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Community and Q&A

New HVAC flex ductwork new plastic smell

ParkLane | Posted in General Questions on

About a month ago we had a new AC handler and new AC ductwork installed in our home.  Since then we have had a new plastic smell coming from some of the vents.  The smell is similar to when you open a new plastic shower curtain or new pool float.  The ac company said the smell is coming from the plastic lining that is used in the flex ductwork.  The maker of the ductwork has stated that is not possible since they use PE plastic and PE plastic does not off gas in anyway.  We were okay with the smell when it was first installed but after a month I would have figured the smell would have gone away.  Any ideas?  Could it be the plastic lining?  And if so, are the off gases hazardous?   Could some of the ductwork be defective, which is why only certain vents are effected?

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  1. Expert Member
    BILL WICHERS | | #1

    PE should mean “polyethylene”. PE is a very stable material, but it burns so you probably have some fire retardants in it in those ducts to make it self extinguishing. You can also have a little bit of plastic smell, but it should be minimal and dissipate quickly.

    I’d suspect maybe whatever they used to seal joints in the ductwork. Some tapes can be pretty smelly.

    You may find that running your system for heat for a while will make it smell more for a little while, and then it will go away. Heat will help to speed up the off gassing process of any residuals in the system.


  2. ParkLane | | #2

    Thank you Zephyr7! The ac company inspected the entire system yesterday and cannot find the source. They were thinking that there might have been an incomplete seal causing some of the mastic to remain wet and produce a smell. It appears they used mastic to seal the joints. I did not see any tape but will ask them about that.
    They found no leaks and all the mastic appears to be dry. They are coming back out on Monday to cut into the duct board boxes to see if they might be able to locate the source. The smell is very strong and doesn't appear to be lessening which I would assume would happen with any off gases from a particular product. They are also going to replace one of the effected duct work lines to see if maybe the duct work is defective. They brought a piece of new duct work for me to smell and it has no odor. But maybe if the product is defective it would have a smell. We are currently not staying in the house because the ac company could not tell us if the fumes were toxic or not. I figure it is best to air on the safe side. I would imagine the products they use in ac install are harmless but you just never know!

  3. Expert Member
    BILL WICHERS | | #3

    It sounds like you have a good contractor that really wants to get this fixed for you. That is a GOOD thing! A lot of places will try to blow off customers in situations like this.

    If you have a defective piece of flex duct you’d need to replace it. I suppose you’d have to give all the duct a sniff test to check for this, I don’t know of any other way to do it. I’d check all the seals first, anything that started out as a liquid (mastic, caulk, glue, etc) is most likely to be the issue since the off gassing is part of the curing process. If the smell smells like acetone then I’d suspect silicone caulk. Silicone caulk can stay smelly for a long time if it’s in a small enclosed space with no air movement.

    Chances are the smell is too low concentration to be a health hazard, but it doesn’t hurt to play it safe and stay out of the house while things get figured out if you have that option available.


    1. ParkLane | | #4

      Zephyr7, Yes thankfully the ac company seems willing to figure this one out. I have had to follow up with them a bunch though but I guess that is to be expected. They are going to come out again today with different techs to see if they can figure it out. I spent the morning in the attic checking out all the duct work. All the mastic seems to be dry, some of it is cool to the touch, but I figure that is normal. The smell is different than caulk. I know that smell very well as I used to use caulk a lot in my old profession. This is more of a new plastic smell. That is what makes me think it might have something to do with the duct lining. I am going to see if the ac company can replace one of the effected lines. If the smell goes away in that line then we know what the culprit is and how to fix it.

  4. walta100 | | #5

    Ok I say it. If the ducts smell is that bad rip out the flex and pony up the money for steel ductwork. Have the contractor bring you a new sealed box of flex duct and give it a smell. PS I hate flex ducts.

    But consider most air handlers have plastic drip trays, spacers and wiring. Also the blower motors electrical windings are in the air stream and can get quite warm, they were likely dipped in epoxy varnish and I would call it plastic. Epoxy chemistry has so many variables a batch that is off slightly is possible.


    1. ParkLane | | #6

      Walter Ahlgrim, thank you. I had never really considered duct work options. For some reason I only thought there was one. After the flex duct work was installed I read about steel duct work and wished we would have gotten that instead. Though I have read that a lot of ac companies won't do steel duct work because it requires trained workers that they do not have. The ac company was back out at the house today with a different tech. This tech noticed that not all boots were sealed completely in the attic. He believes the air is coming from the attic and is giving a plastic smell. They are coming back out on Wednesday to reseal the boots. I am not 100% convinced. The attic doesn't smell like plastic (I have been up there crawling around more times then I can count lately). But I am going to go with their assessment and see if it changes the smell coming out of the vents. If not then it will be back to the drawing board.

  5. MattJF | | #7

    I think you need to find the location of the smell within the system. It would help to know if it is originating from the air handler, the supply ducts or return ducts.

    Have the system running, quickly turn it off, pull the front door off the blower and stick you head in there. See if it smells. If you are lucky, it doesn’t and you can eliminate half the system.

