Why must furnace exhaust slope up?

Discussion in 'Plumbing Building Code' started by HowToLou, Jan 1, 2019.

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  1. Jan 1, 2019 #1

    HowToLou

    HowToLou

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    I know code says the exhaust vent off a furnace must slope up on its way out. Given that all of today’s furnaces have a fan forced exhaust, what it the reason behind that code? Thanks.
     
  2. Jan 1, 2019 #2

    frodo

    frodo

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    because that is the way the appliance was designed
    if you do not slope the pipe you will trip the vent gas sensor, and the unit will shut down
    if you bypass the sensor your warranty is void
    if you bypass the sensor and die, your insurance will not pay off

    technical bullshit
    if pipe is not sloped correctly it causes back pressure and the fumes will not exit the building
    first your eyes will be irritated, itchy
    then you will fall asleep and not wake up
    the fumes are odorless and colorless, and deadly

    did I mention you can DIE?
    I will also Mention that If you survive and a family member dies
    you can. After Investigators find the problem. be charged with man slaughter
    SO, Be sure you die along with your family so you will not be Bubba/RayRays new girlfriend

    Happy New Year
     
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  3. Jan 2, 2019 #3

    HowToLou

    HowToLou

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    Thanks for your answer and wonderful attitude. I actually knew all that. I was looking for a real answer from someone who knew exactly how the furnaces were designed.
     
    Last edited by a moderator: Jan 2, 2019
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  4. Jan 2, 2019 #4

    havasu

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    I believe your answer is within Frodo's post. Carbon monoxide is heavier than air, and will settle to the low point unless thermal dynamics assist you with the emission of these deadly gasses.
     
  5. Jan 2, 2019 #5

    frodo

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    ............
     
    Last edited: Jan 2, 2019
  6. Jan 2, 2019 #6

    Diehard

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    Carbon Monoxide is actually slightly lighter than air. Hence the reason for the required location of the detectors.
    Depending on the type of furnace, it's important to have the condensate to run back to the unit.
     
  7. Jan 2, 2019 #7

    Rickyman

    Rickyman

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    An inducer motor is designed to pull air/combustion gases through the the heat exchangers and push them into the vent then gravity takes over.
     
  8. Jan 2, 2019 #8

    Diehard

    Diehard

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    And how does gravity take over?
     
  9. Jan 2, 2019 #9

    Rickyman

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    Hot combustion gas rises. That’s why the vent has to be sloped upward
     
  10. Jan 2, 2019 #10

    havasu

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    I stand corrected. Thank you!

    Carbon monoxide (CO) is slightly lighter than air, so carbon monoxide alarms are effective when placed on the ceiling or high up on a wall. ... In fact, carbon monoxide is slightly lighter than air and diffuses evenly throughout the room.
     
  11. Jan 2, 2019 #11

    Diehard

    Diehard

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    Thought the exhaust duct would be under positive pressure due to the induction fan?
     
  12. Jan 2, 2019 #12

    Diehard

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    You may of been thinking Hydrogen Sulfide, which is associated with sewer gases.
     
  13. Jan 2, 2019 #13

    chiraldude

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    Code is code and you have to do what it says to pass inspection but I don't see any reason it would have to be one way or the other. Slope down would allow water to drain outside, slope up would have the water drain back to the furnace. The important thing would be slope so condensate couldn't collect in a low spot.
    When the furnace shuts off, there could be a slight back flow of exhaust on a down sloped pipe so a tiny amount of CO could enter the living space but only if using a 1 Pipe system. If you install a 2 pipe system with fresh air intake outside the house there would be no way for CO to enter living space. Additionally, if both pipes were installed sloped the same direction, there would be no convection flow either way.
    Only thing I can figure is that upslope is just "the way we always did it in the past so why change now" type of thinking.
     
  14. Jan 2, 2019 #14

    voletl

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    You never have a condensating furnace drain to the outside the condensate will freeze in the winter and block up the pipe.

    Regardless of what the equipment is the flue pipe has priority over anything else.
     
  15. Jan 2, 2019 #15

    chiraldude

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    Now that you mention it, I think I have seen this. I live in MN now and I have seen large icicles formed around PVC vent pipes during long cold spells. Sometimes it does look like it is getting close to blocking the pipe.
     
  16. Jan 2, 2019 #16

    voletl

    voletl

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    Had a guy terminate the pvc 8 inches from the ground he was afraid of the snow pileing up on it so he 90ed the pipe up and ran it against the house up 7 feet. That 3 inch pvc was frozen solid when I can to see what was wrong.
     

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