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Thread: Musty/basement smell coming through ductwork

  1. #1
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    Confused Musty/basement smell coming through ductwork

    Does anyone know why there is musty smell coming out of the ductwork?
    Our home has a large cemented crawl space. When we bought the home, we noticed the smell in the house, but we attributed it to the house being simply closed for a year. After a year of renovations, the musty/basementy smell did not go away. The smell is coming from the crawl space through the ductwork. The crawl space is fully cemented. Our HVAC contractor (and we had several companies) told us to first deal with the crawl space smell before dealing with the HVAC system. We put the French drains all around the perimeter of the crawlspace. We removed fiber glass insulation and installed extreme block rigid core insulation all along the walls of the crawl space. We have 3 dehumidifiers running 24/7 (one of the is a huge Sante-Fe). So after all of this the crawl space still has a smell but it is not bad at all. More like a subterranean/earthy smell. Not worse than any other crawl spaces I've been too.

    Now, after all of that. Our HVAC guy came last week and reconstructed portions of ductwork, cleaned coils, did something else to the furnace, patched the holes, etc. ($ worth of work). It's been a week and we can still smell the basement smell coming through the supply vents. The HVAC guys are saying that we need to have the fan ON, and run the system for a while to get rid of the smell and for the ducts to dry. But I'm loosing hope at this point. We spent close to $on crawlspace (french drains, insulation, HVAC) and the smell is still there. How is it possible for the basement smell (which is not bad at all at this point) to get into the duct? Is this possible that the ducts are smelly and it need time to air out? We are so desperate at this point. I'm about ready to either sell the house or rip HVAC and reinstall some other heating, like baseboard.
    Last edited by Special-K; 08-10-2022 at 09:29 PM. Reason: Removed Pricing

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    Pricing not allowed per site rules.

  3. #3
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    What is the %RH in the basement and crawlspace??

    <50%RH and the elimination of all the wet spots are important. I assume that the crawlspace part has all the earth covered with plastic and sealed.

    The same applies to the a/c, no damp wet spots in the a/c drain pan. The idea of running the a/c fan on a low speed fan is an acceptable method of drying out the a/c. If you do not have a VS blower on the your a/c, the Santa Fe could be connected to the a/c duct to get a steady small flow throughout the ducts and home.

    Which Santa Fa do you have? What size is your crawlspace and basement in sq.ft. The other dehumidifiers size?? Any Fresh air ventilation? Location of the home?

    Another issue may be your a/c's ability to help dryout your home. A well setup a/c should remove enough moisture during a hot to also contribute to the drying out the entire home including the basement. What is the typical %RH in your home during a hot day with extended a/c operation? What temperature do you keep your home at and typical %RH in the home during the mild seasons of the year? Any carpeting, rugs, or wood flooring in the basement?

    Post the info at your info on the site, it helps us give you good suggestions.

    Regards Teddy Bear
    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"

  4. #4
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    Hi there.

    The home is located in Pennsylvania.
    The crawl space if fully cemented and consisted of 3 sections. The largest section has Santa Fe Advance90. The smaller sections have small dehumidifier units we got from Lowes. My husband has all three dehumidifiers draining into sump pumps.
    There is no ventilation. All the vents are closed so no outside air comes in. I do open the garage door from time to time but only if the humidity is below 50 outside.
    All the perimeter of the walls is covered with rigid core insulation which is supposed to be air tight and mold resistant.
    Also, all of the perimeter of the house has french drains.

    The RH is about 45% in all 3 sections of the crawlspace. No dirt, just cement everywhere. I wonder is some sort of ground gasses can penetrating through the cement?

    Not sure what the humidity inside the house is. It feels pretty dry with AC on. We keep it at 76 degrees. But I do open the windows for about an hour in the morning to let so fresh air in even if it's humid and even when the system is on.

    Yesterday we discovered an uncovered sewer pipe from an old sewer system and we had that capped today.

    The basement smell that comes into the house through the supply vents is not very strong but because the house is closed up, it starts saturating the living area and it feels pretty stinky to me. When the HVAC system was off, there was no smell, but once we turned the AC then the smell started coming in. I wonder if the HVAC guys missed some holes in the ducts? We noticed some minor gaps between the boots and the floors, so tomorrow, I'm planning to foam it.

