Thank you, klem. Your explanation was very helpful.

I just started a service agreement with a reputable hvac contractor who has worked on the house in the past, so they can clean out the AC. A family member strongly recommended a service agreement in this climate, and I have seen first hand (twice) what happens to AC units that are not regularly serviced. I feel comfortable with that decision.

I can dig into the duct cleaning a bit myself.

I ask a lot of questions. It’s what I do. My questioning tends to weed out the snake oil salespeople. It’s my way of learning more about the people I choose to work with, while learning as much as I can from their expertise. I don’t spend any more money than I have to. I DIY as much as I can, and I am always looking to gain more skills. This house is a fixer with good bones, but it has different issues than my previous home that I remodeled, so there is a lot for me to learn.

When I walked into the house the first time, the carpet was moist. It was gross - old, wet, matted, spotted and clearly pet-friendly. My first priority was to dispose of it. The crawl space encapsulation was the right decision for many reasons - the home is much more comfortable, interior humidity is now about 45% making higher temperatures more comfortable, air quality is much better, and I can go into the crawl space and do my own inspections and DIY because it is clean and comfortable in there. I have also noticed fewer, if any, pests in the house. I paid a mid-range price, but there are not that many service providers here and I am happy to contribute to small businesses who have a documented history of serving my community well. I always do a cost-benefits analysis when I outsource. I am more comfortable when I can take care of the problems myself.

There is still a little bit of siding and subfloor that need to be replaced because it gets wet during rainstorms. I don’t think that is related because it is on the edge of the house,

I disagree. That wet subfloor is almost certainly the source or 'a' source of the odor. And if the subfloor is wet, you can be sure that the joists are also getting wet.

The vents were also sealed.

Therefore, no ventilation in the crawlspace to help dissipate the odor from the wet subfloor...

Odor from Air Conditioning Ducts

Maybe. But more likely the ducts are the conduit for distributing the odor that originates from the subfloor & walls thru the entire house.

I agree that the wet subfloor and joists are likely contributing to the odor problem. I also agree that the ducts themselves are likely to be transporting the odor (most HVAC systems operate at negative pressure and most ducts leak) throughout the home.

The terms "sealed" and "unvented" crawl spaces are confusing, even misleading. The applicable section of the International Residential Code (IRC) is section R408.3 which is labelled "Unvented Crawl Space." In it you'll find two alternative prescriptions for how to meet the requirements for an "unvented" crawl space by ventilating it. Silly, right?

If the contractor converted the OP's crawl space in conformance with the IRC requirements, it will either have a fan ventilating the space at a rate of 1 cfm/50 sq. ft. of crawl space area, or a supply from the HVAC system and a return to the common area (a duct or transfer grill.) With the latter option, there's a direct path for odors to flow from the crawl space. I think the code folks need to rethink that option. Returning air from a crawl space to a living area increases the potential for mold spores, insect droppings, soil gases, etc. in the crawl to be well distributed throughout the home. Not the best idea for ensuring the health of the occupants.

Most crawl space retrofits I've seen don't include either of the above provisions for ventilating an "unvented" crawl space.

If the odor is coming from air conditioning ducts then it's most likely dirty sock syndrome. (DSS) I know this sounds imaginery and made up, but Google it... it's well documented.

Basically what causes this odor (IF THIS IS YOUR PROBLEM) is that bacteria collect on the indoor evaporator coil of the AC. This bacteria then feed off the wetness of the coil, darkness of the cabinet.

The bacteria eventually emit a smell. The smell was coined DSS because it smells like a dirty gym locker area.

There have been many things done to aleviate this... such as cleaning the E-coil, even replacing the E-coil

However, cleaning the coil is merely treating the symptom and often times the smell returns a few months to a year later. The same thing occurs if you merely replace the E-coil.

Other more costly options are to remove the coil and have it cleaned then "dipped" in a chemical to prevent the bacteria from growing.

Some methods include adding UV lights to wash the E-coil with. However these things will increase cost as the bulbs must be changed annually to remain effective. Also some E-coil prove rather challenging to be able to hit all areas of the coil with UV light.

