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Thread: Water in Exhaust Vent - Gas Furnace

  1. #1

    Default Water in Exhaust Vent - Gas Furnace

    The 78 year old house I recently bouhgt was converted to a gas forced-air furnace in 1998. the exhaust vent for the furnace has a venter motor and the vent is routed over to and up through an existing brick chimney that extends beyond the roof of the house.

    It has been raining lately - not too heavy or too windy - and I noticed water leaking out of the insulation around of the exhaust vent. I removed it and discovered a rusted out vent pipe, which I quickly replaced - not wanting to have any CO issues. The water seemed to be coming in from inside the vent pipe - it didn't appear to be leaking from the hole in the chimney that it passes into.

    The vent extends to the top of the chimney and the rain cap is in place. Of course, water could be getting in the chimney from another location and coming in through the 90 deg elbow at the bottom of the chimney where the vent pipe enters.

    Before I break out the extension ladder and start climbing to the roof, I thought it might simply be condensation. I live in a humid part of the country and the furnace is in the basement - usually the most humid area in the house. The furnace is in our laundry room, adding to the humidity.

    During the summer when the furnace was not in use, we had some very strong thunderstorms but no water issues. Now that the furnace is in use, I have discovered the water and we have only had light rain but nothing serious. The other thing that makes me think of condensation is the very small amount of water - less than a cup of water over a couple of days. However, when the pipe was insulated, it held most of that water and it was enough to rust out the vent pipe over time.

    The water is clearly getting into the vent pipe somewhere after it enters the brick chimney - it is not from the A/C chiller (hasn't been on for weeks and is far from the vent) or the humidifier (water is not even connected).

    So, any experience with condensation in vent pipes or should I get out my ladder and flashlight?

    Last edited by eduke; October 27th, 2008 at 11:28 AM. Reason: clarity

  2. #2


    The exhaust from a furnace includes water vapor. Therefore it is possible for condensation to occur inside the exhaust vent pipe.

    Does your furnace utilize a heat exchanger to capture heat from the exhaust flow, typically using a concentric exhaust/intake pipe system or a box through which both furnace combustion intake and exhaust are routed? If the exhaust is cooled down too much, water vapor present might condense before the air makes it up the exhaust vent and outside.

    If your furnace does utilize such a heat exchanger, you might disconnect the intake duct from the exchanger to the furnace, preventing the cooling down of the exhaust and letting the furnace (temporarily) get its combustion air from the interior of the room (basement). (Open a basement window also). Now see if the water still occurs in your exhaust vent.
    Last edited by AllanJ; October 27th, 2008 at 10:06 AM.

  3. #3


    Thanks AllanJ.

    There is no heat exchanger for the vent - the exhaust vent exits the top of the furnace burner unit and "snakes" its way horizontally and vertically around the main furnace ducts (8' in all) to a brick chimney where it is then about a 30-foot vertical rise to the top of the chimney and the rain cap.

    The vent ducts leading to the chimney are very hot - if any cooling would occur, it would likely happen in the chimney. The horizontal section (about 5' long) seems to be keeping the water from flowing all the way into the venter motor. The potential for corrosion exists but certainly not to the extent it did when the same horizontal duct was wrapped with insulation that became saturated and eventually rusted out the duct completely.

    Is there another way to help the water vapor excape?

    Also, I simply replaced the ducts and elbows with the same galvanized replacement parts that were originally used - the fellow at Home Depot told me I really should be using a double-walled vent duct - that the construction of the duct provided insulation.

    Given what you have told me, it seems the double-walled duct would protect things close to the duct from heat but would also likely help keep the vent duct from cooling. I noticed the 90 deg elbow coming out of the brick chimney had a corrugated lining - as for the straight ducts that lead to the top, I really can't tell. Could this lining be the second wall of the type of duct the fellow told me about?


  4. #4


    Hopefully, I'm reading your message incorrectly. A single wall vent pipe must not be wrapped with insulation!

    Even a type B, double wall vent will have a 1" clearance requirement to a combustible material (will be stamped on the pipe).

  5. #5



    No, you read it properly - it was wrapped with a foil-backed, yellow fibreglas insulation - foil side out.

    I did not wrap the new duct with anything. I have only sealed the joints with foil tape.


  6. #6


    How big is this chimney? If it is a huge old monster, you may need a chimney liner the same size as the furnace pipe going all the way from the basement to the roof.

    And no, insulation on single wall pipe is a no-no

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