    The first company patched the ductwork but left a bunch of holes behind
    The second company said that even if they fix the duct work, the house will still have the crawlspace smell.
    The third company sold us remi helo LED, and they did not help at all.
    The forth company pathed the holes again, reconstructed part of the ductwork and cleaned the coils. Told us to turn on the system and wait for a month for it to blow out and dry.

    Maybe I'm being impatient since it's been a week since the last company was on the job, but I've been dealing with the smell for a year, and I'm at my wits end.

    Also, I forgot to mention that we cleaned the ducts but that did not help either.

  5. #5
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    Hi there.

    The home is located in Pennsylvania.
    The crawl space if fully cemented and consisted of 3 sections. The largest section has Santa Fe Advance90. The smaller sections have small dehumidifier units we got from Lowes. My husband has all three dehumidifiers draining into sump pumps.
    There is no ventilation. All the vents are closed so no outside air comes in. I do open the garage door from time to time but only if the humidity is below 50 outside.
    All the perimeter of the walls is covered with rigid core insulation which is supposed to be air tight and mold resistant.
    Also, all of the perimeter of the house has french drains.

    The RH is about 45% in all 3 sections of the crawlspace. No dirt, just cement everywhere. I wonder is some sort of ground gasses can penetrating through the cement?

    Not sure what the humidity inside the house is. It feels pretty dry with AC on. We keep it at 76 degrees. But I do open the windows for about an hour in the morning to let so fresh air in even if it's humid and even when the system is on.

    Yesterday we discovered an uncovered sewer pipe from an old sewer system and we had that capped today.

    The basement smell that comes into the house through the supply vents is not very strong but because the house is closed up, it starts saturating the living area and it feels pretty stinky to me. When the HVAC system was off, there was no smell, but once we turned the AC then the smell started coming in. I wonder if the HVAC guys missed some holes in the ducts? We noticed some minor gaps between the boots and the floors, so tomorrow, I'm planning to foam it.

    The first company patched the ductwork but left a bunch of holes behind
    The second company said that even if they fix the duct work, the house will still have the crawlspace smell.
    The third company sold us remi helo LED, and they did not help at all.
    The forth company pathed the holes again, reconstructed part of the ductwork and cleaned the coils. Told us to turn on the system and wait for a month for it to blow out and dry.

    Maybe I'm being impatient since it's been a week since the last company was on the job, but I've been dealing with the smell for a year, and I'm at my wits end.

    Also, I forgot to mention that we cleaned the ducts but that did not help either.

  6. #6
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    Finding the sewer pipe is big. Any untraped drains will allow soil gas odors into the home. This includes traped drains that have dried out. Also soil pipes rot off and allow soil gas to enter the basement. The basement is dry enough stop any odor growing mold.

    Square feet of home?

    I suggest connecting a 6" dampered duct connecting the outside air to the Santa Fe Advnce. Run the fan on the Santa Fe 24/7 a fresh air change in 4-5 hours. Need to adjust damper and a restriction on the inlet to get small portion of outside air combined with basement air for ideal fresh air change rate. The amount of fresh air is like a like a bath room exhaust. Adjust the amount to make the basement fresh. This will work up through the home without odors after air change is high enough.

    https://www.santa-fe-products.com/wp...07/TS-1152.pdf

    This shows the Santa Fe with a return duct kit mounted on the unit. A 10" tee with a 6" dampered duct is connect to outside fresh air. Start with damper open with the fan on 24/7. Adjust with more air until odor is gone. To increase fresh air, restrict the open side of the tee with tape.
    Show this to your a/c tech.

    You have made many improvements and are close to fixing this problem.

    Where in PA are you located? I was there last week. Son lives in Allentown.

    Keep us posted.

    Regards Teddy Bear
    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"

  7. #7
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    Has a blower door test for house leaks been performed?
    or a duct blaster test for duct leaks?
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  8. #8
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    Quote Originally Posted by kdean1 View Post
    Has a blower door test for house leaks been performed?
    or a duct blaster test for duct leaks?
    Hi there.

    We did have a blower door test done that showed a significant amount of air leakage. However, when the second HVAC guy came and we told him about the blower door test, he said that it was done incorrectly by the first guy. I don't remember why, though.

    We did not do the duct blast test. The last contractor that we hired did a lot of work to the ductwork but he did not mention anything about duct leakage test. He did mention that he used his ear device that allows him to hear potential leaks in the ducts (I thought that was weird.

    Would you recommend the blower door test or the duct blast one? I'm so desperate that I'm going to inspect those ducts as carefully as I can myself. I have very little idea what to look for though.