Additional studies were done, certain homes that were known to have the problem were tracked and when the home owners that had the gym locker smell moved, the smells went away in that particular home. This isn't to suggest that you're some sloppy pig... that's typically not how this problem exists.

It's bacteria that causes the smell. Again, if that is what you have. It has to be investigated, process of elimination. Maybe it's two fold the wet sub flooring generates the bacteria, the bacteria gets swept into the HVAC return making it's way to the E-coil and viola... there you go.

An easy way to determine if this DSS might be your problem. Have the E-coil "PROFESSIONALLY" cleaned. This is not DIY. ---- if the smell goes away afterward, then it's DSS.

I service the Katy, Texas area.

"This bacteria then feed off the wetness of the coil, darkness of the cabinet."

Well, not exactly. Bacteria need a food source in addition to moisture. In an HVAC system, that food source is dust, lint, and organic particulate matter that gets by the filters. If you can eliminate the food source, you take away a critical requirement for bacteria to survive. That's why enhanced filtration is a method often prescribed to reduce the potential recurrence of dirty sock syndrome.

That said, dirty sock syndrome isn't often encountered and when it is, it normally presents in HVAC shoulder season (when there is little need for heating and air conditioning.) The long off time between cycles gives the bacteria time to develop in numbers. Judging from the OP's post, the crawl space work was done a month ago which probably puts it in air conditioning season and the change in odor correlates with the performance of the work. While dirty sock syndrome is a possibility, it's not probable.

The higher the value of the filtration, then air flow issues can become a problem. What can you actually find? The highest Merv rating in a one inch filter: around Merv 13. There's still plenty of stuff that gets thru a Merv 13 filter.

So while it's a remote chance that it's DSS, it's still quite possible. Manufacturer's wouldn't have spent all the time they did investigating it. Better filtration: Another issue is cost.

I found a Merv 16 filter but it's a 5 inch wide filter for about $100, so you need a special filter rack to even think about using it.

Even at Merv 16, still plenty of stuff that can get thru. Typical size of bacteria is 2 micrometers. So even a Merv 16 will not stop bacteria, nor the finest of dust particles.

It's a little early to prescribe a solution when the root cause of the OP's problem hasn't been determined.

I do a fair amount of forensic work in homes. I find when investigating problems, it's best to begin by looking for the most probable cause and work down the list.

It's a little early to prescribe a solution when the root cause of the OP's problem hasn't been determined.

Thanks Mr. Charles Ross for that little nugget. Contrary to popular myth, I am not a psychic, fortune teller etc.

I don't investigate problems from the internet. This job is an "in person, on site job".

I believe I made it quite clear in my response that I am not certain what I suspect is accurate. This is why I said: "It needs to be investigated"... maybe in somewhat different words, that essentially mean the same thing.

This is a forum board. Reading and understanding what you are reading... is required.

@Austin Air Companie,

You raised an argument about the efficacy of mechanical filtration with respect to DSS. Since enhanced filtration is one method to reduce the likelihood of DSS, it seems inappropriate at this point to argue about it. Diagnosis comes before prescription.

Charles stick to home building, this subject is over your head. Either that inform yourself and google the subject, it's well documented.

As I stated earlier, homes were tracked... people from those homes moved and problems with DSS went away. That doesn't spell anything about better filtration.

People want to believe there is a "simple fix" to otherwise complicated problems. Anyone can play... step right up.

Consider your sources people, I know you will. ~wink, wink.

Actually, DSS abating after people moving out doesn't dismiss filtration as one method to mitigate its recurrence.

Filtration mitigates the potential for DSS by removing some amount of dust, lint and organic matter that would otherwise get to the coil and serve as food for bacteria. The amount of organic material on the coil would be dependent on the type of filter, the frequency of system maintenance--especially the frequency of filter changes-- whether the occupants had pets, and the particular occupants' level of housekeeping. Those variables all depend on the particular occupants and their lifestyle.

