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updated: 7/14/2013 4:51 PM

Home fix: Condensate drainpipe leak could cause water stain

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By Dwight Barnett

Q. Recently I noticed a stain on the ceiling in the hall on the first floor of my home. Because the home is a two-story design, I had no idea a leak could be in the middle of the hall, but there is no leak on the ceilings of the second-floor rooms. The stain is not under or near a bathroom, but there is a chimney to the furnace in a closet on the second floor. I checked the chimney and could not locate a leak. Is this something you might have information on or a solution to my query?

A. When a furnace/air-conditioning unit is located above a finished area of the home, the installer should have placed a watertight metal "catch" pan under the unit in case there is a future condensate leak. When the air conditioner is operating, the interior coil produces several gallons of condensate water every day. Under ideal conditions, the condensate water should flow through a drainpipe to the exterior of the home, but often I find it flows to an open sewer pipe in a closet.

I have found the condensate drainpipes in crawl spaces where the water simply pools on the crawl space floor. In some cases, the drainpipe is connected directly to a sewer pipe in a basement or crawl space, but when winter comes and the pipe is dry, the furnace fan can pull sewer gases through the pipe and deliver the offending odors throughout the home. Neither situation is acceptable.

What I normally see is that the catch pan is drained to the same sewer pipe opening in the closet as the condensate pipe from the air conditioner. The two drainpipes cannot be connected to the same sewer opening. If the sewer pipe is clogged, the condensate will overflow the catch pan and damage all the ceilings and personal property below the second floor.

I have seen this happen and the damage is extensive. The pipe connected to the catch pan should drain to a viewable area outside the home, not to a gutter and not to a downspout drain.

I would prefer to see the pipe drain to a point above the front door or on the exterior just above the overhead garage door. The reasoning is, when you see water dripping out of the pipe, you know you have a problem that needs immediate attention. Another solution is to install a float switch inside the catch pan and plug any holes in the pan. When the pan begins to fill with water, the float switch will shut the air conditioner off and the production of condensate water will stop.

When the air conditioner is no longer cooling the home, you will most likely call a contractor to see what is wrong. Now that we have a solution to future problems, here's what you can do: Make sure the condensate drainpipe is not clogged. There should be an opening on top of the pipe that serves as a vent. Run a long, stiff wire or straight piece of coat hanger down the pipe to free any blockage. Check the outside of the furnace cabinet for leaks or water stains. Water stains on the cabinet or floor indicate an internal condensate leak.

There is a water tray inside the air-conditioning coil box on the furnace. Have a HVAC (heating-venting-air-conditioning) contractor open the coil box and check the tray for leaks or clogs. If the furnace filter is not maintained, dust and dirt that bypass the filter can clog the tray and drainpipe. If the unit is older, the tray may have rusted through. The tray can be repaired without replacing the entire coil, but if the unit is old, it might be wise to upgrade both the heating and cooling units.

• Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett at

Scripps Howard News Service

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