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  1. #1
    Join Date
    Mar 2009
    Location
    Westborough, MA
    Posts
    20

    Default How long for my house to cool?

    We had central air installed last month for the first time (1800 sq foot, well insulated cape in the Northeast) and in running the AC unit for the first time, it took 4 hours to cool the house 4 degrees... seems like a very long time, and it didn't really make a difference. How long should it take to cool my house 10 degrees, and is there a difference in running it for the very first time as opposed to regular usage? Any information anyone has would be helpful... I want to make sure my AC is working properly.

  2. #2
    Join Date
    Aug 2007
    Posts
    612

    Default Re: How long for my house to cool?

    4 hours of run time for 4 degrees is just not right.

    Just to mention the obvious without trying to offend....
    Make sure your thermostat is on "cool" and you don't just have the fan circulating the air through the house.

    Once you move beyond the "short between the headset" possibility... (I have been there & done that myself)

    There are a few questions to try to answer to try to help...


    What size unit was installed, 5 ton, 4 ton, etc...

    Where are the supply runs, cool basement, hot attic space, etc.

    Are the ducts insulated?

    Where is the return ductwork and is it well sealed & insulated?

    What is the temp of the air coming out of the room vents versus the temp in the main plenum?

    Any leaks noticeable on the refrigerant lines?

    Since it is only a month old, I would have the installer out there answering these questions.

  3. #3
    Join Date
    Mar 2009
    Location
    Westborough, MA
    Posts
    20

    Default Re: How long for my house to cool?

    Oh it is definitely on "cool"... but I thought about that too, haha. Cool air is definitely coming out of the registers, it just doesn't seem to be cooling down the whole house like I expected it should.

    We have approximately 1800 sq. feet of well insulated house to cool over 2 stories, and have a 2 ton unit installed. (quick rule of thumbs seems to be 12000 BTUS (one ton) per 1000 well insulated sq feet, so sizing should be accurate.

    We have plenty of returns, and our ductwork was insulated thoroughly at the time of the install.

    We have only run the AC that one time... the initial startup and it just didn't seem to get the job done. I have read that the slow cooling might have something to do with the removal of humidity for the first time, or just the initial startup of the AC and it might not be calibrated properly? We have the contractor coming out this week, I just want to be prepared with all the right info to ask him questions and make sure he takes care of it.

    Any help would be appreciated... thanks.

  4. #4
    Join Date
    Aug 2007
    Posts
    612

    Default Re: How long for my house to cool?

    Why not just run it again cooling from current temp to about 5 degrees less and see how long it takes. Obviously you wouldn't normally run AC in the weather we're having, but for a test it wouldn't hurt to try it.

  5. #5
    Join Date
    Oct 2007
    Posts
    175

    Default Re: How long for my house to cool?

    Check the air temperature coming out of the registers - it needs to be in the 40-50 range for effective cooling.

  6. #6
    Join Date
    Oct 2007
    Posts
    175

    Default Re: How long for my house to cool?

    Quote Originally Posted by birdball32 View Post
    We have approximately 1800 sq. feet of well insulated house to cool over 2 stories, and have a 2 ton unit installed. (quick rule of thumbs seems to be 12000 BTUS (one ton) per 1000 well insulated sq feet, so sizing should be accurate.

    There is no "average" that will get you a good design. There are many factors that have to be considered. But, 500 sf/ton is probably a better number for a more conventional house.

    I my area I have always used 350 s.f. per ton. This assumes more than the average amount of perimeter wall, more than the average amount of glass, and a certain amount of 2 story volume. The custom houses I was designing a few years ago all fell in this category and the actual HVAC capacity was often in the 350-400 sf/ton range.

  7. #7
    Join Date
    Feb 2009
    Posts
    693

    Default Re: How long for my house to cool?

    Just throwing in here but I have a 3 ton unit for 1500 sq/ft. I can cool from 85 to 75 in about 30 minutes to an hour.

  8. #8
    Join Date
    Mar 2009
    Location
    Westborough, MA
    Posts
    20

    Default Re: How long for my house to cool?

    My contractor performed all the necessary heat gain/loss calculations before sizing the unit, so I WOULD HOPE that the unit is appropriately sized.

    Does the removal of the humidity from a house that has never had air conditioning before play any role in why it may have taken so long the first time?

    I appreciate everyone helping out in this forum... you guys are great!

  9. #9
    Join Date
    Aug 2007
    Location
    Maryland
    Posts
    1,586

    Default Re: How long for my house to cool?

    Yes removing the humidity also known as latent heat does take a while before the actual temperature starts to come down.
    Oversizing the A/C will cool faster, but also in the heat of summer the unit will run less and not remove the humidity as well. The house may not feel as comfortable because of more humidity and because some rooms will tend to not get cooled down properly.
    My a/c takes a while to cool the house from 85 or 90 degreees, but I can't give a time. It works fine cooling the house in the summer. It's best to turn the a/c on before it gets too hot.

  10. #10
    Join Date
    Nov 2007
    Location
    Sand Springs, OK
    Posts
    467

    Default Re: How long for my house to cool?

    4 hours for a 4 degree drop in a 2 story house sounds right to me.

    Where are you expecting it to cool?

    I'll describe my house for you to compare. I have a 2070 sq/f house with a cathedral ceiling in the living room. Only one story but I have a 5 ton 12 seer unit.

    My office, sewing area is in the SE corner with a bank of windows and rock wall to bring in heat. The Kitchen is on the south side and the garage on the SW corner. These are the warmest rooms in the house during most of the day. As the sun sets in the west the 2 western bedrooms and the entry way heat up through sunlight streaming in through the windows. The bathrooms are on the northern wall. The center of all of this is split equally the living room with a high ceiling and my bedroom which has no windows at all. I have door to the hallway, one to the master bath, and where the windows should have been there's an addition/florida room/studio with windows on 3 sides and skylights.

    Once again, my office is the hottest room in the house until I start cooking

    The kitchen will heat up fairly quickly when in use. In order to stay cool in the kitchen I have to lower the ac temp 3 degrees starting at 5 and finally quitting at 7:30.

    Meanwhile the boys' bedrooms are staying a continual temperature to accomodate the radiant heat coming in through the windows.

    My room and the northern bathrooms drop 10 degrees during this cool down phase.

    In other words some of your rooms aren't going to cool as efficiently as others.


    My suggestion is keeping window shades and curtains closed in the rooms that heat up more, especially the upstairs rooms. My rule is after the sun the shades can raise.
    Debby in Oklahoma

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