Chimney condensation

barrychanJanuary 4, 2010


I bought this new construction in 2007 at new york city and have been battling with lots of leaks since we moved in. Luckily most of the leaks are resolved by the builder who built it and those that can't be resolved are resolved with the awning solution (that solved my patio leak).

Just when I thought all the leaks have been resolved; my chimney started to leak around the flue pipe area in the basement. I have two water heaters and 1 furnace connected to the chimney. It is leaking around the where the flue pipe connects to the chimney. Pictures located at

It doesn't leak in the spring,summer,fall and would leak throughout the winter. It started leaking when there is no precipitation on the outside and as the temperature gets colder outdoors, the leak starts getting worse and will continue until the March time frame. Do you guys have any idea of what can be done to fix this?? The contractor caulked and check the flashing around the roof and added more weep holes in the chimney but that still did not resolve the problem and has been leaking for 3 winters. I am suspecting that newer efficient appliances makes it worse and I have a pretty tall chimney around 40 feet. Thanks for your help.

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hi barry -

I'm a fairly recent diy'er so you'd have to take anything I say with a grain of salt. I got into maintaining my own heating system (oil) in the last few years.

I believe you can get a lot of condensation in your chimney if the flue gases aren't hot enough coming off your furnace. I know heater service guys check the gases with a thermometer they stick in an access hole in the vent pipe - I think before the damper on the furnace side. The condensation I believe is bad for the chimney and can cause deterioration.

I can be going off on a tangent here. Hope some real pro's come to your aid.

Good luck!

    Bookmark   January 4, 2010 at 11:29AM
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You need to contact a good heating co. that can do a stack
sizing for you. You will probably need a flue liner put down
your chimney, and out into the basement and then have all your flues put into it. Your cement chimney is probably to large and also if it is a outdoor chimney it does not get hot enough to draft good. A new aluminium liner sized right will make it work right. later paulbm

    Bookmark   January 4, 2010 at 8:24PM
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Thanks champipple and paulbmfor the inputs. I was away for two weeks. I had a contractor come in and another guy from a heating company this past weekend. The contractor wanted to put another wall up where the flu connects in an attempt to keep the flu where it exits the wall as close as to the outside temperature.

This other guy from a heating company said to he would like to use double wall flue since he thinks the condensation is occurring where the flue exits into the wall. Now I understand what they are attempting to do and it sounds right to me (since i really don't know that much) but which solution would work the best?

Another detail I would like to add is the furnace and two water heaters are enclosed inside of a 5' by 10' room. Thanks!

    Bookmark   January 18, 2010 at 12:49PM
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I haven't gone with either solution. I asked around and someone told me they make a fan that would help blow the flue gases and maybe that would solve the problem. Do you guys know if I could buy such a fan at home depot? Another thing I did is I bought this foil insulation wrap to wrap around the flue pipes of the water heater and furnace. It seems to help but hasn't stopped the condensation.

any help is appreciated. Thanks.

    Bookmark   March 1, 2010 at 12:06PM
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chimney liner is the way to go. What is the size of the actual inside diam. of the chimney. how many btu geoing into the chimey, wat is the chimney pade out of on the inside? take that info to a supply house. Talk to a person behind the desk. Then have him recomend someone to check it out.

    Bookmark   March 1, 2010 at 12:23PM
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CJ Mechanical of North jersey llc.

chimney liner is the cheapest and best way to go.As wisehvac said the current size of the chimney you are using is most likly to large. calculations are in order. doublewall bvent chimney will work a bit better due to the double wall but more exspensive.

    Bookmark   March 15, 2010 at 4:14PM
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I am hoping that you figured out why your chimney was creating so much condensation and what you ended up doing about it? I have the EXACT same problem in Baltimore and would love your advice.
One HVAC person said I probably need a new chimney liner- the other thinks the flue is clogged and that should fix it? HELP!

    Bookmark   December 14, 2011 at 9:23PM
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