    Drill a hole in the supply plenum (make sure you are well away from the coil) and smell the air the come out. It may be hard to smell the blowing air, so you could try use a tube that doesn’t smell and a non smelling shopping back to capture an air sample.

    Stick your nose right up into each supply vent. You might need to take breaks, go outside for a bit if you become desensitized.

  6. walta100 | | #8

    The move away from steel ductwork is not about the lack of skilled labor it is about the cost of materials and the amount of labor required. Few people are willing to pay the cost for steel hidden away never to be seen. If there was demand the labor would get trained almost no skill needed to run 6-8 inch round pipe.

    The biggest problem with the flex after a few years and 3 or 4 non HVAC people crawl thru the attic the ducts end up disconnected, flatten or kinked and they never flow as well.

    I doubt whatever you are smelling is the ducts.


  7. ParkLane | | #9

    Matt F, Thank you, we took the door off the blower and it doesn't smell at all. I will suggest drilling the hole idea to the ac techs when they come out tomorrow.

  8. ParkLane | | #10

    Walter Ahlgrim, thank you, Yes I am reading now all the negative things about flex duct. If this issue doesn't get resolved with all of our troubleshooting we might do as you say and get it replaced with steel. In regards to the plastic wiring and such used in the motor, wouldn't that cause a smell in all the duct work, and not just some?

    1. Expert Member
      BILL WICHERS | | #12

      Any smells from the motor or wiring would only be while it was running and hot most likely. The enamel insulation used in the motors is NOT epoxy based, it's a nylon and urethane varnish material. Those are very stable materials. You're only likely to smell anything from those if the motor is WAY overheating, in which case you'd need a new motor anyway. You'd get that distinctive "burning electrical smell".

      Most wiring is PVC insulated, and those materials are generally pretty stable too. Any "new machine" smell would only last a few days to a few weeks I would think.

      I doubt very much you have smells coming from the wiring. It's much more likely you have some partially cured caulk somewhere, or some gooey tape -- something like that.


  9. MattJF | | #11

    Good to hear that half the system is ruled out.

    An AC tech could also pull sections of ducts off whatever plenum box there is. It would be worth looking in there to see if someone left a roll of foil tape inside. That would certainly cause a plastic smell.

    Are there specific registers or rooms where the smell occurs?

    Flex is not all bad, it actually can be very good. It does a good job of not transmitting noise, so it makes a lot of sense to do the final run outs in flex. Flex gets a bad rep for being used poorly so often. It is great when pulled straight and tight, it does not turn well. Flex also tends to have less leakage as there are less seems to seal.

  10. ParkLane | | #13

    Zephyr7, the ac techs were out at the house this morning. Thankfully it was the same tech that installed the system. He reviewed all the boots and found they were all in pretty good shape with the exception of two. He resealed the boots with mastic. He believes the smell is not coming from the unsealed areas. He agrees with me that it might be a defective plastic lining in some of the flex duct lines. He said if the smell continues our next plan of action would be to replace one of the effected duct lines and see if the smell goes away. I asked about what other materials were used to create the system and he said mastic and spray adhesive (to create the boxes). He said no spray foam or PVC glue was used. Chaulk was used to seal the vent on the outside (close to the ceiling). I didn't ask about tape but I will the next time he is out.

  11. ParkLane | | #14

    Matt F, The ac tech was out this morning to reseal the boots. I will ask about the plenum boxes next time he comes out. That is a good thought, thank you for that. If they used foil plastic tape on the insides of the box do you think the plastic smell could be still coming from the tape? There are a total of 10 vents in the house, 6 of those vents are effected. Some are stronger smelling then others. Some of the 6 vents have the same plenum box and some have a different box. However, there are some vents that are not effected but they share a plenum box with vents that are effected. It is very strange. I would think if the plenum box were the culprit then all the vents associated with that box would be effected.

  12. MattJF | | #15

    The spray adhesive for the boxes seems like the likely culprit. Are the boxes steel or ductboard? Are they insulated inside the box or wrapped outside? I imagine the spray adhesive was used to attach insulation, but maybe it has a use for assembly with ductboard,

    Is there insulation glued inside the register boots?

    Foil tape could cause the smell issues if a lot of adhesive surface is exposes to the air flow. It would need to be a lot of surface and I would imagine would dissipate quickly.

    Some pictures would be helpful. If the smell is distinct enough you can tell which vents it is coming from, you should able to track it down. I would draw a map and the try to smell test sections to trace back to the source.

  13. ParkLane | | #16

    Matt F, I have attached some photos of the duct work. I have also attached photos of a piece of the duct board (front and back) that was used to create the boxes. The black side is the interior side. I think the ac tech said he used spray adhesive for the box but I also remember him saying something about a tape with foil. I am not sure about the register boots. It appears to be the same duct board with mastic. I think I am going to have them bring over a sample of the tape so that I can smell it to see if it is the same smell.

