Bryan teams up with Jesse to explain why air conditioning ducts, units, and vents sweat. Bryan explains how that condensation happens, and Jesse shows how to fix it in the field. Moisture issues on vents, ducts, and equipment can cause fungal growth and may indicate a larger humidity problem.
A vent will sweat because its temperature is hitting the dew point of the air, and it may be caused by humid air intrusion around the vent. To fix that, you can remove the vent and try to seal the boot to make sure humid air can’t seep out through cracks. If you still have problems with a sealed vent, you have to start thinking about the temperature of the vent and the dew point of the air in the space. The dew point can change as the air stratifies, so a destratification fan can help. Otherwise, you’ll have to control the humidity in the space.
Running lower fan speeds will result in a cooler vent, but it also removes more moisture from the air (by running the air over a colder coil and improving the SHR) and reduces the overall relative humidity. Many technicians increase the fan speed to warm up the vent, but that’s not a fix-all approach because it doesn’t reduce the relative humidity as much. Overall, you want to assess air leakiness (with a blower door) and ventilation, and you’ll want to see if the occupant behaviors are introducing excess humidity. Keep the dew point in mind as well; to see what the dew point is, you will need a high-quality psychrometer.
Sweating ducts in an attic, crawl space, or basement is a common problem in our market, especially in ventilated attics. In cases like the one Jesse shows where the air handler is also in the unconditioned attic or crawl space, the air handler can sweat so aggressively that the secondary drain pan fills up. Jesse determined that the attic had a 77-degree dew point, which is very high and will cause condensation to form on any surface below 77 degrees. To control the temperature and humidity in an attic, we would typically seal the attic completely, remove the blown-in insulation, and install a dehumidifier. This video shows an UltraAire SD12 dehumidifier, which does NOT increase the sensible heat and has managed to keep the attic at 40% relative humidity.
As the attics get hotter, the dew points can get higher even if the relative humidity stays low. To prevent supply ducts from sweating, you might consider using ducts with a higher R-value or doubling up on the insulation. Gaps in insulation will result in condensation, so the insulation needs to be consistent. You can also drop the dew point in the attic, which can be achieved through ventilation. However, poor ventilation can make the situation even worse. (Those cases include dryer vents, bath fans, or kitchen exhaust leaking or venting into the attic.) Poorly vented attics can be vented better (NOT with power-vented attics). If you seal the attic with foam, you must dehumidify and condition it as Jesse showed. Dehumidifiers provide constant dehumidification, though they typically remove less moisture than the A/C system; proper equipment sizing is vital to make sure the occupant gets the most reliable dehumidification possible.
Low airflow can also contribute to duct sweating, as it keeps the surface temperatures cool. Poor airflow can be due to a dirty filter or coil; those problems have simple fixes.
In general, the worst duct sweating we’ve seen has occurred in pretty well-insulated attics with radiant barriers. Moisture tends to get trapped in those attics, and the low temperature caused by the radiant barrier keeps the duct and unit surfaces below the dew point. Ventilation (done right) allows more heat to conduct into the duct jacket, keeping the surface above the dew point.
The equipment usually sweats when it’s kept in an unconditioned (or poorly conditioned) space. When there are gaps and cracks that lead to the attic, moisture can come in. An enclosed space that is otherwise sealed tends to cause equipment to sweat, especially if the equipment also radiantly cools the surrounding walls and causes condensation to form on the walls. If you’re going to seal off a space, it should be conditioned; ventilation will leave you at the mercy of the outdoor dew points.
Here are some other great sources for information on this topic:
Videos from Retrotec on Blower Door and Duct Testing
https://retrotec.com/youtube/gallery/
The Energy Conservatory (TEC)
Ultra-Aire Dehumidifiers
https://www.ultra-aire.com/
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/
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Awesome tips Bryan and Jesse. This sweating seems to be more of an issue than it used to be with the lower efficiency units. Also, slowing the blower speed seems counter-intuitive but makes perfect sense. Thanks!
I just opened up my furnace and I immediately see rust and water spots inside of it.. my furnace & air handler is in an upstairs closet. When I looked in the closet the other day, I saw condensation on the refrigerant lines (I believe that's what they are? 2 copper pipes leading into where the evap coils are).. is being In a closet causing this? Or what other issue might be causing this? I started looking into this because I'm getting high humidity(nearly 70%) in house & my a/c seems to be running almost constantly.
