Musty/moldy smell from vents in home

2 years ago

Hello,

Here's a real stumper for yall that, so far, no one can figure out.

In early 2020, we had our HVAC unit replaced as well as all new ductwork. We live in Mooresville NC, a humid climate, and have a crawlspace. Our HVAC unit is a heat pump. A month or two after getting the new unit, we kept smelling strong musty smells from our vents. There was a 100+ sq foot section of the crawlspace, under a hall bathroom, full of mold. It was explained to us that the new hvac unit was strong enough now to pull in the smell whereas the old unit was not and therefore the mold was probably there for some time. We bought our house in October of 2018 and the seller did have a burst pipe fixed in that location. The south is notorious for crawlspace issues due to the intense humidity so none of this is a shock. Fast forward a few months-November 2020, and we have our crawlspace encapsulated, mold remediated, and a dehumidifier installed with the hopes that this would stop future growth should Another pipe burst, etc. Spring of 2021, we notice the smell come back when using the heat. We are told we need a "soil odor fan" as the smell may be coming from organismic growth between the soil and the vapor barrier. We sit on it over the summer because the AC does not smell and we're hopeful come winter 2021 we do not need it. Fast forward to October 2021, the heats on and it smells. Also, given we're in the south we do sometimes need AC in October and now the AC also smells. We try the soil odor fan and to no avail, more smell. So, it was uninstalled, and we were refunded. We discovered that the original mold is back, likely because it was not fully treated the first go-around...something was missed. The company comes back out, treats it, and it's gone. However, the smell is still present. My husband and I verified the mold is indeed gone and a moisture meter reads 10% everywhere in the crawlspace where wood is present. Now, six days ago, we have an HVAC air scrubber installed...this works for 4 days...and 2 days ago, the smell is back. All filters have been replaced multiple times, the HVAC unit has been cleaned, and the ductwork has been inspected thoroughly....no issues are noted with the HVAC system but the smell is only present when air is circulating. At this point, NO ONE knows what to do or who we should call for help.

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Charles Ross Homes

2 years ago

Hi, Lindsey,

Most homes operate at slight negative pressure due to a combination of HVAC system issues and stack effect. Duct work seams should be sealed with mastic designed for that purpose, but even meticulously installed HVAC ducts will leak. A leaky return duct can indeed pull moisture, odors, and soil gases from the crawl space and the air handler will dutifully distribute them throughout the home. Duct leakage can be tested (and quantified) with a duct blaster test.

Most HVAC replacement work and most crawl space encapsulation work are performed without the benefit of permits or inspections. And the majority of both are performed by individuals who don't know anything about basic building science principles, in my experience.

It may not make sense at first, but a sealed, conditioned crawl space actually requires mechanical ventilation in order to comply with the building code. I'm betting your crawl space doesn't have any ventilation-- which would otherwise dilute the concentration of soil gases and odors-- or has a return opening from the crawl space which is the non-sensical of the two code-prescribed methods to ventilate a conditioned crawl space.

The International Residential Code (IRC,) which forms the core of most statewide building codes, requires ventilation of "unvented" crawl spaces at a rate of 1 cfm/ 50 sq. ft. of floor area--which is not a lot. The prescriptive methods to achieve that are listed in section R408.3 of the code. Of the two prescribed methods, I recommend continuous mechanical exhaust. I've found that the other prescribed method--supplying air from the HVAC supply can create localized conditions which favor growth of mold and mildew.

Some of the earliest work evaluating methods to construct conditioned crawl spaces was performed by John Tooley in your home state of NC. I believe he's retired now, but he worked for Advanced Energy in Raleigh. I suggest you give them a call.

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Comments (13)

klem1
2 years ago
"It was explained to us that the new hvac unit was strong enough now to pull in the smell whereas the old unit was not"

That is a crock. There is no exchange of air in properly installed ducts. I don't know that ducts are causing it but if the problem is caused by duct it's because old duct didn't leak air and new duct does.
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Lindsey Hill
Original Author
2 years ago
Charles Ross Homes-makes sense to me. Something like this? Our home is small- 1200sq ft https://www.crawlspacedoors.com/store/fans/portable-fans/portable-fan-exhaust-system/
Lindsey Hill
Original Author
2 years ago
Also I did call them and he's retired But, they give free consultations s they're reimbursed by the power companies to do so. Thank you! I'm willing to hear anyone put at this point!
PRO
Charles Ross Homes
2 years ago
Hi, Lindsey,

I have the Fantech FR100 installed in my conditioned crawl space. It operates 24/7. I pull air through a floor register in a closet. Broan/Nutone makes a comparable in-line fan. I think you can purchase either as a kit including a mounting bracket, damper, clamps, etc.

Do check to make sure storm water is drained away from your foundation. It's a root cause of many crawl space issues.
Lindsey Hill thanked Charles Ross Homes
Lindsey Hill
Original Author
2 years ago
charles ross homes: Do you think we could install it ourselves? I'm so tired of spending money on contractors- My husband and I are both fairly handy.
PRO
Charles Ross Homes
2 years ago
There are lots of YouTube videos on how to install an in-line vent fan. Search by make/model to see if there's one for the fan you've selected.
Lindsey Hill thanked Charles Ross Homes
Lindsey Hill
Original Author
2 years ago
charles ross homes: How is this different from a soil odor fan? We tried that and it didn't work...
PRO
Charles Ross Homes
2 years ago
A soil gas vent fan would be connected to a perforated pipe in the soil under your soil vapor retarder or under a concrete slab. Its purpose is to exhaust soil gases (e.g., radon) before they enter the crawl space.

A vent fan for a sealed, conditioned crawl space introduces a continuous flow of conditioned air to prevent the volume of air in the crawl from getting stale, musty, humid, etc. It deals with the air volume above the soil vapor retarder.
Lindsey Hill thanked Charles Ross Homes
Lindsey Hill
Original Author
2 years ago
Charles Ross Homes: So, it's not connected to anything apart from the foundation, right? Thanks SO much for the help =)
PRO
Charles Ross Homes
2 years ago
You can install it at the foundation and pull air from the living space through the crawl space and exhaust it to exterior. Mine is installed with a flex connection to a supply grill in the floor of a closet. It "pushes" rather than "pulls" air into the crawl. It's the same configuration recommended by John Tooley a number of years ago. I suggest you confer with the consultants at Advanced Energy in Raleigh about the configuration they've found to work best in your area.
Lindsey Hill thanked Charles Ross Homes
cat_ky
2 years ago
Make sure your crawl space has plenty of vents in the foundation, and make sure all your new vents are sealed at all seams. You shouldnt be getting any mold under your house, if there is plenty of ventilation.
PRO
Charles Ross Homes
2 years ago
Ventilated crawl spaces are subject to mold/mildew growth depending on the temperature and relative humidity. The point of encapsulating a crawl space is to control the conditions in an effort to minimize the potential for mold/mildew. You don't want "plenty of vents in the foundation" in a sealed, conditioned crawl space. You want a single vent which is the exhaust of a ventilation fan exhausting at the code-required ventilation rate.

Musty/moldy smell from vents in home

Charles Ross Homes

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klem1

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