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)
https://energyconservatory.com/
Ultra-Aire Dehumidifiers
https://www.ultra-aire.com/
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