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  1. #1

    Question

    I have a detailed question please bear with me. I have a Trane XL 90 model TDC060B930AC furnace (downflow/horizontal). With a matching XE 800 model TTJ724A100BO A/C condenser unit. The evaporator unit sits bellow the furnace. The air intake is on my ceiling and the coil sits on a raised duct system, which sits on the floor (cement slap construction). The duct system then returns up into the attic. Once in the attic, each room is feed and the air is blown down from the ceiling vents. My opinion is that there was no planning with this set up. It should of just been an upflow to start with.

    I want to move the furnace into my attic. I want to draw the return air from the floor, through a duct, to the attic. Then, have the furnace blow the air through the ceiling.
    The return air draw for the furnace is 17” x 20 ¼”, drawing about 1200 CFM. My confusion starts here. The hole coming out of the furnace is only 14 ¼” x 16 ½”, which is sitting right on top of the A/C coil. The A/C coil openings on both sides are 17” x 20 ¼”, same as the return air. Problem is, the air is only in direct contact with half of the coil. 3” on the sides and 4” on one end is missed.

    Here’s the question(s); Couldn’t I mount the A/C coil in the return duct before it goes into the attic? Electric furnaces and Air Handlers have the A/C coil mounted before the blower. They draw the air through them instead of blowing the air through them. If the blower, blows 1200 CFM through it, then isn’t it safe to figure the it has to Draw 1200 CFM? If the return air duct was sealed air tight except through the return filter. The duct insulated on the inside and on the out side. Wouldn’t this work just as well or good as the coil mounted on the outlet side of the furnace? Plus I would have all of the air going through the coil instead of just 2/3eds.

    The system is a good quality and working system 92%+AFUE, although the A/C is only SEER 10. I haven’t discussed this idea with a contractor as of yet. I don’t want to sound stupid or be taken . With the way that it’s set up now I have questions of reliability for the craftsmen in this area. My reasoning for this set up are for easier access to the freon lines, charging, checking Superheat. The freon lineset wouldn’t need an oil trap, condensation blockage wouldn’t destroy my ceiling and the A/C coil would have total air flow through.
    I would like to make my own decisions of the work I want done before I contract someone to do it.

    Any replies are welcome, thank you for your time.

  2. #2
    Join Date
    Oct 2006
    Posts
    42
    The air handlers don't have a heat exchanger, you don't want to pull moist air through the heat exchanger.If you want the system in the attic it will have to have a-coil on the supply side.If you put the furnace in the attic you will have to worry about it freezing.Call a contractor and have them tell you your options.

  3. #3
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Having the evaperator coil on the return end of a gas furnace will cause condensation inside the heat exchanger in the cooling mode. The coil always needs to be on the supply end of the furnace.
    If more government is the answer, then it's a really stupid question.

  4. #4
    Join Date
    Nov 2005
    Posts
    104
    The coil must be after the furnace or the moist air will rust out your heat exchanger. Are you saying that you have a downflow on a slab. is there ductwork in the slab? Downflows are typically used when the supply duct is in the floor below the furnace. It sounds like you have a downflow for no reason. If that's the case wait until you need to replace it and put an upflow in its place and modify the ductwork. You will get better airflow and you won't have to relocate everything. ductwork, gas, electrical, drains,flue pipes, copper, build a platform, attic access big enough for the unit, filter change, service ability...efficiency in unconditioned space, blah, blah,blah.

  5. #5
    Good points, never thought about the heat exchanger rusting. Yes a downflow on a slab with the duct work on top NOT in the slab. Freezing, you talking about the furnace condensation drain line. The A/C wouldn’t be on in the winter to freeze. Yes there’s lots of room up there. I need the room in the house for a second bath. I’ll shop around for a contractor, see I would of sounded stupid. Thanks.

  6. #6
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    Bite the bullet, if you are going to all the trouble to put your unit in the attic, put the correct furnace or airhandler with it. Using a downflow furnace on a slab setup is just plain dumb. What happened in this install? Did someone have a left over furnace or a used one from somewhere?
    "Go big or Go Home"

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