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Started By
Message
AC System Moisture/Mold Issue
Posted on 3/13/25 at 2:38 pm
Posted on 3/13/25 at 2:38 pm
In Fall 2023, I got new American Standard units to match older but same size Trane units for both upstairs (3 ton) and downstairs (4 ton) for my 3100 sq ft house. Long story short, I’ve suddenly got a moisture/mildew issue in my plenum/duct work for my downstairs which I discovered after it started smelling when running in last 2-3 months.
Original installer came back and quoted me $12k to replace plenum/duct work/reseal return/reseal air registers/etc.
Second company came and give me their thoughts and suggested none of the above but said UV light to kill any current mold/mildew. Also pointed out that downstairs might be overpowered and wasted my time talking about all the shite the other company did wrong or not up to code.
Anybody ever deal with this before? How should I proceed? I am clueless when it comes to AC system troubleshooting.
Original installer came back and quoted me $12k to replace plenum/duct work/reseal return/reseal air registers/etc.
Second company came and give me their thoughts and suggested none of the above but said UV light to kill any current mold/mildew. Also pointed out that downstairs might be overpowered and wasted my time talking about all the shite the other company did wrong or not up to code.
Anybody ever deal with this before? How should I proceed? I am clueless when it comes to AC system troubleshooting.
9
Posted on 3/13/25 at 6:01 pm to CalcuttaTigah
Bumping, hoping someone can weigh in
Posted on 3/13/25 at 6:30 pm to CalcuttaTigah
Specifically, where is the mold? Is it on the outside of the ceiling registers? All over the house or just in bedrooms, bathrooms?
Short story is, you have a moisture problem and it has to be coming from somewhere.
Did the tech take a look at your inside coil?
Short story is, you have a moisture problem and it has to be coming from somewhere.
Did the tech take a look at your inside coil?
Posted on 3/13/25 at 8:28 pm to CalcuttaTigah
You probably do have a moisture problem that definitely needs to be addressed, but UB lighting is a pretty cheap investment to keep your units clean. We have a remi halo lights on the plenum side of the air handler that is supposed to naturally create peroxide that cleans the ducts. It also is supposed to cause any dirt/dust particles to clump up and become heavier which keeps the junk from floating in the air. We have noticed a difference with this system. We also have a cheaper uv light shining on our coils to keep them clean. They were only a few hundred dollars and we’ve been pleased so far.
Posted on 3/13/25 at 8:35 pm to damonster
Yes I will probably do that but trying to attack the source of the moisture issue is the challenge right now. I do wonder if there is a way to get definitive answer without spending $12k to figure it out.
Posted on 3/13/25 at 10:51 pm to CalcuttaTigah
Sounds like insufficient insulation and sealing of ducts. Not enough of either can create condensation at the register.
Posted on 3/13/25 at 10:52 pm to CalcuttaTigah
Just spitballing here as I don't have all of the data from your post.
Is it possible the newer units are higher SEER and since they are the same tonnage are now short cycling and not dehumidifying the space as well?
In souther climates, we count on a delicate balance of cooling, and not over-cooling, to maintain indoor climates at acceptable (IE 50% or less) humidity levels.
Have you gotten a cheap temp/humidity gauge and measured what the relative humidity is before an after the change?
Is it possible the newer units are higher SEER and since they are the same tonnage are now short cycling and not dehumidifying the space as well?
In souther climates, we count on a delicate balance of cooling, and not over-cooling, to maintain indoor climates at acceptable (IE 50% or less) humidity levels.
Have you gotten a cheap temp/humidity gauge and measured what the relative humidity is before an after the change?
Posted on 3/13/25 at 11:43 pm to CalcuttaTigah
There is always going to be some moisture with an AC unit. It’s just the nature of them and being so humid in LA. We have 2 units. One downstairs and one upstairs in the attic. The one in the attic can get like 120+ degrees in the summer. The environment is just conducive for mold growth. The uv lighting just kind of evens the playing field and keeps the units clean. Good luck!
Posted on 3/14/25 at 7:13 am to AndyCBR
I suspect this is an issue. What can be done if my system is basically too efficient? My smart thermostat says humidity is mid 50s or so.
This post was edited on 3/14/25 at 7:15 am
Posted on 3/14/25 at 7:20 am to CalcuttaTigah
quote:
wasted my time talking about all the shite the other company did wrong or not up to code.
You can't be this dumb?
Previous installer says $12k to fix an issue for a system he installed, you have mold and mildew, different tech comes out and points out problems with install, and HE wasted your time?
You need to call back the original installer and MAKE them do the work correctly.
