On the Level: Air conditioning drip due to condensation

I called a plumber who suspects it's condensation. He confirmed that the water meter isn't moving with all the inside water off so it's not a pipe leak. He punched nail holes into the dripping drywall to allow it to drain overnight so it won't form mold or mildew, but it didn't dry out in one night and is slowly dripping again today gaining more wetness and spreading the wet seam.

A: The plumber is right - it's condensation. Over the last week and a half or so we had almost record-breaking summer heat and humidity. I glanced at the Weather Channel early last week to see a dew point of 74 degrees at 8 a.m. That air is wringing wet and should such air encounter anything less than 74 degrees it will condense its moisture out and things that temperature or cooler will get wet.

I'm betting that you waited until it got to almost uncomfortable in your home before you switched the air on. The time until you felt the need for cooler air allowed warm, moist air to completely engulf the interior of your home, including the air inside the enclosed bulkheads that cover the metal air-distribution system. Those old air ducts are fabricated from galvanized sheet metal and as soon as the air conditioning lowered the temperature of the outside of those ducts to below the dew-point of the hot air inside of the bulkhead, the ducts started to sweat - and drip. It's not the cool air inside of the ducts that's causing the problem - it's the air surrounding the ducts.

Modern heating and cooling contractors like to use insulated flexible ductwork to distribute the conditioned air throughout the house for two very good reasons. The insulated duct greatly reduces the likelihood of causing the problems you've encountered and, secondly, the round flexible plastic sheathed ducts quiet the system down to a whisper and that appeals to homeowners who want to avoid air-conditioning systems that sound like a jet plane flying through the house as the system runs.

As the A/C continues to run and dehumidifies the air inside the house - including the air inside of the bulkheads - the dripping will eventually stop. The plumber did you a favor by punching the little nail holes to drain the condensed water out and as soon as it dries completely the opportunity for mold and mildew to form will evaporate, too.

My further suspicion is that there is a small dip or belly in the metal duct just above where you've seen the wet and the condensed water runs down the duct to the lowest point where it drips off and onto the bulkhead's drywall and shows up at a seam. If you never want that to happen again under almost any condition then you'd have to open the bulkhead and insulate the ductwork or, while keeping an eye on the dew point, as soon as you note that the outside dew point is creeping up near 70 degrees, turn the air on. One of the key advantages of air-conditioning is dehumidification. I recommend that you set the air at your desired interior temperature level and leave it alone until October. A 76 degree to 78 degree setting won't break the bank and dry air won't be uncomfortable. Then the house won't fill with hot, wet air while you're away from home only to cause mischief when you return and turn the air back on.

Duct cleaning doesn't really impact this situation but take a flashlight and peek into the ducts and see if they are really dirty. If so then consider it as part of general good housekeeping, especially if you have, or have had, pets.

Keep the mail coming. If you've got a question, tip, or comment let me know. Write "On The Level," c/o The Capital, P.O. Box 3407, Annapolis, MD 21403 or e-mail me at