Q.I have an air-conditioning duct located inside a ceiling bulkhead covering the cool air distribution which when the air conditioner is on now leaks at a slow drip along the taped seam. It must be condensation but I don't know for sure. What else cou ld it be? Does this mean I have to have all the ducts cleaned? Should I take out the portion of saturated dry wall seam and see what's leaking? Who should fix this supposed condensation leak? The home is less than 20 years old. The air is electric and t he heat is gas. All units are new but the ductwork is about 18 years old and never had the ducts cleaned. I change filters regularly.

I called a plumber who suspects it's condensation. He confirmed that the water meter isn't moving much so it's not a pipe leak. He punched 5 nail holes into the dripping drywall to allow it to dry overnight so it doesn't form mold or mildew but it didn't dry out in one night and is slowly dripping again today gaining more we tness and spreading the wet seam.

A. The plumber was right-- it’s condensation. Over the last week and a half or so we have gone from a cool spring straight into almost record-breaking summer heat and humidity. I glanced at the the Weather Channel early last week to see a dew point of 7 4ºF at 10 O’clock in the morning. That’s air that is wringing wet and should such air encounter anything less than 74º 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 insid e the enclosed bulkheads that cover the metal air-distribution system. Those 18 year 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-poin t 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 the air surrounding the ducts. Nowadays heating and cooling contractors like to use insulated flexi ble 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 th e system down to a whisper and that appeals to homeowners who want to avoid air-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 of the house-- including the air inside of the bulkheads-- the dripping will 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 go away.

My 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 ne ver 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 of the dew point, as soon as you note that the outside dew point is creeping up near 70ºF, turn the air on. I recommend that you set the air at your desired interior temperature level and leave it alone until October. Then the house wont fill with hot, wet air while you’re away from home only to cause mischief when you return and jam the air back on.

Duct cleaning doesn’t really impact this situation but take a flashlight and peer 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.