  9. #9
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    Hi there!

    Thanks so much for your suggestion. We will definitely look into your suggestion with the Santa-Fe unit. I'll have my husband look at it carefully and see if he could rig it.

    We are in Danville, PA, which I think is about 100 miles away from Allentown.

    Our house is 3600 square feet, and is all one level ranch. Because is so large and long there are actually 2 separate units, and I'm assuming 2 separate ductwork systems.

    We will keep working on getting the musty smell down to a minimum in the crawl space. My question is why does the basement air comes through the vents? I'm assuming if the ductwork is tight, then no matter how the basement smells, none of the air should be getting in? Does it mean that there are holes in ducts that suck it up? Or no matter how tight it is, it will always suck some basement air in? I have no idea, since we never had HVAC before. I apologize for the stupid questions in advance.

  10. #10
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    Air mores slowly throughout the home including the basement/crawlspace through the floor and door undercuts. Floors are not air tight but have many imperfections.

    Duct kits for the return and supply are available for the Santa Fe dehus. Your house is large but the Santa Fe and 2 other dehus should handle the load. Get fresh moving through the dehu into the basement crawlspace during the mild seasons. During winter, the wind and stack effect should provide enough natural infiltration, not need the mechanical ventilation.

    Keep passing info on to the a/c techs.

    Regards Teddy Bear
    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"

  11. #11
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    Quote Originally Posted by beachpotatoe View Post
    Hi there.

    We did have a blower door test done that showed a significant amount of air leakage. However, when the second HVAC guy came and we told him about the blower door test, he said that it was done incorrectly by the first guy. I don't remember why, though.

    We did not do the duct blast test. The last contractor that we hired did a lot of work to the ductwork but he did not mention anything about duct leakage test. He did mention that he used his ear device that allows him to hear potential leaks in the ducts (I thought that was weird.

    Would you recommend the blower door test or the duct blast one? I'm so desperate that I'm going to inspect those ducts as carefully as I can myself. I have very little idea what to look for though.
    I would not discount significant leakage measured with a blower door test. Do you have a report from the test? If so, please share it with us. Only if an exterior door or window was open, or using the wrong ring on the blower door could it be badly in error. The blower door test may also have included some duct leaks since they're open to the house. You may be able to locate significant duct leaks with a visual inspection. There are other methods to identify duct leaks with some temperature measurements at the equipment and at the registers, but this doesn't locate leaks.

    I'm a big proponent of dehumidifiers as they will improve indoor air quality, even when the cooling system isn't in use.

    Still, it would be best to identify the source of the problem before undertaking any potentially expensive remedies.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  12. #12
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    Quote Originally Posted by kdean1 View Post
    I would not discount significant leakage measured with a blower door test. Do you have a report from the test? If so, please share it with us. Only if an exterior door or window was open, or using the wrong ring on the blower door could it be badly in error. The blower door test may also have included some duct leaks since they're open to the house. You may be able to locate significant duct leaks with a visual inspection. There are other methods to identify duct leaks with some temperature measurements at the equipment and at the registers, but this doesn't locate leaks.

    I'm a big proponent of dehumidifiers as they will improve indoor air quality, even when the cooling system isn't in use.

    Still, it would be best to identify the source of the problem before undertaking any potentially expensive remedies.
    Hi there,

    The first HVAC guy who did the blower door test did not give us any report and just sent some pictures of what he did. One was a picture of the door with a big fan that he mounted on it. The second was a pic of his measurements. It showed 3448 CFM, and another number of -25.8 Pa. I have no idea what that means. But the second HVAC guy said that that it seemed suspicious to him and he does not think it was done correctly. The third HVAC guys came in and visually inspected ductwork and patched some holes, but it did not do anything. The forth HVAC guy came with a crew of 3 people and worked on the ductwork patching and reconstructing it for 2 days. But the air still stinks of basement coming out of the vents. We also have Remi Halo LED but they did absolutely nothing.

    The last HVAC guy said to give it a month for the air to run threw the ductwork and the smell would go away. It's been a year since we began dealing with this. I'm losing hope that a month of waiting would do anything. It's been 1.5 weeks since the last HVAC company and the house still stinks of basement air.

    Also, we are at a loss why there is a smell in the basement. The basement is bone dry. The 3 dehumidifiers are running non-stop. The walls are insulated. The floor joists are nice and dry. Today I checked the humidity and it was at 45%.