Well you're not going to mitigate bacteria completely with a filter. I posted the size of it further up and that size I posted is on the "larger" end of the spectrum for bacteria.

SO if you have even smaller particulate of bacteria than what I mention above... even more so that a filter will not stop it. Just as much a filter will not stop all dust. We're talking very small particulate here.

Again... this problem is well documented. Follow the science.

What people do in their own homes. What is a good source of bacteria in a home? (eliminate that source... often times this isn't realized until people move. -- it's been noted, but you have to "read it" and then understand what it is you're reading.")

Let's argue some more? It's a daily routine for some people. (reading between the lines can save you so much time... I make it really simple for you. ~wink, wink)

@Austin Air Companie,

DSS is real, but rare. The likelihood of DSS is small relative to odors originating in the crawl space being pulled into poorly sealed return ductwork. When troubleshooting potential root causes of any problem, it's best to look for the "horses" before looking for the "zebras."

Your assertion "This bacteria then feed off the wetness of the coil, darkness of the cabinet." suggests you don't know much about biological systems. Bacteria need food and water. Limit the amount of food and you limit the amount of bacteria. A filter doesn't need to filter out bacteria to be effective; it simply needs to filter out organic materials to the extent the bacteria don't survive.

While it's always fun filling in the gaps in your education, we need more information from the OP about what's going on in the crawl space if we're going to advance the troubleshooting task at hand.

A filter doesn't need to filter out bacteria to be effective; it simply needs to filter out organic materials to the extent the bacteria don't survive.

So the manufacturer's that spent all this time investigating this problem wasted their time?

You as a builder design systems for homes you build on paper. Within a year, maybe two you are long gone building something else?

This problem is a service type problem. I can go to nearly any home here even as young as 2 years old... pull the panel to the blower compartment and find fine dust. If it's in the blower compartment, it's on the coil.

However, just because I find dust does not make DSS. Bacteria is required to do that.

So your backward thinking is to say: let's ignore the bacteria and focus on stopping dust? Only on paper does that sound reasonable.

I don't live in an "only on paper" world. I know you do though and there's the rub.

@Austin Air Companie,

It seems you're obsessing over the solution to an unlikely problem. While it's not likely from the information related by the OP that they have a DSS problem, if someone does have a DSS problem, I'd recommend a stepwise approach:

1. Consider the existing, if any, routine HVAC maintenance schedule. Recurrence of DSS may be preventable by more frequent cleaning of your system's evaporator coil.

2. Consider housekeeping methods and frequency, particularly if you have pets. Our robo vacuum cleaner has markedly reduced the amount of dog hair in our home. They're cheaper than teenagers and they don't talk back to you.

3. Consider increasing the efficiency of your HVAC system filters You're not looking to filter out bacteria, but rather, to keep as much organic matter out of the system as is practical (i.e., without excessive pressure drop and loss in air flow.) LImit the bacterial buffet table, if you will.

4. Where the above measures are not effective on their own, consider adding UV lights at the coil to kill bacteria. These provide the additional benefit of killing many viruses and fungi so they may provide a health benefit in addition to reducing the potential for DSS.

As always, you'll be well served to consult an HVAC design professional who can review your particular needs, your particular system, and discuss the various options and costs with you.

@Austin Air Companie,

It seems you're obsessing over the solution to an unlikely problem. While it's not likely from the information related by the OP that they have a DSS problem.

Reading and understanding what you are reading?

The topic to this thread states:

Odor from Air Conditioning Ducts

Where do odors from HVAC equipment typically originate from? The OP stated these odors are coming from more than one duct / insinuating all ducts.

Do I know for certain that it's DSS? No. But it is a possibility... again you need to read things carefully and then understand what it is you're reading.

All you're doing Charles is digging a bigger hole for yourself. Go build something.

@Austin Air Companie

I don't suppose you have many crawl spaces in Texas, but they are typical here in coastal Virginia as are the attendant problems if they are vented crawl spaces. I do some forensic work, and I've spent more time than I care to in both unconditioned and conditioned crawl spaces. We've been designing and constructing homes with unvented/conditioned crawl spaces since 1999.