  14. ParkLane | | #17

    The ac company now thinks that the ac handler coils might have some glue, from the factory, on them which could be causing the smell. They are going to send a tech out tomorrow to clean the coils using a sanitation/cleaning fluid. Has anyone heard of this happening before? I am confused as to why the glue smell would only come out certain vents but I am willing to have them try it to see if it works.

  15. Expert Member
    BILL WICHERS | | #18

    I’d suspect either the spray adhesive or the caulk. Silicone caulk will smell of acetone while curing, and I’ve sometimes seen that curing process take weeks in a confined space. If it’s the caulk, the smell will be strongest in whatever part of the system the caulk is in.

    Glue on the coil from the factory sounds strange to me, but it shouldn’t be too difficult to check.


  16. ParkLane | | #19

    Zephry7 thanks so much! I am having the tech bring out the tapes and other adhesives they used in the install out to the house today so that I can smell them. If one of those match the smell I am smelling then we will know where it is coming from. The layout of what vents smell and what vents do not smell is very strange as they come off different boxes. I have draw up a diagram and have attached it to this post. The black boxes are the duct board boxes they created. The green lines are larger flex duct lines that join the boxes. The red lines are flex duct lines that have the smell. The blue lines are flex duct lines that do not have the smell. The yellow lines are the return lines. The spare bath and spare bedroom are the lines that smell the strongest.

  17. ParkLane | | #20

    The ac company was out at the house today and the sprayed the coils. It didn't help. They are going to come out next week and replace the spare bath line to see what that does. I am hoping that is the answer because I cannot figure out anything else that would be the culprit. I smelled the tape and adhesives they used to create the boxes. None of them had the same plastic smell. The tech did mention the clear condensation pump line. It is made out of clear plastic and does have a plastic smell. I am not sure how the condensation pump works and if the smell of the plastic hose could infiltrate the system. If that was the case I would imagine it would effect the whole system and not just some vents.

  18. ParkLane | | #21

    The ac techs were back at the house today. The replaced the spare bath duct line and the smell went away!  I am still not sure why. I smelled the duct work they removed and I smelled inside the box that was connected to the spare bath line.  I smelled the boot that they created using duct board and there was an odor to it.  It didn't match exactly the smell I was smelling but it was close.  They said they used spray adhesive to create the boxes and I asked them not to use it when they made the new boxes.  I am not sure if that was the change that made the difference but I am happy nonetheless that the smell is gone.  They are going to come out tomorrow to change out the rest of the effected duct lines.  I have my fingers crossed that the change solves the problem for the rest of the ducts.  I asked the ac techs why only certain vents would be effected if the same materials were used for all the vents.  They said they had 3 techs creating boxes that day.  Some may have used the spray and others didn't.  I am not sure why the spray adhesive is still giving off such a potent smell 5 1/2 weeks after the install.  Have my fingers crossed that we can finally have our home back tomorrow!  

    1. Expert Member
      BILL WICHERS | | #22

      My guess is that “spray adhesive” was the source of the smell. It’s possible it wasn’t fully cured when the system ran the first time, with some of the adhesive getting into the ductwork. You may have even had a reaction between the spray adhesive and the duct material that caused the smell. Solvents and plastics sometimes have “compatibility” issues. An example I can think of is the vinyl outer surface of old HP test equipment. If you clean it with formula 409 cleaner, the vinyl gets sticky and stays that way permanently. There is something in formula 409 cleaner than chemically alters whatever HP used in that vinyl cover material and makes it sticky/gummy.

      The “clear tube” from the condensate pump is probably vinyl tubing — the same stuff any hardware store will carry and sell by the foot. I doubt you’ll have any problems with that material, especially used as a condensate line. All the line is for is to drain away condensate which is moisture that condensers on the air conditioner coil in humid weather and runs out through that drain line.


  19. ParkLane | | #23

    Zephyr7, yes I think you are completely right. I think something might have happened with the spray adhesive and the plastic lining. The techs also made a comment today remembering that it was raining when they installed the first set of boots. He said the boots might have gotten wet and that mixed with all the sprays, tapes, etc, might have created the perfect smell mixture. I am guessing if it was chemically altered that is why it never stopped smelling. The techs replaced all the effected boots and the smell seems to have lessened if not gone all together. The spare bathroom line that they completely replaced (boot and line) yesterday still has a bit of a smell, but I think it is new plastic liner I am smelling and not the same chemical plastic smell I was smelling before. The plastic liner has a smell that reminds me of raisins for some reason and that is what the vent smells like now. I am assuming that smell will go away with time. I am excited that the problem seems to be fixed. I am hoping it doesn't happen to anyone else but at least maybe they can use this troubleshooting information to do the same with their house. Thank you everyone for your advice, input and thoughts!

    1. kman3728 | | #24

      I realize this is two years later, but worth a shot. Did your raisin smell go away very quickly? We're in a new home and one duct has a similar raisin, almost sweet, smell. Curious if it will go away eventually on its own. Contractor told me to wait 6 months. It's been about that long. Thanks

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