    I would appreciate any suggestions.

  13. #13
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    Any ducts routed through basement soil? Occasionally we find supply ducts under the basement slab. The pan in the a/c that collects condensate not draining well. Un trapped condensate pipes that can allow back into the a/c ducts or dehus are all possible we spots that will growing odor producing bacteria.

    Another suggestion was mentioned, running the a/c blower 24/7 to dryout any damp spots.

    Don't forget fresh air ventilation, 100 cfm minimum 24/7 while maintaining <50%RH must be tried.

    A damp spot or major plumbing drain is lost the moisture seal to stop soil gas from entering the basement is also a possibility.

    A small dead animal in the duct or basement also possible. You mention french drain, are there wet spots in the open drain?

    Let us know what solves the problem when you fix it. Is the odor present during the winter?

    Keep us posted.

    Regards Teddy Bear
    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"

  14. #14
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    Quote Originally Posted by beachpotatoe View Post
    Hi there,

    The first HVAC guy who did the blower door test did not give us any report and just sent some pictures of what he did. One was a picture of the door with a big fan that he mounted on it. The second was a pic of his measurements. It showed 3448 CFM, and another number of -25.8 Pa. I have no idea what that means. But the second HVAC guy said that that it seemed suspicious to him and he does not think it was done correctly. The third HVAC guys came in and visually inspected ductwork and patched some holes, but it did not do anything. The forth HVAC guy came with a crew of 3 people and worked on the ductwork patching and reconstructing it for 2 days. But the air still stinks of basement coming out of the vents. We also have Remi Halo LED but they did absolutely nothing.

    The last HVAC guy said to give it a month for the air to run threw the ductwork and the smell would go away. It's been a year since we began dealing with this. I'm losing hope that a month of waiting would do anything. It's been 1.5 weeks since the last HVAC company and the house still stinks of basement air.

    Also, we are at a loss why there is a smell in the basement. The basement is bone dry. The 3 dehumidifiers are running non-stop. The walls are insulated. The floor joists are nice and dry. Today I checked the humidity and it was at 45%.

    I would appreciate any suggestions.
    The standard depressurization test is -50 Pa. If it was 3448 CFM at -25, then the real number would be considerably higher. A decent (not great) CFM50 number would be equal to the sq. ft. of your floor area. I prefer CFM50 rather than ACH50 (Air Changes/Hr.).

    I don't recommend any IAQ gadgets such as i-wave or UV for residential application. The important things are humidity control, filtration, and ventilation.
    But your issue seems to be more related to infiltration.
    I suggest getting a more accurate blower door test. Check the link in my sig for someone in the HVAC 2.0 group. They are more focused on practical solutions than the majority of home energy raters and utility company testers.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  15. #15
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    Quote Originally Posted by teddy bear View Post
    Any ducts routed through basement soil? Occasionally we find supply ducts under the basement slab. The pan in the a/c that collects condensate not draining well. Un trapped condensate pipes that can allow back into the a/c ducts or dehus are all possible we spots that will growing odor producing bacteria.

    Another suggestion was mentioned, running the a/c blower 24/7 to dryout any damp spots.

    Don't forget fresh air ventilation, 100 cfm minimum 24/7 while maintaining <50%RH must be tried.

    A damp spot or major plumbing drain is lost the moisture seal to stop soil gas from entering the basement is also a possibility.

    A small dead animal in the duct or basement also possible. You mention french drain, are there wet spots in the open drain?

    Let us know what solves the problem when you fix it. Is the odor present during the winter?

    Keep us posted.

    Regards Teddy Bear

    Thank you! Will keep working on getting this resolved and will post with any updates!

  16. #16
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    Quote Originally Posted by kdean1 View Post
    The standard depressurization test is -50 Pa. If it was 3448 CFM at -25, then the real number would be considerably higher. A decent (not great) CFM50 number would be equal to the sq. ft. of your floor area. I prefer CFM50 rather than ACH50 (Air Changes/Hr.).

    I don't recommend any IAQ gadgets such as i-wave or UV for residential application. The important things are humidity control, filtration, and ventilation.
    But your issue seems to be more related to infiltration.
    I suggest getting a more accurate blower door test. Check the link in my sig for someone in the HVAC 2.0 group. They are more focused on practical solutions than the majority of home energy raters and utility company testers.
    Thank you for your suggestions!

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