Condensation on our Venting

July 18th 2002

Question from Barbara Kelly in Sherwood Park, Alberta -----

Dear Shell,

We moved into a new home almost 5 years ago. The past 2 winters I have noticed a white, milky liquid leaking down the outside of the metal chimneystack in the basement whenever the temperature is very cold outside. (Previous winters were not as cold.) This liquid would drip onto the basement floor and was sticky. There are also white "drip marks" down the sides of the chimneystack. I checked with the company that installed the furnace and their serviceman said that the liquid was the by-product of natural gas condensation. He suggested that I put a "drip tray" below the elbow on the furnace stack to catch the liquid. I did this for the first year. The liquid eventually dried to a powder form. However, this past winter there was an increase in the amount of liquid coming down the stack - also longer spells of cold weather. I called ATCO Gas and explained the problem, hoping they could give me an explanation and/or solution. They suggested I contact a company that installs furnaces. This time I called another furnace company, one that I have used for several years. The serviceman that came out agreed that the liquid was the result of natural gas condensation, and if I understood him correctly, the exhaust gases were not clearing the chimneystack before condensing. He took the stats for the furnace, hot water tank and the boiler we have for radiant floor heating in the basement. He told me that he would calculate the height the chimney should be for the existing appliances and asked if I knew how tall the chimneystack was from the roof up. I did not know the height. When he called back he said that he thought the height was close to being correct but he would have to go onto the roof and actually measure the chimney. He said that we may need a "kit" to correct the problem or worse, have to go through the drywall to correct the problem. This occurred in the early spring and so far I have not had him back to do the measuring. Before I proceed any further, I would like your opinion. When I listened to your Saturday show a few months ago, a gentleman called in asking about ice build up around his chimney at roof level. You advised him to call Bob Tessier. I wrote down Mr. Tessier's cell number but do not want to call him unless I could use you as a reference. Any help or advice that you can give me would be greatly appreciated. I would like this problem fixed before this winter begins.

Thank you.



Dear Barbara,

Your problem is caused by condensation as your second service man explained. It is not however, caused by “natural gas condensation” but is the result of the combustion byproducts not venting properly. The venting gases are being allowed to cool too much before they make their way out of your chimney. Water is one of the byproducts of combustion and if the gases are allowed to cool, the water vapors turn to liquid. Improper venting will result in condensation regardless of the fuel used.

To correct the problem, they may have to properly size the venting/chimney by installing a chimney liner to reduce the chimney vent size. Please feel free to go ahead and call Bob Tessier for assistance. He is very knowledgeable and will be able to help you.

It’s Just That Easy!

Shell Busey