Upstairs too hot, air conditioner runs in winter
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  1. #1

    Upstairs too hot, air conditioner runs in winter

    I have a house in CA. Most of the house is downstairs with a master bed/bath upstairs. For various reasons, it is not possible to run ducting between levels. So the upstairs is on a completely separate system.

    The problem is the upstairs is always too hot. Even in the dead of winter, the outside high was probably 50 F today, the upstairs AC turns on (thermo set at 77)

    Any time anyone heats up the downstair (shower, cooking, furnace) all the heat immediately goes upstairs. Unless someone babysits opening and closing a window upstairs to regulate the temp, the upstairs AC will run. End result is the upstairs AC is costing a lot in electricity.

    I hired a HVAC contractor to completely redo the downstairs system (which needed to be redone anyway, and was not done by a HVAC contractor initially). Presumably did a load analysis considering the upstairs. Doesn't think there's any problem since both systems are functioning (functioning meaning if it's >77 upstairs the AC will come on and cool it to 77)

    Another person (same company) said the thermo was set too low. I don't think 77 is unreasonable. Any higher I'm afraid will get humid and musty.

    I could install a door in the downstairs room the stairs are in. But the registers in that downstairs room would still blow upstairs, there doesn't seem to be a good way to completely isolate the two levels.

    I am perfectly willing to hire another HVAC contractor. But I already went through a round of estimates for the new downstairs system and no one really said anything different (just that the downstairs needed to be replaced). I don't want to drag them all back unless I know how to ask what I want.

    Thanks so much for your help!

  2. #2
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    If the downstairs furnace is over sized it is creating some of the problem.

    A high discharge air temp causes more air to rise to the second floor. A smaller correctly sized furnace with a lower discharge air temp will help to minimize this.

    The room that has the stairs to the second floor may also have more supply going to it, then it needs. reducing it to what it needs will also help.
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  3. #3
    Join Date
    Mar 2006
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    tex
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    You will have to look a Laws such as Charles and Boyles Laws Something that Architects do not know about , we keep it a secret from them , so they keep design stupid crappy floor plans that the home owner has to live in and pay 2 or 3 times as much in utility cost.
    Heat rises and cold falls that's it, that's the secret and don't tell anybody. You will have to have better control of the down stairs unit and maybe undersize it or get a variable capacity furnace

  4. #4
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
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    Heated air rises and cooled air falls. When you have a 2 story house where both floors are open to one another you're going to experience this.

    Your downstairs system is reacting to not only the heat loss of that floor it is also reacting to the heat loss of the second floor (cool air flowing down). The downstairs thermostat sees the total of the house and is turning on the heat. The heated air rises until it can't go any further up.

    As a simple thing to try is to turn your blower on your lower furnace to continuous on and see if that has any effect.

    You're probably either going to have to door off, or isolate the upstairs or install some form of de-stratification duct work to eliminate your problem.

    Had one 3 story house with a staircase from the basement floor to the top floor open. At the basement level the stairs stopped in a landing area that was walled off and a door isolated it from the basement. The 2 upper floors would be nice and comfortable and you could see your breath it was so cold at the bottom of the stairs in the basement.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  5. #5
    Join Date
    Dec 2010
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    Down by the river
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    sounds like you need to have an economiser for upstairs to bring in fresh air instead of running the a/c.

  6. #6
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190
    How about asking your contractor if they can see a place to install an exhaust/supply fan that a t Stat would turn on to take some of that warm air and take it back to the first floor. What ever area on the firs floor that is most difficult to heat would be most logical or any are where 'dumping' some extra air won't be a problem

    PS beenthere, Your buddy brian should read this one and see if low returns are the answer
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  7. #7
    Join Date
    Dec 2010
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    The Quad-Cities area (midwest).
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    Arzel Zoning make a nice economizer. Watch the video on arzelzoning.com

    It'll be under "Coolmizer". It might be just the solution for your situation.

  8. #8
    Join Date
    Jul 2005
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    5,296
    Quote Originally Posted by prct View Post
    I have a house in CA. Most of the house is downstairs with a master bed/bath upstairs. For various reasons, it is not possible to run ducting between levels. So the upstairs is on a completely separate system.

    The problem is the upstairs is always too hot. Even in the dead of winter, the outside high was probably 50 F today, the upstairs AC turns on (thermo set at 77)

    Any time anyone heats up the downstair (shower, cooking, furnace) all the heat immediately goes upstairs. Unless someone babysits opening and closing a window upstairs to regulate the temp, the upstairs AC will run. End result is the upstairs AC is costing a lot in electricity.

    I hired a HVAC contractor to completely redo the downstairs system (which needed to be redone anyway, and was not done by a HVAC contractor initially). Presumably did a load analysis considering the upstairs. Doesn't think there's any problem since both systems are functioning (functioning meaning if it's >77 upstairs the AC will come on and cool it to 77)

    Another person (same company) said the thermo was set too low. I don't think 77 is unreasonable. Any higher I'm afraid will get humid and musty.

    I could install a door in the downstairs room the stairs are in. But the registers in that downstairs room would still blow upstairs, there doesn't seem to be a good way to completely isolate the two levels.

    I am perfectly willing to hire another HVAC contractor. But I already went through a round of estimates for the new downstairs system and no one really said anything different (just that the downstairs needed to be replaced). I don't want to drag them all back unless I know how to ask what I want.

    Thanks so much for your help!
    The only way warm air can be migrating to the upstairs is for air to be leaving the upstairs space. Otherwise you'd keep piling air up, perpetually increasing its pressure, which is obviously not going to happen, right? Heat doesn't travel through the air very well, it is contained primarily as kinetic energy of the air molecules themselves, so where the air goes is where the heat goes. Heat moves in air primarily by "convection". There is also heat transfer by conduction and by radiation, but radiated heat energy transfer occurs in a line of sight fashion, just like light, and air is a very weak source. Since you don't have line of sight from the downstairs rooms to the upstairs rooms radiation isn't how the heat is getting up there. Conduction of heat in air, the other heat transfer method, occurs at a snails pace. Air is a good insulator.

    So the heat, by default, is getting upstairs primarily by riding along with the air stream that is migrating up the stairs.

    So where is all that air going?

    It's going to the outside, around window frames or through open windows, out through leaking ducts in the attic, out through the bathroom exhaust fan, out through cracks or gaps around the attic access hatch or door, through switch boxes and ceiling boxes, etc,. etc,.. Try sealing the windows and any other cracks or openings to the outside or attic, and see if that doesn't help alleviate the problem.

    Look up "stack effect".

    Since you mentioned trying to regulate this problem by cracking the upstairs windows, look especially at the last paragraph (in the pink box, bottom of page) of this article:

    http://www.greenbuildingadvisor.com/...s-act-chimneys

    HTH.
    Last edited by hvacrmedic; 01-01-2011 at 12:07 AM.

  9. #9
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    Jan 2004
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    Quote Originally Posted by genduct View Post

    PS beenthere, Your buddy brian should read this one and see if low returns are the answer
    Returns are always the answer for/from him.
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