Ask the Engineer: Sweaty Ductwork

I love that scene in “Along Came Poly” when Philip Seymour Hoffman and Ben Stiller challenge two other guys to a friendly game of basketball. If you remember, Ben’s character gets matched up to the gentleman that Philip’s character refers to affectionately as “Sasquatch.” The name plays well with his physique, as he is rather rotund and covered in hair. You discover this when he removes his shirt to make himself more agile and aerodynamic as he matches up with Ben Stiller. There is that epic scene when the sweaty sasquatch goes up for a layup and Ben gets stuffed as he tries to hold his ground. The end result is Ben’s face plants into this guy’s hairy, sweaty, jiggly stomach. Ben executes a perfect look of disgust as the sweat drips from his lip.

Like Sasquatch, ductwork can also sweat in crawlspaces and attics. What’s going on with this? Why is it happening? Can it be fixed?

Usually when ductwork is sweating the same thing that happens to your glass of water when you are in a restaurant. The temperature of the glass is below the dew point of the grains of moisture in the air, causing the water molecules in the air to slow down and attach themselves to the surface of the glass. If your attic or crawlspace is hot or of high humidity or both, then the same thing  happens to your ductwork.

The normal cause of this happening is a dirty filter. What? Something as simple as a dirty filter can cause something that can ruin my ceiling or cause mold growth or lower the life of my unit? Yes, yes, and yes.

Let’s do a mental exercise: If your filter is dirty, what is happening? It is harder for air to pass through (ironically your filter is actually picking up more particulates when it is dirty). Less air means the volume of air passing through your coil is slower. Imagine taking your hand and dunking it quickly into a cold glass of water, if you felt your hand it would barely change temperature. But if you took your hand and slowly put it in the cold water, let it set for a few seconds and then pull it back out, you would feel a noticeable difference in your hands surface temperature. Well, as the air passes through the coil it gets colder than it normally would (by the way, you are killing your fan as it works to overcome the static of a dirty filter). That colder air then passes through ductwork with insulation (usually R-8) that is sized for the temperature delta of the cold air in the duct and the hot air outside the duct, the end result? Sweating duct.

A more difficult problem to remedy is that your ductwork is undersized or not sealed properly or insulated correctly. These all can be fixed but it isn’t nearly as simple as replacing a dirty filter.

If you run into this problem, and need some help, give us a call, we are a licensed and insured residential HVAC provider in Columbia, South Carolina.

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