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Thread: a coil on top?

  1. #1
    Join Date
    Aug 2005
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    2

    Question

    Nice site!

    Ok guys, I have talked to numerous people about this question and googled for hours....

    I have a friend with a 2 yr old york electric, downflow furnace...

    He installed an a/c a-coil box on top of this furnace...He has been told by numerous hvac people that it would work better if the a coil was under the furnace (Air being blown through the a-coil)

    He did it this way because the installation instructions said to put it on top because it is an electric furnace, and so that air can be "pulled" through it..

    Some techs say it can go either way...And all say that on a gas furnace it would always go on the bottom.....

    Could a pro please help us end this argument and set the facts straight?

    If it has to be on top, could someone explain why it needs to be this way?

    Why coil before blower on electric downflow, and coil after blower on a gas downflow....

    Thanks to all who answer!


  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,761
    i always was taught coil after furnace and blower. many air handlers are made with coil before blower and electric heat strips. i figure that a gas or oil furace with burner section open to room air there is more chance of warm air condensing inside heat exchanger. i think???

  3. #3
    Join Date
    Nov 2001
    Posts
    1,874
    In the electric pull through position,
    Any water drops will not fall onto the electric elements.


    On the gas units it almost always goes after the heat exchanger. The blower will take moisture away from the metal.

    an important part of the pull vs blow through,
    The pull through condensate drain must have a trap.

    This was short and simple, I don't like to act like a white coat (engineer) and make things more difficult then they need be.
    If you try to fail, and succeed.
    Which have you done ?



  4. #4
    Join Date
    Nov 2000
    Location
    Eastern PA
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    68,964
    There is no problem with a coil being installed before the blower or electric heat coils in any position. It is only fossil fuel furnaces that will have a problem with condensate due to the outside air entering the heat exchanger.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  5. #5
    Join Date
    Aug 2005
    Posts
    2
    So I am assuming that in all your prof. opinions, it does not matter?

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    It matters with Gas,either way is fine with eletric,other then conflicting with the mfrs. instructions.

  7. #7
    Join Date
    Jul 2004
    Posts
    322
    I thought with gas you don't want any water spray on the heat exchanger, so coil should be downstream of furnace to prevent rust. No idea about electric.

  8. #8
    Join Date
    Nov 2001
    Posts
    1,874
    What fan coil has electric strips and puts the evaporator above the strips ??
    If you try to fail, and succeed.
    Which have you done ?



  9. #9
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,115
    Originally posted by Toolpusher
    What fan coil has electric strips and puts the evaporator above the strips ??
    None I know of lately, maybe some stone age machines.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  10. #10
    Join Date
    Jul 2002
    Location
    Texas
    Posts
    276
    Two part answer:

    1. Fossel fuel: The R22 coil shall be installed downstream the heat exchanger. If there is a refgerant leak and the heat exchanger is hot enough Phosgen gas will be produced.

    2. Electric strips: coil will work either upstream or down stream the stripsif system is cooling only. In heat pump systems, the evap will be upsteam or before the strips. The strips are used to heat the supply air duiring defrost and low ambient conditons.


  11. #11
    Join Date
    Jul 2002
    Location
    Texas
    Posts
    276
    Originally posted by RoBoTeq
    There is no problem with a coil being installed before the blower or electric heat coils in any position. It is only fossil fuel furnaces that will have a problem with condensate due to the outside air entering the heat exchanger.
    RoBo, How would the avap see any combustion air?

  12. #12
    Join Date
    Jul 2004
    Location
    midwest
    Posts
    2,868
    Originally posted by Toolpusher
    What fan coil has electric strips and puts the evaporator above the strips ??
    Coleman, in mobile homes are all like this.

  13. #13
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,964
    Originally posted by fat bob
    I thought with gas you don't want any water spray on the heat exchanger, so coil should be downstream of furnace to prevent rust. No idea about electric.
    The main issue with a fossil furl furnace is that the combustion/flue side of the heat exchanger is open to the outside air which in the summer is warm and humid. When the cooling system cools the blower side of the heat exchanger, it cools the heat exchanger to below the dew point which causes the moisture in the air in the combustion/flue side of the heat exchanger to condense which in turn rusts or otherwise eats up the heat exchanger.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


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