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  1. #1
    Kelly williams's Avatar
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    Default Negative slope - furnace flue

    I just had an inspection done on a home we are buying and one item was to repair the negative slope in the furnace flue. The sellers are offering us a lump sum to take care of the issues - this is one of them. Can my somewhat handy husband do it himself? What on average would this cost?

    Thank you!!

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    Default Re: Negative slope - furnace flue

    Quote Originally Posted by Kelly williams View Post
    I just had an inspection done on a home we are buying and one item was to repair the negative slope in the furnace flue. The sellers are offering us a lump sum to take care of the issues - this is one of them. Can my somewhat handy husband do it himself? What on average would this cost?

    Thank you!!
    I don't think anyone here would know what your husband can do, even if he is handy.
    Get estimates from an HVAC company.

    ' correct a wise man and you gain a friend... correct a fool and he'll bloody your nose'.

  3. #3
    Kelly williams's Avatar
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    Default Re: Negative slope - furnace flue

    Here is a picture from the report in case it helps anyone give advice.

    Thank you

    Attached Files Attached Files

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    Default Re: Negative slope - furnace flue

    Quote Originally Posted by Kelly williams View Post
    I just had an inspection done on a home we are buying and one item was to repair the negative slope in the furnace flue. The sellers are offering us a lump sum to take care of the issues - this is one of them. Can my somewhat handy husband do it himself? What on average would this cost?

    Thank you!!
    Get an estimate from a HVAC company, ask for that amount from the seller, and have the HVAC company fix it. In their inital inspection ask them to take a look at the HAVAC unit while they are at it. You never know.


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    Mark Fisher's Avatar
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    Default Re: Negative slope - furnace flue

    Looks like a forced draft unit to me; is slope even an issue? Or isn't a slight down-slope to the exterior beneficial to keep condensate from running back to the furnace?

    Mark Fisher
    Allegany Inspection Service - Cumberland MD 21502 - 301-722-2224
    Home Inspections, Mold Testing, Thermal Imaging

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    Default Re: Negative slope - furnace flue

    I'm kinda leaning towards agreeing with Mark - you won't want the condensate running back into the furnace......Best to check with a licensed and qualifier HVAC installer / contractor, but NOT the one that did this work 'cause you already know what "their" response will be.


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    Default Re: Negative slope - furnace flue

    Disagreeing (against my better judgement) with those above...

    High-efficiency furnaces *are* designed to have the condensate flow back into the furnace, where it is then drained by gravity out of the heat exchanger and into the condensate pump you can see on the lower-right of the photo(I wonder where the line leading to the left goes?).

    The risk (as I just read on another thread here today) of draining/sloping to the exterior is freezing and ice damming in winter.

    As for whether your husband can do it, probably a question better addressed to him. It is, in essence, basic plumbing. Nevermind that it's attached to a furnace.

    Aside: man I would hate to have/work on a crawl-space unit like that.


  8. #8
    Bill Barnes's Avatar
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    Exclamation Re: Negative slope - furnace flue

    I inspected a home last December. The furnace was a down flow unit installed in a small room just off the garage. The PVC flue and combustion air pipes ran up into the attic then through the attic horizontally about 25 feet. The pipes terminated at the gable end wall and were connected to a concentric vent connector. At the furnace I heard what was a gurgling sound but no condensate was being drained. I went back outside for a closer look with the furnace operating and was the recipient of a gush of water from 12 foot overhead. It seems the vent pipe would collect condensation until the exhaust blower would dispense of it through the concentric. After a closer look, the pipes were on an ever so slight downward pitch toward the concentric vent so condensation could not be drained back to the condensate pump at the furnace. I made recommendations and continued to "drive-by" monitor the house through the winter. I saw a mass of ice building up on the brick ledge of the wainscot directly below the vent on several cold days that I had driven by. I saw the new owners out one day and asked if they had done anything with the recommendations I made. The answer was a disturbing no. But I knew what the answer was. I showed the owner a photo of the ice build up on my phone and a short time later I saw that they had made a change. It appears now that the vent and combustion air pipes are vertical and through the roof right above the furnace. I can't wait to see the owners out again to ask what the impression of the HVAC tech was when he saw the original install. I hate to think that the condensate could have actually frozen in the vent and blocked it but I'm sure with the right wind and extreme cold temps it would be possible between cycles.

    My point to this long story is to have that flue pipe looked at by a trustworthy, qualified, experienced HVAC technician.


