Q: About four years ago I replaced my central air-conditioning system because the duct work rotted out due to condensation forming there. The company that replaced the old unit said the condensation was caused by me when I closed the grate louvers in each room over the winter months. I no longer do that, and yet I am experiencing the same problem. I didn't realize this until the paint started to blister in my kitchen, hall, bath and bedroom. Can you help?
-- C.P.
A: Condensation forms on cold surfaces. To stop condensation, either reduce the moisture levels going to the cold surface or warm the surface above the dew point. Insulation helps warm the surface of the ducts, but may not solve all the problems.
A vapor barrier on the outside of the insulation around the duct will stop moisture from reaching the ducts and condensing out. The ducts have to be sealed at all joints and seams to lower the flow of air to the ducts. Tightly sealed and insulated ducts should not leak.
There is the possibility that moisture forms inside the ducts, then leaks at a low joint or seam. A properly installed duct system will drain all the ducts toward the coil and fan system. Moisture in the coil box is drained through a pipe to a sewer or other suitable location. If there is too much moisture for the coil box, then water will leak out in the fan system and pool around the unit.
Because this is a known problem, all condensing units installed above a finished area of a home have to have a secondary drain pan drained through a pipe separated from the condensate pipe. Apparently you have not been talking to a professional installer. If I can figure this out, he certainly should be able to.
(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at PO Box 268, Evansville, IN 47702 or send him e-mail at d.barnett@insightbb.com.)
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