By the time it reaches the supply vent, it might be a bit warmer, depending on where the ducts are located and how well they’re insulated. How cold is conditioned air? When it comes off the cooling coil, it’s usually about 55 to 60 degrees Fahrenheit. That means as long as the vent itself is warmer than 55° F, you shouldn’t see any sweating. If your indoor temperature and relative humidity are at or close to any of those rows in the table above, your dew point is about 55° F. A supply vent at a temperature of 60° F or below will sweat in that case. At a temperature of 75° F and relative humidity of 60%, for example, the dew point will be 60° F. That means you’re more likely to get condensation because the supply vent doesn’t have to be as cold. Conversely, if you keep the house warmer, you have to keep the relative humidity lower or the dew point will rise above 55° F.īut what if you’re not on or close to one of those lines leading to a 55° F dew point? The higher your relative humidity is, the higher the dew point is at a given temperature. As you can see, the colder you keep the house, the higher the relative humidity can be to keep your 55° F dew point. If you don’t keep your house at 75° F and 50% relative humidity, the table above shows the relative humidity levels that correspond to a 55° F dew point when you go below or above a temperature of 75° F. Those conditions correspond to a dew point of 55° F, our special number. Those conditions are 75° F for the indoor temperature and 50% relative humidity. Well, it’s special if you keep your house at or close to the indoor design conditions recommended by the Air Conditioning Contractors of America (ACCA). Dew point and the temperature of conditioned airīut how do you tell which is the culprit? Is the dew point too high or the supply vent too cold? The special number here is 55° F. That means the fixes are simply reducing the humidity of the air or raising the temperature of the vent, depending on which of the two reasons is causing the problem. Either the air is too humid (dew point too high) or the supply vent is too cold. Now, as for the reasons, we can start with the two big ones. That’s simple, right? Humid air with a dew point above the temperature of the supply vent causes condensation. The threshold for condensation to occur is that the temperature of the cold surface has to be below the dew point of the water vapor in the nearby air. If your air conditioner supply vents are sweating, you have a cold surface (the vent) and near it you have air with water vapor in it. Condensation occurs only when humid air is in contact with a cold surface. There are a lot of reasons why this happens, but there’s only one cause. And if you’re really lucky, you wonder why I’m even writing about this because you’ve never seen it. If you’re lucky, it’s just an occasional nuisance that doesn’t lead to long-term damage to the supply register, indoor air quality issues, or the need for towels on the floor. Other times it leads to the paint coming off and rust forming on the vent (photo at bottom). Sometimes it leads to mold, as you can see in the photo above. It didn’t take me long to find the problem.Īir conditioner vents get cold. Another one was on the other side of the bed, and those two vents were dripping steadily. It hadn’t just fallen out of the laundry basket it was there to catch the water dripping from the air conditioner supply vent in the ceiling directly above. The homeowners took me into the master suite, where the first thing to catch my eye was the towel folded neatly on the floor. Back in the summer of 2005, I got a call to figure out a moisture problem in a new house.
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