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  1. #1
    Join Date
    Jun 2009
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    5

    Question Adding AC to forced hot air not working

    Hello,

    I read some posts with similar issues to mine and I've learned a lot from this forum already. However I want to describe my specific issue.

    We have a 2 family 2 story home + full basement. Home was built in 1959 with forced hot air. We replaced furnaces with 2 new York HVAC's - located in basement. We got 3 estimates and decided on the York guys.
    The heat works very well however now in the hot months the cool is not working well at all. Cool air is comming out of the registers but it takes hours to drop 1 degree.
    The registers and returns are next to the floor which I think is the biggest part of the problem i.e. the cool air never gets a chance to rise. The problem is bigger in the second floor.
    What do you suggest?
    Move the registers up next to the ceiling?
    Add higher-up on the wall returns controlled by dampers for winter/summer?
    Should we have placed the AC for second floor in the attic?
    Please help, this is our first home and we're still learning.

    Thank you,
    Donnie

  2. #2
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
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    6,004

    Default Re: Adding AC to forced hot air not working

    Could be the unit installed is not large enough, needs to be recharged, the high speed on the blower fan was not connected, or the system needs to be rebalanced for cooling.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  3. #3
    Join Date
    Jun 2009
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    5

    Default Re: Adding AC to forced hot air not working

    Hi Jack, thanks for the quick reply.
    Floor 1 - 900sq foot - 2 ton 95.5% efficient York unit (NEW)
    Floor 2 - 900sq foot - 2.5 ton 95.5% efficient York unit (NEW)
    For both units the fans have only 1 speed. I'd have to get the contractor to see if they need recharging.

    Do you think the location of the registers & returns could have a lot to do with this problem

    Thanks again,
    Donnie

  4. #4
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
    Posts
    6,004

    Default Re: Adding AC to forced hot air not working

    I've had floor registers for my HVAC for over 20 years with no problem heating or air conditioning. The blower has a 3 speed motor. It runs on low speed for heat and high speed for air. My home is about 2000 sq. ft. and the 3 ton unit is more than adequate.

    Cold air doesn't rise. With 5 tons of AC and you are unable to cool such small sq. footage either the cooling units aren't working properly, you have no insulation, insufficient air return, poorly designed duct system, or the fan is running to slow.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  5. #5
    Join Date
    Jun 2009
    Posts
    5

    Default Re: Adding AC to forced hot air not working

    Thanks Jack, that pretty much puts things in prospective doesn't it.
    You're right, our system is far from being ideally sealed and No I haven't seen any insulation on any of the ducting (including the trunk(s)).
    The design of the ducting from what I've seen seems pretty standard with one or 2 odd modifications from the previous owners.
    Well I guess I'm looking at a pretty big project aren't I?

    P.S. What I meant about the "Cold air rising" is that since it's properties are to stay on the bottom, the registers are on the bottom and so are the returns then the hot air at the top will never cool - unless I install ceiling fans to stir up the air in all rooms. I don't know if that makes sense.
    Last edited by Donnie_D; 06-15-2009 at 12:21 PM.

  6. #6
    Join Date
    Jun 2009
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    5

    Default Re: Adding AC to forced hot air not working

    So the contractor from York came and checked out the system.
    He showed me how to change the speed of the fan (low, med, high).

    Now he said the problem was the system had too much gas inside so he released some of it. His explanation was that during hot weather the pressure would rise so much and cause the compressor to shut down - which I think was true. Now I got to wait for some hot weather to stress-test the system.

    Question: What should be the temperature of air coming from the registers? he said ~59 ~60 ~61 is very good. Is that right?

  7. #7
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
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    6,004

    Default Re: Adding AC to forced hot air not working

    The temperature coming out of the registers will depend on the temperature of the air going into the return. If I remember correctly the coil lowers the temp of the air passing through by 8 degrees.So the air gradually lowers in temp but to be effective it needs to move large volumes of air.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  8. #8
    Join Date
    Jul 2009
    Location
    Northeast
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    661

    Default Re: Adding AC to forced hot air not working

    Temp differential should be 15-20 degrees, they usually run the mean (17 degrees) depending on the charge. I've seem them as high as 23 degrees and as low as 13 degrees. All your duct work has to be taped sealed and insulated. To check your temp diff. drill a 1/8 hole in the supply plenum (be careful of the coil) and one in the return plenum and use a digital thermometer to check. You must remember that the longer your A/C runs the lower the diff will be, this is because the room temp is dropping as will the return air. Good Luck!!

  9. #9
    Join Date
    Feb 2010
    Location
    Iowa, Quad-Cities area
    Posts
    129

    Default Re: Adding AC to forced hot air not working

    The second floor system should have been a 2 ton. The first floor system a 1.5 ton. Just my opinion. Bigger isn't better.

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