Posted on 3/14/25 at 7:32 am to jmon
The guy was clearly pointing stuff out that was unrelated to the issue at hand and likely from original install (e.g. banding holding up flex duct work is supposed to be solid instead of perforated) as a way to build credibility and get me to pay him to do stuff which I already mentioned doesn’t even address the source of the problem.
The guy was telling me to bring them to small claims court.
The guy was telling me to bring them to small claims court.
Posted on 3/14/25 at 11:49 am to CalcuttaTigah
I'm not an A/C guy but worked on plenty of them. Here's where I'd start.
Check the owners Manuel for the correct intake/filter size, if it's too small for the new unit it may be an issue. This is a common problem when installing a bigger unit in an older house. You may need to increase the filter size or install another intake to increase air flow.
Get an instant read temperature gun and check the temp at all of your vents, the initial duct install may be wrong and they put in dampers to get more air to other vents.
Check the drain pan and drain pipe for blockage, if it doesn't drain correctly the system will pick up moisture from the coils sweating.
Check your smart thermostat wiring. Most new units have variable speed motors and if it's wired wrong it won't work correctly. Based on the tonnage of your units I suspect it's over sized and not running long enough to pull all the moisture out of the house. The smart thermostat when working properly "should" lower the blower motor speed to run long enough to bring the humidity down.
Most blower fans have three different speed settings: high, medium and low. When your system was installed, there is a good chance that the fan speed was set to high. If so, changing it to the medium setting may help reduce your high humidity issues by allowing the system to run for longer periods of time. You can find this in the installation manual.
There's also a possible duct leak pulling in humid air.
Good Luck, keep us posted.
Check the owners Manuel for the correct intake/filter size, if it's too small for the new unit it may be an issue. This is a common problem when installing a bigger unit in an older house. You may need to increase the filter size or install another intake to increase air flow.
Get an instant read temperature gun and check the temp at all of your vents, the initial duct install may be wrong and they put in dampers to get more air to other vents.
Check the drain pan and drain pipe for blockage, if it doesn't drain correctly the system will pick up moisture from the coils sweating.
Check your smart thermostat wiring. Most new units have variable speed motors and if it's wired wrong it won't work correctly. Based on the tonnage of your units I suspect it's over sized and not running long enough to pull all the moisture out of the house. The smart thermostat when working properly "should" lower the blower motor speed to run long enough to bring the humidity down.
Most blower fans have three different speed settings: high, medium and low. When your system was installed, there is a good chance that the fan speed was set to high. If so, changing it to the medium setting may help reduce your high humidity issues by allowing the system to run for longer periods of time. You can find this in the installation manual.
There's also a possible duct leak pulling in humid air.
Good Luck, keep us posted.
Posted on 3/14/25 at 1:55 pm to CalcuttaTigah
quote:
I suspect this is an issue. What can be done if my system is basically too efficient? My smart thermostat says humidity is mid 50s or so.
Mid 50-s RH is not terrible in South LA depending on how well sealed the house is. My system is properly sized and I see that in shoulder seasons but get into the low 50's and even 48% or so when the unit is running a lot in summer.
Pay attention to Radrob's post above and see if you can find out what the setting for the blower fan was set to at installation. A single speed unit has jumpers where the installer can set the blower to a couple of different speeds.
Your unit needs to run longer, at a lower flowrate, to dehumidify better since it is more efficient at the same tonnage as before.
This won't entirely correct a grossly oversized system but since you say you are mid 50's for RH you are in the ballpark to begin with.
Hope this helps.
Posted on 3/14/25 at 2:08 pm to CalcuttaTigah
I
looks like this person is going to need new systems and new duct work/air registers, etc...
quote:I am seeing this same issue in a friend's house in New Orleans...black all over the air registers...units are over 20 years old.
AC System Moisture/Mold Issue
looks like this person is going to need new systems and new duct work/air registers, etc...
Posted on 3/14/25 at 8:48 pm to CalcuttaTigah
I had 2 new Trane units installed during a recent remodel and we started smelling this musty smell when we switched from heat to a/c. Called my guy and he said it was prob “dirty sock syndrome”. Researched and found it’s a real thing caused by some bacteria or whatever growing on the coils. Factory ended up sending 2 new coils for the units and they were epoxy coated which supposedly inhibits the growth. So far so good. It’s been abt 2 months now.
Posted on 3/15/25 at 7:49 pm to CalcuttaTigah
It’s prob condensation
Posted on 3/15/25 at 8:25 pm to Duck Island
quote:this is common...happens every year from me when we transition from heat to cool...
we started smelling this musty smell when we switched from heat to a/c.
Posted on 3/16/25 at 10:14 am to Chicken
A/C guy said it dries up when you run the air consistently. I checked out the old coils when they brought them down and saw some growth on them. They smelled like arse, worse than the air coming out was.
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