heard the same from many of my customers (btw, I represent a ms manuf, so bias noted)

    The ms gave us comfort, efficiency, zone control, no duct losses, proper sizing for the space, quiet operation and as they are pretty much all heat pumps, tremendous heating capability.

    On the new, to us, house with a four yr old ducted system with massive problems. this time my wife told me to put in a ms. I tore out the entire system and have replaced them with ms. I have an upstairs system and a downstairs system giving excellent zone control and comfort.

    Your current system is a basket case as you acknowledge. The unit is oversized and the ductwork is undersized and not properly placed. You can spend a lot of time/money trying to resurrect this thing and it will still not be right, it will not cool well, it will be expensive to operate and it will not last. Also, it being a family thing, you need to get out from under it...right? Put in a ms. You will be done, it will be a big improvement and you will be a hero!
  • Bart Vaio
    Bart Vaio Member Posts: 56

    AirFlow is Airflow...  Have you done a heatloss calc? Manual J....  Cooling is airflow intensive you will likely need between 1 and 3 cfm per sq ft. depending on construction for the top floor.  4 inch duct does 40 cfm, 6 inch duct does 100 cfm, 8 inch does 200 cfm, those numbers are best case without registers.  Add them all up, your probably short on CFM's...
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