  9. #9
    Kelly williams's Avatar
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    Default Re: Negative slope - furnace flue

    Thank you all! Looks like I will have to wait until I get in town to find an havc guy and get a quote- thank you for all your responses! You have given me a lot of questions to ask when he comes!!


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    Default Re: Negative slope - furnace flue

    Quote Originally Posted by thaddeus cox View Post
    Disagreeing (against my better judgement) with those above...

    High-efficiency furnaces *are* designed to have the condensate flow back into the furnace, where it is then drained by gravity out of the heat exchanger and into the condensate pump you can see on the lower-right of the photo(I wonder where the line leading to the left goes?).

    The risk (as I just read on another thread here today) of draining/sloping to the exterior is freezing and ice damming in winter.

    As for whether your husband can do it, probably a question better addressed to him. It is, in essence, basic plumbing. Nevermind that it's attached to a furnace.

    Aside: man I would hate to have/work on a crawl-space unit like that.
    I agree with Thadeus. Every installation of a high efficiency furnace I have seen has the exhaust pipe sloped back towards the unit as they have built-in condensate drains to collect the condensate. The location of the ducts is going to result in extra work for somebody to correct the run.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

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    Default Re: Negative slope - furnace flue

    Kelly I agree with Mr.Cox! High Effieciency furnaces "are" condensing furnaces with slightly corrosive condensate. You don't want to dump that outside your house. Furthermore, as he stated freezing up of condensate can be an issue in winter weather if it gets that cold where you live. I would hire a pro to do it and have the seller pick up the cost whether your Hubby can do it or not.


  12. #12
    Bob Harper's Avatar
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    Cool Re: Negative slope - furnace flue

    As with any appliance, refer to the listed instructions but yes, they usually prefer the condensate drains back to the unit. If there is a low point, it will form a trap.

    Can't see any primer on those joints. Inadequate pipe support.

    Is that unit listed for multi-poise mounting? How is the secondary heat exchanger condensate drain run? Any leaks? This looks like a crawlspace. Is this unit approved for this application? How is the condesate managed so it doesn't freeze up? What about the termination?

    Intake not ducted to outdoors and does it have a screen that complies with mfrs specs?

    Keep the fire in the fireplace.

  13. #13
    Mark Fisher's Avatar
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    Default Re: Negative slope - furnace flue

    Quote Originally Posted by thaddeus cox View Post
    Disagreeing (against my better judgement) with those above...

    High-efficiency furnaces *are* designed to have the condensate flow back into the furnace, where it is then drained by gravity out of the heat exchanger and into the condensate pump you can see on the lower-right of the photo(I wonder where the line leading to the left goes?).

    The risk (as I just read on another thread here today) of draining/sloping to the exterior is freezing and ice damming in winter.
    Yes, I can see that sloping away can be bad if there is a way for the condensate to be trapped (as in an elbow or riser). But I've seen several units that have rusted due to backflow of condensate. There is a difference between 'forced-draft' and sealed combustion 'condensing' furnaces; I need to review my knowledge of how condensing units' heat exchangers work (beyond the fact that they extract additional heat from the water vapor present in the flue gases by condensation). Specifically, how they handle the condensate from that process as well as any backflow, in a system with both positive air pressure and passive (gravity) condensate drainage to the pump or drain.

    Mark Fisher
    Allegany Inspection Service - Cumberland MD 21502 - 301-722-2224
    Home Inspections, Mold Testing, Thermal Imaging

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    Default Re: Negative slope - furnace flue

    Quote Originally Posted by Mark Fisher View Post
    Yes, I can see that sloping away can be bad if there is a way for the condensate to be trapped (as in an elbow or riser). But I've seen several units that have rusted due to backflow of condensate. There is a difference between 'forced-draft' and sealed combustion 'condensing' furnaces; I need to review my knowledge of how condensing units' heat exchangers work (beyond the fact that they extract additional heat from the water vapor present in the flue gases by condensation). Specifically, how they handle the condensate from that process as well as any backflow, in a system with both positive air pressure and passive (gravity) condensate drainage to the pump or drain.
    My Carrier shuts down the blower for a brief period every so often to allow the condensate to drain out... and drain out it does! A surprisingly significant flow of water runs right out.

    This is one of the reasons why the orientation, plumb and level of the unit is important (drainage), otherwise you may end up with standing water in the HE.

    This is also why the HEs are warrantied for long periods/lifetime.


  15. #15
    Kelly williams's Avatar
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    Default Re: Negative slope - furnace flue

    Someone asked where this was located and the answer is: I assume crawl space bc next to it is a picture of the PVC pipe coming out of the porch is listed as the furnace flue.


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