The OP reported a "musty odor" and conditions in the crawl space that are consistent with it being the most probable cause. Their HVAC ductwork is located in the crawl. Most duct work leaks and it's most likely that the HVAC system is pulling the musty odor into leaky return duct work and distributing it throughout the home.

Further, the OP said work to install a soil vapor retarder and dehumidifier was done a month ago. That puts it in air conditioning season. DSS normally presents itself in shoulder seasons where the HVAC system may not operate for long periods and conditions for bacterial growth are favorable. The smell is transported throughout the home when the system eventually turns back on.

When troubleshooting, look for the "horses" before deciding to look for "zebras." While DSS can occur, it's rare. Here's what the good folks at Texas Air have to say about it:

Dirty sock syndrome (DSS) occurs when your air conditioning unit begins to emit a moldy smell that is usually compared to dirty or sweaty socks. Dirty Sock Syndrome plagues 0.5 to 2 percent of heat pumps in the southern states, with Texas being on the lower side of the percentage.

https://www.texasairrepair.com/dirty-sock-syndrome/

Full disclosure: I have no ownership or other interest in Texas Air. Their service territory includes San Antonio, Houston, and the areas in between. That should provide them with a whole lot of real-world data on DSS, albeit that DSS is even rarer in Texas than other southern states.

We won't know the root cause of the OP's problem without additional information from them. They may have left the building. In the interim, I'll go with the more probably cause.

There is a saying in diagnostics: When you hear hoofbeats, don't go looking for zebras.

OP said that the subfloor & siding is wet. And that it has been raining almost every day.

If the siding & subfloor are wet, then the framing is wet. The insulation behind the siding is wet. And it is getting wetter because of ongoing rainfall.

Plastic was installed in the crawlspace and the vents were sealed. So no cross ventilation to disperse odor.

First things first. Deal with the known moisture problem. Wet subfloors are nothing to dilly dally around about getting fixed. Mold, rot = stink. And both are outside of the crawlspace encapsulation envelop.

Plus, we need to know if the crawlspace is conditioned space (with supply & return ducting) now that it has been encapsulated and the vents have been sealed.

ci_lantro is correct in pointing out that all sources of potential moisture need to be dealt with first. You need to tear out that siding and figure out how water is getting to that subfloor. Parts of that subfloor will probably need to be torn out, too.

Water is the enemy. Doing this may still not solve all your problems. But, this water issue needs to be dealt with first. I have a couple of additional ideas that may help. These come from my personal experience.

Like you, we encapsulated our crawl space in a home that had a combination of crawl and in-ground basement. The encapsulation company doing this for us warned us that if we began to experience a nasty smell, they could come back out and add an additional ventilation piping system underneath all that plastic. They said that it was 50/50 whether we would experience the phenomena. It depended on organic elements in the dirt underneath the house. It would not be an expensive fix, but they didn't recommend doing it unless we began to experience the smell. It's something that you should investigate. Make a call to your encapsulation contractor and discuss it.

I live in the Deep South and high humidity is a fact of life. Fighting mold and mildew is a constant issue. You said that you had a dehumidifier. Is it one of the freestanding models? If is is, I suggest the addition of a dehumidifier such as the Aprilaire system be added to your HVAC array.

Even with a permanent dehumidifier, you probably have mold or mildew in your ductwork. Once you reduce the humidity, the mold spores will become inert.

As one of our other posters has pointed out, UV lighting is ineffective. There is too much volume of air passing through the duct work for a UV light to do any good. I'm certain one of our contractors will suggest it and you may end up spending a lot of money to install one. It won't do the job.

The solution to mold in our home and ductwork was provided by my HVAC guy. Along with the Aprilaire dehumidifier and really good filters is the addition of a GPS system to your HVAC unit, Global Plasma Solutions. It's about $800. It's not a do-it-yourself unit. It does the trick. In about a week or so of running it in conjunction with the dehumidifier, my DH's mold allergies are gone. My seasonal allergy symptoms are reduced.

