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  1. #1

    Should I insulate the furnace and evap coil case?

    Have a question in regards to an attic mounted horizontal gas furnace and evap coil. Attic is ventilated and considered unconditioned space. Have been systematically replacing the old R-4 flex duct with R-8 along with upgrading the attic insulation to R-38+. Is there any product on the market that can be used to insulate the outside of the furnace case and the cased coil in order to minimize temperature losses or gains through these cabinets? Is there any data to support or disprove this?

    Thanks

  2. #2
    Join Date
    Dec 2010
    Location
    New Orleans
    Posts
    1,012
    Well that would make it hard to service if you sealed it with insulation The coil should have insulation on the inside of the case. The furnace gets too hot internally to insulate. So keep weatherproofing your house and you should be fine.

  3. #3
    There are clearances to combustables for the furnace, do not insulate it... the coil could be as long as the insulation used can withstand the highest temp. of the plenum ... If you do not know, Dont do it.... As long as the trunk line and branches are well insulated, you should be fine.

  4. #4
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,046
    Attic radiant barrier for heat gain or make attic part of thermal envelope by spray foaming under side of roof instead of attic floors

  5. #5
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,560
    I think you're getting a bit extreme. I'm pretty sure that the equipment manufacturers have thought this through somewhat and insulate and do what is necessary to maximize efficiency and insulation in said equipment. You might save a few pennies a month, but not much more.
    I like DIY'ers. They pay better to fix.

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,046
    Quote Originally Posted by bmathews View Post
    I think you're getting a bit extreme. I'm pretty sure that the equipment manufacturers have thought this through somewhat and insulate and do what is necessary to maximize efficiency and insulation in said equipment. You might save a few pennies a month, but not much more.
    They don't do a very good job most are lined with 1/4 in fiberglass prob R1 or less and most blower compartments leak very badly. Try sealing a system 100%. Or even 90% without addressing the equipments leakage

  7. #7
    Join Date
    Jan 2011
    Location
    Sevierville, TN
    Posts
    7
    U Can use duct wrap insulation R8 and wrap the case coil and sides and back of furnace but do not wrap the front of the furnace so it can be got to to service.
    Time would probly be better spent sealing duct leakage. Would save more money in the long run and help indoor air quality.

  8. #8
    Join Date
    May 2008
    Location
    Myrtle Creek. Oregon
    Posts
    182
    most furnaces have a reflective insulation on the inside. so no need to insulate the out side

  9. #9
    Join Date
    Jan 2011
    Location
    Albuquerque,N.M.
    Posts
    2
    No! you should not insulate.I think you will be fine.
    Semper Fi

  10. #10
    Join Date
    Jul 2010
    Location
    Maryland
    Posts
    35
    I would have said there was no need to insulate the coil cabinet. But a few days back while installing a Bryant system I read in the cased coil instructions recommending the coil to be insulated on the outside of the cabinet. This was the first time I had ever seen such a recommendation.

  11. #11

    Thanks for replies

    These are all good replies and good information to ponder. I know I am getting a little extreme but every little bit helps. Since my original post I replaced the old undersized supply plenum with a new one with 1" inside insulation. Repositioned the flex duct openings, new R-8 flex duct and it made a big difference in airflow. After moving to Phoenix from cold country I have had to put up with a whole lot of cheap HVAC systems. Seems like the motto here is not "Well, we got it to work" it is more like "At least we got it in there".

    I am still looking into wrapping the supply plenum and evaporator case as they are exposed to the environment in the vented attic. Just looking for a good deal on duct wrap now as most CertainTeed rolls are 50 to 100' long and I will never use that much - probably need 25 square feet at most.

    Any thoughts on using 3M fyre wrap? That way I could wrap the furnace case and not worry about clearance to combustibles. Just need to make access doors accessible.

    Keep sending in ideas. I appreciate them.

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