No return on 3rd floor... problems with humidity?

Discussion in 'HVAC Heating & Cooling' started by thecheester, Jun 17, 2009.

  1. thecheester

    thecheester New Member

    Messages:
    4
    Hello everybody,

    Been reading these forums for a while but inactive up until now!

    In the process of buying a condo now and there is no return air duct for the central AC/heat system on the 3rd floor of a roughly 1,000 sq. ft apartment.

    I've read the system will be inefficient but also I've heard that something about the pressure being created will cause extra humidity, potentially causing trouble with mold creation.

    Has anybody had any experience with this?

    Another person said I shouldn't worry, given that the return is on the common area stairways on the first floor so the air will eventually escape through the door and all will be good.

    Any thoughts?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    The HVAC system is fairly good about trying to blow air into a room IF the ducts are sized properly, AND there's a way for that air to escape...it takes the easiest path out of the ducts, which may be a room below. Depending on how much room there is under the door, you migth get some flow under there, but it is unlikely the proper size. There are several alternatives...add an air duct either in the wall, or the door. If the doors are never closed, it's not a problem.

    The other issue is, especially when running the A/C, that heat rises. If you don't get any flow, the room will be uncomfortably hot in the summer. I've found my system is more efficient if I have a return duct drawing air from the high point in the house. Rather than drawing cold air out of the bottom, it draws off the very hot air at the top, whether from the hot air during heating season accumulating at the top of the house or from heat rising during the cooling season...drawing that hot air off from the top (and running the fan most of the time; i.e., not on auto), evens out the air temperature. As to humidity, it is normally the same around the house except when using something like the shower or dryer. But, different temperatures will reflect different relative humidity levels, and a cool room might feel clammy verses a warmer room with the same actual humidity in the air (note the difference between relative and actual humidity in the air). Warmer air can hold more water, so the same amount will represent a lower relative humidity level.
  3. thecheester

    thecheester New Member

    Messages:
    4
    Interesting... thank you!

    I've gotten mixed reviews about the possibility of humidity being developed in the apartment..

    There is no way that I can add a return duct (at least that I'm aware of) without going through the condo association and making changes to the outside of the building ,which will be a pain to get approved.

    Anybody else has had different experiences?
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,503
    Location:
    Cave Creek, Arizona
    return

    IT is a matter of physics. Warm air rises, period. It WILL accumlate on the upper level Summer, Winter, Spring, or Fall, so that upper area will ALWAYS be hotter than the lower ones, UNLESS you find some way to draw it off and mix it with the rest of the system. If the air cannot circulate, other than by natural convection as the lower strata cools and flows downwards naturally, any moisture in it will remain there, and accumulate since warm air can hold more moisture than cool air..
  5. thecheester

    thecheester New Member

    Messages:
    4
    Thank you guys... it sounds like it should be fine
  6. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE
    Where is the furnace/AC coil/fan unit? If it's in the basement, it may not be able to push cool air up to the 3rd floor.
  7. thecheester

    thecheester New Member

    Messages:
    4
    Yeah, it's in the basement but it's dedicated to the unit (i.e., it's not split between the 3 floors)

    Do you mean it wont be able to push because the air has nowhere to escape?
  8. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE
    It's partially that, but even with a return vent, it would be difficult for most residential AC units to push a column of dense, cool air up 2 or 3 floors.
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