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  1. #1

    Condensation on outside of Plenum

    I recently built a home in summer of 2010, and my air conditioning this summer (2011) is having a condensation problem on the OUTSIDE of the plenum and ducts (at least for the first 3-4 feet running past the plenum).

    I have a trane furnace and air conditioner, it is 3 zones total one for the basement, main floor, and upstairs) so there is three main branches going out of the plenum with dampers.

    The furnace is located in my finished basement in my mechanical room, and there is no unusual humidity problem down there or anything like that. I have even run a dehumidifier from time to time but it doesn't fix the problem.

    The plenum itself condenses on the outside of itself, as well as one of the main branch ducts (that runs the most which goes upstairs up a flu chase) which also condensates on the outside for about the first 3-4 feet running out of the plenum. The condensation problems start at the beginning of the plenum in my mechanical room.

    All of this drips on the floor making a big mess anytime the temperature gets above 90 degrees outside. To make it worse it drips on an electrical subpanel which I am worried about as far as shock hazard.

    The plenum is NOT insulated inside (according to the installation HVAC folks) or outside in anyway, and neither is the ducts. They are sheet metal.

    I have had the folks that installed the system take a look at it three different times, and I forked out a lot of money for an entire air conditioning / furnace system to them last year, and they have not been able to solve the issue.

    I do know last year the builder "forgot" to have an air filter running in my air conditioner for the first month or so after construction. Could this be caused by a dirty evaporator problem? Or a low freon type issue? Or maybe just the fact that the ducts or plenum should have been insulated.

    Other then that the drain line works normally and drains. The condensation is on the outside starting at the plenum and then disappearing.

    Thank you for any help and advice. I am trying to ask for some expert help in guiding me in the right direction.

    Thanks for your time and responses,

    Matt

  2. #2
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    "The plenum is NOT insulated inside (according to the installation HVAC folks) or outside in anyway, and neither is the ducts. They are sheet metal."
    And there is your problem.

    The only way it won't sweat is if it never gets below dewpoint.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  3. #3
    Thank you for the reply.

    What should have happened for a proper installation? Should the inside of the plenum have been insulated somehow? or the outside of the plenum, and with what type of material?

    For a corrective measures should they take the duct work apart and insulate the inside for the first few feet, or just insulate the outside somehow for a few feet. Hmm the zone damper condenses too coming out one of the main branches.

    The HVAC installation folks looked me straight in the eye and said that noone EVER insulates plenums when I asked him. Guess I can't trust them.

    Should the sheet metal plenum, and all of the sheet metal branches coming out the plenum, have been insulated everywhere? Or just the first few feet of run?

    I live in Iowa in case anyone needs to know the climate I live in (snow in the winter/hot in the summer)

    Thanks for your help everyone.

  4. #4
    also I was wondering, can this situation create a mold problem inside on the ducts (maybe it is condensing inside also?, or only on the outside and I just have to put up with a mess. Or possibly can equipment replacement early on?

    my wife runs a preschool in our finished basement where the furnace/ac is located

  5. #5
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    You can sometimes get away with not insulating the ducts in well insulated and conditioned areas of the home. I believe the plenums should always be insulated and any ducts when installed in any mechanical room. I would have them wrap the outside of the plenum and ducts in that room. May look better if insulated with a duct/fiberboard on the inside but good luck getting them to tear into it now.

    Do you notice a temperature difference in that room than the others? They may be able to give that area a little more supply and do without insulating.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  6. #6
    Hmm my zone system used to have a "pressure relief" duct with a door in it that bleed off extra pressure, if only one zone was turned on. This pressure relief duct releases extra pressure (cold air) itself in the mechanical room just into the empty space. It was condensing this summer in the mechanical room )on the outside of the plenum and ducts) and it would get really cold in there.

    I had them add this "pressure relief" duct back into the actual return air before the air filter (that is what it said to do in the zone manual), and that solved it getting extra cold in the room. But the ducts and plenum still condense unfortunately.

  7. #7
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    The plenum sweating and creating a mess is unacceptable in any case and even more so in a finished room. The plenum will rust out early if continued sweating occurs. I don't know about the statement that no one insulates ducts and plenums. I would call him on it. You always insulate ducts and plenums in un-conditioned areas like in attics and crawl spaces and in areas that may not be well insulated and sealed.

    I would call and speak with the owner and demand the problem be fixed or he will be held responsible for any accidents or damage caused by the condensation on the floors.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  8. #8
    Thank you for your help RCB, I really appreciate it. This will cause my wife and I a lot less headaches...

    Whew I am so glad I found this board, and feel privileged to receive help from you.

    Thanks for the explanations and for making this simple!

  9. #9
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    I am not completely familiar of how things are done in your area. Most all of the systems supply side here is insulated through the entire home. Someone with plenty of experience with your install situation should be along shortly that will be able to give you better help than I can provide on this one.

    I would suggest having them resolve the issues as it is their job and you didn't pay for a system that you have to mop up after daily. You need to keep calling them on this until it is fixed before the warranty gets outdated.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  10. #10
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    You said the company has been out three times looking it over so I didn't mention the air flow, charge or dirty evaporator coil but either of these can cause the supply to run colder than designed which will aid in causing the plenum and duct condensing.

    You also mentioned that two zones are often shut. Were they closed when they came out and did they check the CFM during that situation? Might want to consider proper ventilation for that room as well. If you are having poor air recirculation there the system can't dehumidify it, just forcing cold air in doesn't work.

    Hope you get it sorted. It's pretty late but someone may come along later with better insight on the issue.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,437
    Insultating the first few feet only moves the condensate further down the ducts. What is the temp/%RH in the space? What is the temp of air in the ducts?Maintaining <50%RH in the basement space is important avoid mold and condensation on ducts.
    Get a good temp/%RH monitor and check your home throughout.
    Keep us posted.
    Regards tB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #12
    Join Date
    Sep 2009
    Location
    Hampton, GA
    Posts
    568

    Unhappy

    You need to get that plenum insulated with fiberglass for sure. Thats the only way it will stop sweating. I can;t believe the contractor doesn't even know that. That is sad. Also ask them to check the airflow is setup properly per tonnage for your AC unit.

  13. #13
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,170
    In the midwest & Ohio Valley that I'm familiar with, supply work in a basement is not insulated. If it is so humid there that the ductwork sweats, it is too humid period. You should get good (www.thermastor.com) dehumidifier to prevent mold other places in the basement.

    Ducts in any unconditioned space like a garage or attic certainly should be insulated.

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