HVAC Design -- returns necessary on each floor?

Discussion in 'HVAC Heating & Cooling' started by homey6660, Mar 18, 2008.

  1. homey6660

    homey6660 New Member

    Messages:
    18
    Location:
    Virginia
    I have a two-storey, 87 year-old rowhouse that I'm having central heat/air installed in. The house is 1500 sq ft. The HVAC contractor I'm thinking of going with has worked with my on a potential design for the system that would have the return venting on the first floor only (the system itself is in the basement, vice the roof). Have the supply on both floors, but the return only on one is mostly a factor of the limited options for unobtrusively adding ductwork (I looked into Unico, but not going that direction).

    Anyhow, I'm wondering what all of the downsides of this might be. I understand that with a return on one level only, it might make the temperatures on each level uneven. Is that right? Are there other downsides?

    Thanks, in advance, for your help.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,319
    Location:
    New England
    If a door is closed to a room, and there's little space for air to come out underneath (say a plush carpet is there), it's sort of like blowing up a balloon - you won't get much air going into the room. It is much easier to get the air to flow where you want it if there is a return. Few houses are built that way, especially old ones. You can put a grill in the wall into the hallway to allow air to be exhausted from the room, but that also allows more sound in and out.
  3. jdoll42

    jdoll42 Computer Systems Engineer

    Messages:
    68
    Location:
    In Illinois near St. Louis, MO
    I'm not a pro, but common sense tells me if you are putting more air into a room than you are sucking out, that room will become positively pressured. That can cause all sorts of fun things, such as doors to slamming unexpectedly. Then you just lost all airflow through room. Like jadnashua said, you could put grills in the doors or walls, but then you have your sound issues. Might as well just take the doors off the rooms without returns.

    Then again, none of my bathrooms have returns in them (but they have exhaust fans). Hmm....

    Maybe the pro's here could confirm my suspicions.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,319
    Location:
    New England
    For optimum control over any room that can and will have its door shut, closing it off from the rest of the house, should have a return. Now, many times the door may have an inch of clearance under it until the homeowner adds a nice plush carpet, where it may be essentially closed. When the air finds resistance entering the room, it goes to other, easier branches which messes up the balance of the system.
  5. PEW

    PEW DIY Senior Member

    Messages:
    487
    You can get by without a return in every room, though it would be best to have a return in each room. But, you do want a return on every floor. Summer return for ac should be close to the ceiling and the winter return close to the floor.
  6. Nubbin

    Nubbin New Member

    Messages:
    10
    many happy returns

    A single return in a two story house can work well as long as the doors are kept open between floors. To prevent stratification of air in the rooms, do not buy the cheap stamped metal supply grills. Because you have only one return you will not be able to use the return duct to pull as the season requires so the supply will need adjustable vanes to throw the air correctly and mix the air without a draft.
    With only one return duct I hope you will have a path that is about one square foot per ton of cooling at the most narrow part of the duct work. Also, if in the 4-5 ton range, try to get two 20x25 filters set as parralle. If the fan coil/furnace is a max .50"w.c. static pressure model, the restriction prevents enough air to be sucked in to deliver your cooling needs.
    Good luck, I have had a few of those "you can't get there from here" older houses!
  7. homey6660

    homey6660 New Member

    Messages:
    18
    Location:
    Virginia
    return

    I just had the AC installed and I went with two returns--one on each level of my 1600 sq ft house. RIght now, the ducts are still exposed b/c the contractor is working with me to fine tune the install based on how it's been operating. The first floor gets cool very quickly and easily. The return is pulling a lot of air, based on my hold-a-papertowel-in-the-return-and-watch-it-flutter test. The return upstairs pulls less so and it is much warmer upstairs (split system wasn't doable, so I have to figure out how to make do with what I have). Now, as far as I know, there are three reasons for it being warmer upstairs...

    1. it's further from the source, so it's inevitably going to be pushing less cool air to the far reaches of the duct line.

    2. b/c the ducts are exposed and the registers are not on, a lot of the air is blowing out the lower level--more than it would if there were registers.

    3. size of the returns (and this is my real question). I had a 2nd contractor come in to take a look and he suggested that the ducting where it connects with the blower should be narrower for the 1st floor return than it is for the 2nd floor return (right now, they're both 10 inches). His rationale is that if the return is going to pull the same amount of air, if you restrict the one on the 1st floor, it will allow more air to be pulled from the 2nd floor. Do you guys agree?
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,319
    Location:
    New England
    The better way to do this is to size the returns and supplies. It is good to insulate the ductwork as well so you are putting equivalent temperature air into the rooms. I tend to leave doors open. In the summer, I restrict the output of the air on the lower rooms some, and make sure the upstairs registers are fully opened. I do the opposite in the winter. Then, I'm putting more air volume upstairs, and letting it fall down. Pushing cold air up is tougher.

    If you have a damper on the supply lines, you can force more air where you want it. Restricting the returns probably isn't a great idea. You could put one on the lower return and see if you can pull more from upstairs, but there are limits on the backpressure and total flow.

    I have a return way at the top of the house in the stairwell. While I have one downstairs near the floor, it is mostly closed. This pulls hot air from the ceiling out of that well and runs it through the a/c or heating and resdistributes it, either being cooled in the summer, or warmed in the winter. I don't have a cold air at the floor problem since my primary heat downstairs is radiant floors- the forced air is a backup and for a/c.
  9. homey6660

    homey6660 New Member

    Messages:
    18
    Location:
    Virginia
    solution?

    I think what I will do is keep the returns the same size. Instead, I will manipulate the vents (e.g. -- close at least some of the vents on the colder, lower level), while using a filter or some other material to limit the intake on the lower return...which would allow the upstairs return to pull more. How's that sound?
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