Your spambot missed the point of this post, Mr Weil. It's a NEW plastic smell, not a BURNING plastic smell.

OP: It's probably fine but if it doesn't go away in another month or two, then I'd get the installers back.

I have never known that new flex duct to give off any smell.

If duct board has been used, this can give off some slight smells... but in all my years never got complaints from it.

So my suspicion is your installers used something to seal with that might be giving off smells. If I had to guess I would say they probably used a foam sealer.

If this is the case, it may take upwards of 90 - 120 days for the smell to stop.

Take some pictures if you can and post them, but realize in order to smell inside the home (whatever it is) has to be on the interior side of the duct work.

Thank you! Yes duct board was used to create 2 boxes in the attic. 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.
But 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!

Here are some photos of the duct work in the attic. It is hard to get photos of inside the duct work. I found some pieces of the duct board that was used to create the boxes. It is silver on one side and black on the other. The installers said they used spray adhesive and tape to create the boxes. It appears they did not use any spray foam. I have heard from others that the tape they use can be smelly. I am going to ask the company if I can smell some of the tape to see if it is the same smell. The ac company is at a loss as to where the smell is coming from. The strange thing is that it is not decreasing with time. Tomorrow it will be 5 weeks since the system was installed. I would have thought by now the smell would have decreased, even just a bit, but it hasn't. It is still just as strong as it was the day it was installed. I wish we would figure out what is causing the smell.

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.

“...confused as to why the glue smell would only come out certain vents...”

Voila. Now you need to do the following:

1) Which vents have the smell?

2) What element of the ducting do the “smelly” vents share?

3) Where is the first common duct point where a “non-smelly” vent breaks off from the otherwise ”smelly” vent duct?

For example: You have 10 rooms; 4 bedrooms, 2 bathrooms, a kitchen, dining room, living room and a study. The 3 small bedrooms and the guest bathroom all have smelly vents, the others don’t smell. The smelly bedrooms and the bathroom are all at one end of the house and on the same vent run. The living room has a branch off this vent run, just after the plenum, but it doesn’t smell. You now know the issue is likely something between where the vents start beyond the living room split and the first spot that another vent breaks off to serve a bedroom or the bathroom.

Nearly every product out there that goes into the home has off gassing of the chemicals used to create the product. Some of these off gassing things make smells. If it's not due to a sealant they used it may be the fashion in how you are operating the system.

The system runs, the smells (usually) tend to be less obvious the longer the system runs. (This truly depends on what or where the smell is coming from, because the duct work is new / smells did not exist before it's not a hard leap to know that the smells are likely coming from the duct work.)

If you run the system for a long time does the smell remain or does it dissipate the longer the system runs? (You can test this by turning the fan switch from auto to on --- so the system fan doesn't shut off but continues to run while the cooling cycles.)

What does the smell mimic? or smell like. Does the smell remind you of something?

North Texan, thank you! 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. It is confusing to me how there are lines that come off a box that are both smelly and not smelly.

Makes no sense at all that the smell is coming from the duct boxes, then. That only leaves the individual flex duct runs. Is the flex duct the same throughout the house? How about the mastic at the register boxes?

sktn77a, They used the same brand flex duct throughout the whole house. There is a larger size for the joining duct lines and I think the return lines might be bigger as well. Mastic was used on the boxes but it is all dry and they had me smell fresh mastic the other day. It didn't match the smell I am smelling. The mastic smelled like plaster and this smell is more like brand new plastic. That is why I am believing it has to be the individual flex duct lines since they are lined with plastic. The flex line manufacturer says that is impossible but maybe they are defective. If the coil cleaning today doesn't solve the problem then I am going to suggest that their next step be to replace one of the effective flex duct lines to see if that solves the issue. They seem reluctant to do that at this point and want to try other things first. I guess I can understand that replacing a duct line costs time and money.

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.

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!

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!

Did this ever resolve completely? we just had an addition put on that has flex duct an the smell is

awful in that part of the house

I wish it had been resolved but it never was. The smell never went away and no one could ever figure out what was causing it. We moved out of the house a couple months after the new ac was installed so I am not sure what it is like today. If you have an internal water pump that is attached to the system and not one of the external ones, I always thought that might have something to do with it. It is the pump that goes up through the attic to the outside of the house. I hope it gets resolved for you! I know how much it can drive a person crazy! I surely did to me!

I‘m sorry it was never resolved while you lived there! Thankfully, our builder is helping us investigate. We do not have a water pump attached to the system, other than the house humidifier. This drains to the sump pump. The strange part is that the smell (strong heated plastic smell, smells just like the flex ducting) is strongest throughout the part of our home with the original ductwork, where supposedly no modifications were made. It is very frustrating, not to mention our eyes burn and other ENT irritation.

Just curious, did ENT symptoms include very dry nose, occasional dry/sore throat, headache, cough? We’ve been suffering these for years since replacing our hvac. have spent tens of thousands troubleshooting (replaced again with heat pump, replaced all ductwork), only making things worse. Suspect overzealous mastic application, but why does it take years to offgas?

@Rebecca Wilding this is probably way off your radar by now but were having similar issues can you share your contact info so we can pick your brain?

Suspect overzealous mastic application, but why does it take years to offgas?

mastic smells like elmers glue, I've never known that to have any serious smell for long as it's dry within a few hours and a few hours beyond that I've never smelled or had anyone complain of smell for that in over 30 years of HVAC service and repair.

Duct Board does have a smell to it and largely is going to smell the most when the system starts up after being off for a long period of time like maybe 6-8 hours or so. Reason if the duct board makes up a large return box or several of them -- the smell becomes concentrated enough to smell it when the system starts up after being off for many hours.

Typically within 30 days or so the duct board is not likely to give off much smell unless maybe it some how gets wet / stays wet enough for bacteria or mold growth to develop.

Spray foam: yeah it will off gas for a while and creates a strong smell when doing so. But in my experience with it -- after about 30 days or so it shouldn't be smelling. I am talking about the spray foam you buy at home depot in a can, not the kind of spray foam that is typically used to spray foam a roof deck. -- if the substance was mixed outside of a can? you could have different experience.

Spray adhesive: yeah I would say that will give of a smell but I would think 30 days would be it. I don't have any experience with this as all duct work I use comes in a box / insulation on those pieces installed prior to putting in box. Typcially in the form of a plenum -- sheet metal with insulation on interior.

Some people's noses are more sensitive than others? maybe. Sounds odd to me.

Was there an electronic air cleaner or air scrubber installed?

those can be irritating and smells are like an electric odd burning smell very faint but irritating. If you have one of those installed disconnect power to it and note the changes.