CHECK FOR UNHOOKED OR DAMAGED DRYER VENTS
My HVAC had a 6 inch duct with a blower pulling air from out side in to the main ducts. South Texas humidity is horrible year round. Vents sweating from day one of new house. Rust and mold all the time
I live in an apartment in TEXAS, and too many times in the last 9 months it's been raining in my hallway from the ceiling AC unit. The main units (one for each apartment) are outside the building (I'm guessing they are the heat pump type since I get heat and AC from the same unit).The outside units aren't too old, but these apartments are 30+ yrs old, and I'm sure the inside wirings are old and outdated. I live on the ground floor of a building with 8 900+ sq foot apartments–4 upstairs, 4 downstairs.They keep "fixing" it, but it's a chronic problem. I'm sick of it–it's over 100 degrees outside, and the only way I can get it to stop raining inside is to turn off the AC. I work nights, and the constant intrusions of workmen trying to figure it out are seriously interfering with my sleep.
What would you do with a damper handle that is condensating? I thought about just cover the entire damper with another section of the ductwork.
With the infinity system in the attic and system and ductwork were sweating in dehum mode. Should the ductwork be sized for the 1st or low stage compared to maximum out put to prevent sweating?
"when you sweat too much :)"
Nearly every system I have ever seen that suffrered sweating vents had one of two [if not both] problems: the conditioned space suffered from negative ventilation, and/or some jackwit upsized tonnage too much, and the a/c doesnt stay on long enough on average to dehumidify. In the case of the latter, temps drop so fast the t-stat clicks off before the space is…conditioned. They were probably chasing problem #1 to arrive at the mistake, combined with substandard insulation.
If you ever want to see a bad setup… the maritime industry is littered with them: kitchens with exhaust running 24/7 and no make up air, engine room exhaust causing massive air changeout in adjacent spaces every time a door opens, etc.
I measure the doodoo point.
How about taking a diffuser in the attic with low cfm from the duct in order to lower dew temperature point and balancing the temperature in the attic with room below it.
I have a commercial building in very hot very humid region . The design is providing humid air via fresh air fans without any means of cooling or drying . It just pumps the humid outside air into DX Concealed FCUs plenums.
The result is vents are sweating, relative humidity is high …
How about checking the statics pressure in the supply and return
I'm in Central Florida with the air handler in the garage. There is not a chance in hell of getting humidity down to the 40-50% range. It's straight up 80-90% humidity most of the year. My ducts on the air handler are sweating bullets and it's ruining the ducts, there is water all over the floor and the bottom and corners of the ducts. What do I do? I already have some big water damage on them.
We have a 2005 Norris single wide. We got a new 3 ton Train inside/outside unit with Train Cleaneffects whole home air purifier. We got it because we love the house very cold and didn't want to wait hours for it to get to set temp. At first we kept it 67° day and 59° night. The temp. difference caused our new LVP flooring to have gaps so we stopped doing that. We set the temp. to 70° all the time and got a really good portable ac unit for our bedroom to keep it ice cold. A few months later we noticed the drywall is a little mushy- the inside of exterior walls in bedroom are condensating. Do you know how I can fix this issue (with out increasing the temperature of ac)?
Well done sir, the more you get into hvac you realize know body knows for sure but who is the smartest in the room suggesting goes as fact based on theory 🤔. Temperature pressure relationship is an exact science. Sizing and troubleshooting is hypothesis based on that,no one gets it perfect, too many variables.
Bust out the sling sychrometer
So the moral of the story is you're rarely going to fix this problem unless you remove the equipment and prep the space, equipment and ductwork like mentioned. Alternatively make sure the contractor specs the space to be conditioned prior to new equipment. Very nicely done video and thank you for all the time you spend preparing these.
Thanks for this video! I have this problem. Attic is vented but now sweating for the first time.
Lanten heat is change of state
Hey Bryan, I'm a Building Scientist in Florida as well. Great video! I wanna add something for anyone in the comments that has helped me.
I typically like to grab the dew point outdoors along with the attic dew point for a good comparison. Dew point in a vented attic can only, at best, be the dew point outside. If DP seems really high, but is the same as outside, at least you know the attic itself is performing well. It's a good isolation strategy. I would note that if you just use a hygrometer while on site, it's only a snapshot in time and you won't have the whole picture of what could be happening in other parts of the day. Using data loggers over a period of time are better to catch trends (putting one in the attic, in the conditioned space on each floor, one outside, etc.).
Yeah great video. I see this often in texas. Mostly vents sweating inside the home. The boots also must be insulated well.