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  1. #1

    Plenum Adapter - Conflicting Advice

    I'd posted earlier about getting a furnace properly sized and issues with running new 5 wire thermostat wiring. I appreciate all the helpful comments and I think that I've got that all sorted out.

    Here is my next dilemma. I'm replacing a huge old updraft furnace with a 95% efficient furnace which is much smaller dimensionally. The plenum has an A coil for an old AC unit that I only use a couple of days a year. Contractor #1 said I should reorient the A coil 90 degrees which requires extensive sheet metal work and added expense. Contractor #2 said the whole plenum should be replaced with a factory unit with a new coil installed. Contractor #3 said he would just make a funnel shaped adapter to go from the top of new furnace to the bottom of the old plenum.

    I want the furnace to function quietly and efficiently while heating but in the AC mode, I'm not really concerned because it is so seldom used. I'd appreciate any thoughts on how to make this decision. Naturally, I want to minimize my costs, but I don't want to do something I'll regret later.

    Thanks in advance for your thoughts.

  2. #2
    Join Date
    Nov 2008
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    Colorado
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    Do you have any pictures of what is currently installed?

  3. #3
    Quote Originally Posted by james80031 View Post
    Do you have any pictures of what is currently installed?
    Attached are photos of the right front side of furnace, left front side and back right side. I'm trying to show the plenum to old furnace interface and the bumped outed panels that allow clearance for the A coil that is installed.

    Hope this clarifies things a little.
    Attached Images Attached Images    

  4. #4
    Join Date
    Nov 2008
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    Colorado
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    This is an odd configuration that makes cooling not very efficient, but if you only use it a couple days out of the year, you're not really worried about efficiency. The coil is installed 90* off kilter, but that is probably because the coil was smaller than the size of the plenum opening. Is there any whistling or loud air noises that come from the air coming across the coil in this position?

  5. #5
    Quote Originally Posted by james80031 View Post
    This is an odd configuration that makes cooling not very efficient, but if you only use it a couple days out of the year, you're not really worried about efficiency. The coil is installed 90* off kilter, but that is probably because the coil was smaller than the size of the plenum opening. Is there any whistling or loud air noises that come from the air coming across the coil in this position?
    Yea, I don't know the history - it is a Sears installation. I pulled off one of the bump out panels today and noticed that there is only a baffle on one side of the A coil, so air freely passes it by the A coil on the left side. I guess this explains why the AC only works so-so. The furnace does make kind of a roar when the fan is running, but I don't have anything to compare it to. I would not say that it whistles, but of course it is partially by passing right now.

    If I leave the A coil like this, will the furnace operate efficiently in the heating mode?

  6. #6
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    if the coil is in good shape reinstall it properly in a new plenum.

    if not replace it with a new cased coil.

  7. #7
    Join Date
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    Quote Originally Posted by t527ed View Post
    if the coil is in good shape reinstall it properly in a new plenum.

    if not replace it with a new cased coil.
    I think you're right, but if I was going to cut the lineset to reorient, I'd just put in a new coil. It sounds like the labor they would charge him on sheetmetal work to make this one proper would cost more than a new coil.

  8. #8
    Quote Originally Posted by james80031 View Post
    I think you're right, but if I was going to cut the lineset to reorient, I'd just put in a new coil. It sounds like the labor they would charge him on sheetmetal work to make this one proper would cost more than a new coil.
    Yes - I don't have a quote yet, but that is basically what the contractor said that looked at it today.

    What is the downside to just leaving it as it is and adapting the new furnace outlet to the underside of the plenum? Would the A coil in this orientation cause excessive restriction?

    Thanks.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,058
    Depends how much lower, and how much less air the new furnace moves.
    Some installs like that, had one of the baffles removed, because with it in, it slow the air down to much, and the furnace rode the high limit.

    If the new furnace is short enough, a transition to that plenum can be made that won't hurt the air flow.
    If the new furnace is close to the same height, then it may have trouble moving air.
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  10. #10
    Join Date
    Sep 2002
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    Northeast Ohio
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    If cost was paramont, we would just make a transition. We would material and labor the cost if you opted to reorinent the coil for a cleaner look and the last option would be a coil change out. Obviously if there were issues with air flow we would push for option 2.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  11. #11
    Quote Originally Posted by beenthere View Post
    Depends how much lower, and how much less air the new furnace moves.
    Some installs like that, had one of the baffles removed, because with it in, it slow the air down to much, and the furnace rode the high limit.

    If the new furnace is short enough, a transition to that plenum can be made that won't hurt the air flow.
    If the new furnace is close to the same height, then it may have trouble moving air.
    Thanks, been there. The new furnace , which will most likely a Lennox G61MPV would be 14 1/2" lower. As a two stage, variable speed unit it would move 725 to 1400 cfm. The old belt and pulley blower is rated at 1400 cfm.

    Sketch below is not to scale, but dimensions are correct.

    Thoughts?
    Attached Images Attached Images  

  12. #12
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    as old as that metal work looks i don't think i would want that old coil over my new high end furnace...........

  13. #13
    Join Date
    Jun 2007
    Location
    Round Rock
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    I think I'd just put a new coil in and be done with it. Otherwise you'll be on here next summer asking questions about what coil to put in and why does it cost so much for a new coil or the drain pan on the old coil rotted out and did $$$ damage to your new furnace and it should be under warranty, but the contractor won't warranty it because you didn't buy a new coil, etc....
    I like DIY'ers. They pay better to fix.

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