Many HVAC contractors are unfamiliar with the GPS unit because it is more frequently used in commercial applications.

My HVAC guy is a wizard. He used to instruct at the Carrier school. He suggested this to us two houses ago. It does a great job in conjunction with the dehumidifier and filter. I have recommended this unit to friends and they are happy with it, too.

Bottom line, you have to eliminate the moisture sources internal and external before you can eliminate the smell.

I don't have experience with the GPS system, but I do have experience with UV lights.

First, there are two different types of UV lighting systems: air sanitizing systems and coil sanitizing systems. The effectiveness of each depends on dose and duration. The air sanitizing system attempts to kill bacteria, viruses, etc. in the air stream. I had one in each of two different HVAC systems in my home for years and I remain a skeptic, but more powerful commercial systems have been proven effective. By contrast, a coil sanitizing UV system attempts to kill bacteria, viruses, etc. on the surface of a coil. Because it is continuously focused on a surface rather than an air stream, it's more likely to be effective, in my opinion.

5 days ago... the very first post that I posted that started all this furor over DSS.

People read what they want to read. As I said before, contrary to popular myth I do not possess a single ounce of psychic / telepathic / fortune telling power.

This job (HVAC) is an on-site in person job.

"IF THIS IS YOUR PROBLEM" --- yes in big all cap letters. But clearly even with that people still read what they want to and ignore the rest.

So because of all these other things... you're going to ignore DSS as a contributing factor that "might" be caused indirectly of wet subfloor? It's common sense you would correct these other problems as well as determining if DSS is a problem or not?

click to enlarge if necessary...

@Austin Air Companie,

At the risk of continuing the madness, this was the very first sentence in your very first post on this thread. You introduced the notion--despite evidence of more common causes--that the problem is most likely DSS:

"If the odor is coming from air conditioning ducts then it's most likely dirty sock syndrome. (DSS) I know this sounds imaginery (sic) and made up, but Google it... it's well documented."

I'm glad some others jumped in to help fill the gaps in your education, Ray. There's a lot of heavy lifting needed, and I was getting tired of doing all the work.

I'm familiar with Dirty Sock Syndrome in aviation. Sometimes hydraulic fluid, solvents or oil can get into an aircraft ventilation system and cause this event. It is described as smelling like a gym bag filled with dirty socks. The fumes can be deadly or cause permanent brain damage. I doubt that is what is happening in this instance. We know of water, mold and mildew in the structure. Those are causes of the issues. They must be remediated first before anything else is done. Then, solutions for the HVAC system can be found.

At the risk of continuing the madness, this was the very first sentence in your very first post on this thread. You introduced the notion--despite evidence of more common causes--that the problem is most likely DSS:

Charles Ross you started the madness by ignoring the title to this thread. Read & understand much?

Even in all that "likelyhood" I *CLEARLY* stated some doubt. No? Mr. Ross loves to defer you away from that "content" for what? Argue.

But you Mr. Ross, like to argue for the sake of arguing. That is your M.O. (Modus Operandi) in nearly every thread you participate in.

DSS is rare, doesn't occur that often... so we'll just ignore it as a possibility? This is Mr. Ross's advice to you. Let's not consider everything, funnel ourselves into oblivion.

Your choice: HVAC pro OR Builder

I make it really simple for you. (the silent reader who doesn't say a word, Charles Ross is not the reason for this post don't kid yourself.)

--------

Mr. Ross, I don't have a problem with what others posted. Only you and your constant drum beat against me. Because I know you like to keep thumbs in the backs of people like me... sitting in your ivory tower barking orders to the various sub contractors & GC's that you like to call "partners". I am much wiser to what really goes on in the home builder realm.

The main problem you have with me: You can't put a thumb in my back. I can read between the lines just as much as anyone else visiting these boards.

@Austin Air Companie

Troubleshooting is a systematic investigation of possibilities. The most efficient troubleshooting approach is to investigate the most likely cause first, and work down the list to the least likely cause stopping when you identify the source of the problem. Follow the evidence, if you will.

