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  1. #1
    Join Date
    Jun 2008
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    PVC vent pipe for gas equipment NG?

    Recently I've heard and read some troubling stories about PVC pipe breaking down (turning brittle and breaking) when stack temperatures are over 140. Just wondering if anyone has experienced this type of catastrophic failure?

    Apparently polypropylene is the safe alternative.

  2. #2
    Join Date
    Feb 2010
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    In a boiler room
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    7,135
    The PVC pipe manufacturers do not recommend their pipe be used for venting combustion gases. But that is what everyone uses. Go figure.

    On properly tuned condensing furnaces, the stack temp should never exceed 140, but boilers are a different story.

  3. #3
    Join Date
    Jan 2006
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    muncie Indiana
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    510
    Im not sure that Ive ever had a condensing furnace run a stack temp over 140.

  4. #4
    Join Date
    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by racingfan View Post
    Im not sure that Ive ever had a condensing furnace run a stack temp over 140.
    Several reasons can cause the flue temperature to exceed 140. This is why every furnace should have the airflow varified and set correctly, along with combustion analysis performed.

    PVC manufacturers have not tested the piping for use with LP and natural gas furnaces, but the furnace manufacturers have.

    Condensing boilers should be installed with CPVC and not PVC, most if not all have changed there literature to this requirement, but should also be commissioned with a combustion analysis.

  5. #5
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    Apr 2006
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    Barrie Ontario
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    318
    Our code here requires we use PVC and CPVC venting for condensing gas fired appliances

  6. #6
    Join Date
    Jun 2003
    Location
    Chicago, IL
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    I have never seen PVC fail on a condensing appliance.


    I have, however, seen some pretty scary looking PVC on power vent water heaters. It usually turns brown and gets pretty brittle.

  7. #7
    Join Date
    Dec 2007
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    4,711
    Quote Originally Posted by turkey View Post
    Our code here requires we use PVC and CPVC venting for condensing gas fired appliances
    I thought there was a special pipe, 636 or something, that was required in Canada.

    we used to do pvc now it's DWV/black pipe, not sure(did not pay attention) if it is something special for furnaces or not.
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  8. #8
    Join Date
    Aug 2004
    Location
    S.E. Pa
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    6,204
    In honor of the Canadian std. 636, they approved a listing for polymeric vent pipe, S363 for use with Cat V appliances when approved by the mfr. and installed to the listing. We in the US have not adopted such a listing. There are many problems with glued plastic venting. One product that shows promise is polypropylene such as Inno-Flue from CentroTherm. It uses a sliding joint and is rated much higher. The main problems seem to come, not from proper operation but abnormal operation. It doesn't take much overfiring to begin damaging glued PVC. If it is not given room for thermal expansion, it cracks.

  9. #9
    Join Date
    Jul 2004
    Location
    Massachusetts
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    There are various rules that seem to be routinely broken. For example, the distance from an operable window or door for the exhaust vent for a Categoroy IV appliance is 1-foot or manufacturer's specification, whichever is greater, right? Hmmmm. That's true but only up to 100,000 Btu's. Above 100,000 Btu's the distance increases to a minimum of 3-feet. What about return air for a furnace. Use the existing ducts? Most manufacturer's require (2) returns on furnaces of 100,000 Btu's and greater. So it is with PVC venting. The specs we've seen are as long as the stack temperature doesn't exceed 159F, the Sched 40 PVC is allowed. At 160F and above, CPVC or Poly pipe should be used. Some boilers have a selectable return water temperature or supply water temperature. The exhaust temperature for PVC can be exceeded if the water temperature is improperly selected by the installer or subsequently by a service tech.

    This can get to be dicey when replacing a gas boiler. For example, if the house has aluminum finned copper convection heat, at what water temperature and flow rate was the baseboard sized? And what temperature drop? If the return water temperature is restricted to 160F, then the likelihood of exceeding 160F is eliminated but arguably is running on the ragged edge. Of course, S-40 PVC is significantly less money than CPVC or Poly pipe. IMO, it's just a matter of time before S-40 PVC is eliminated as an exhaust pipe at all, just to remove the opportunity of someone over firing it and creating a bad situation.
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  10. #10
    Join Date
    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by skippedover View Post
    There are various rules that seem to be routinely broken. For example, the distance from an operable window or door for the exhaust vent for a Categoroy IV appliance is 1-foot or manufacturer's specification, whichever is greater, right? Hmmmm. That's true but only up to 100,000 Btu's. Above 100,000 Btu's the distance increases to a minimum of 3-feet. What about return air for a furnace. Use the existing ducts? Most manufacturer's require (2) returns on furnaces of 100,000 Btu's and greater. So it is with PVC venting. The specs we've seen are as long as the stack temperature doesn't exceed 159F, the Sched 40 PVC is allowed. At 160F and above, CPVC or Poly pipe should be used. Some boilers have a selectable return water temperature or supply water temperature. The exhaust temperature for PVC can be exceeded if the water temperature is improperly selected by the installer or subsequently by a service tech.

    This can get to be dicey when replacing a gas boiler. For example, if the house has aluminum finned copper convection heat, at what water temperature and flow rate was the baseboard sized? And what temperature drop? If the return water temperature is restricted to 160F, then the likelihood of exceeding 160F is eliminated but arguably is running on the ragged edge. Of course, S-40 PVC is significantly less money than CPVC or Poly pipe. IMO, it's just a matter of time before S-40 PVC is eliminated as an exhaust pipe at all, just to remove the opportunity of someone over firing it and creating a bad situation.
    Clearance to windows or doors that can be opened is 6'', 9'', or 12 " for direct vent in the US. Canadian standards specifications list 36 inches for above 100K.

    Most all manufactures use 1800 cfm of air to determine if two side returns or side and bottom air passages into the furnace are required for up flow applications. There are plenty of applications with 100K that do not require two inlets.

    Maximum operating temperature for PVC is 140, ABS 160, and CPVC at 180 degrees F.

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