While the OP's problem could be DSS, it's rare, so I wouldn't suggest you lead with that. Further, when the OP relates that subfloor and joists are wet, and there's an odor in the HVAC system, it doesn't take a genius to connect the dots.

Moisture, and the attendant mold, mildew, and rot problems it can cause are fairly common threads here on houzz.com They are learning opportunities, Ray. I suggest you'll benefit much if you use them as such.

@Austin Air Companie

Troubleshooting is a systematic investigation of possibilities. The most efficient troubleshooting approach is to investigate the most likely cause first.

This is a forum board, there is no investigating to be done from a forum board. Only listing possibilities of those "to be investigated".

Clearly Mr. Ross you lost sight of this fact.

Investigations are done on location and in person, not from a forum board. I just listed a possibility to be investigated. You just like to argue without an ounce of investigating.

Now we'll wait and see if you finally understand this "simple logic" I've written here....

@Austin Air Companie

This hand's been played out, Ray. Time to make your way over the thread on combustion, ventilation and dilution air. It's another learning opportunity. Think of these as "halocarbon" moments.

https://www.houzz.com/discussions/6108237/should-this-screen-be-blocked-to-stop-air-flow-between-attic-house#n=21

Yeah nothing new here folks ---- the argument king (Charles Ross) is focusing his attention on a new argument.

Because? he's a failed 18 year Chemical Engineer who currently can't focus his attention on his current career choice of building homes.

So his occupation now is trying to argue with someone whom he fails to argue with on numerous occasions. His attempt now is just another failure on so many levels it's ridiculous.

I make it so simple for those of you who only choose to read and not get involved in the politics of this board. So simple.

I service the Katy, Texas area.

Wow. Thank you everyone for all the technical information!

I have been away from the forum working on electrical and other projects, so I am just now catching up. I saved all your comments and will study them before the hvac people come for their first maintenance service call.

The hvac company highly recommended the encapsulating service so those two companies are in synch. Both companies have 5-star reviews. The hvac consultant will talk me through any questions I have because I will need new A/C units once these go bad. I think the youngest unit is 10 years old and he knows I need to pay for a new roof, so he’s not going to recommend replacement until necessary. (We have already talked at length about a ductless unit upstairs and have decided it is not an option.) The crawl space also has a maintenance service agreement. I feel really good about the crawl space decision - the living space is comfortable, no pests to speak of, energy consumption has stabilized, and the crawl space is comfortable as well.

I still need to get the siding and subfloor fixed (as well as gutters after the new roof is installed). Everyone is having trouble finding workers now. I agree that the wet wood could be part of the problem, especially since the smell seems intermittant. It does seem to be the same wet dirt smell that is outside after a rain shower. In the house, I only notice it after I have been gone for awhile. So it could be wet wood smell circulating, in addition to whatever is going on in the A/C units and ductwork.

Thanks again!

Hi, kelli_ga,

Thanks for the update. Please let us know if the odor problem goes away after addressing the wet subfloor and siding issues or if it's the dreaded (and statistically improbable) "dirty sock syndrome." inquiring minds want to know.....

It does seem to be the same wet dirt smell that is outside after a rain shower. In the house, I only notice it after I have been gone for awhile. So it could be wet wood smell circulating, in addition to whatever is going on in the A/C units and ductwork.

Kelli_ga, Yes entirely possible. Eliminate sources and continue testing. While DSS is less common, it does occur. I've seen it at least a dozen times (in person, not just mentioned on a forum board) in my career. Not very much considering litteraly thousands of homes I've been in 'physically' over the previous 27 years.

I service the Katy, Texas area.

Not going to send you a sub-contractor. I will come in person when you reside in my service area.

Yeah, you might find a dead unicorn in the ductwork. Not likely, but possible.

Yeah, you might find a dead unicorn in the ductwork. Not likely, but possible.

If you do find one it's probably because he didn't get his jab.

I love to laugh probably a bit too much....