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  • HWH exhaust backdrafts into furnace

    My parents' HWH vent (4") is spliced into the furnace vent (9"?). When the HWH is heating, there is some backdraft into the furnace vent - despite an active vent gate controlled by the furnace - and I can feel the hot air backup and smell the burnt gas. They called the gas service and got an all-clear - no CO, normal operation was the diagnosis.
    If a little backdraft is OK, how much is OK? I still don't like it in any case.
    Any advice appreciated. Thanks.

  • #2
    Re: HWH exhaust backdrafts into furnace

    It is illegal to vent two appliances into the same flue.
    sigpic

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    • #3
      Re: HWH exhaust backdrafts into furnace

      Originally posted by NHMaster3015 View Post
      It is illegal to vent two appliances into the same flue.
      Really? I have seen it many, many times, even in new homes. Water-heaters almost always share a vent with the furnace in homes I have seen.
      Water Heater Reviews & Water Heater Information

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      • #4
        Re: HWH exhaust backdrafts into furnace

        Originally posted by NHMaster3015 View Post
        It is illegal to vent two appliances into the same flue.
        Not under my Codes but you have to be careful to size them properly and design them so one will not impede the other.

        Mark
        "Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony

        I never lost a cent on the jobs I didn't get!

        Comment


        • #5
          Re: HWH exhaust backdrafts into furnace

          Originally posted by ToUtahNow View Post
          Not under my Codes but you have to be careful to size them properly and design them so one will not impede the other.

          Mark
          Exactly, as long as its sized and fitted correctly then you can have more than one appliance on a gas flue.
          Water Heater Reviews & Water Heater Information

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          • #6
            Re: HWH exhaust backdrafts into furnace

            a previous home of mine had a gas fired furnace, gas fired boiler and a gas fired HWH. all were installed by a licensed plumber and inspected by the local plumbing inspector. all were vented into the same chimney. when i questioned the inspector about this, he noted that the chimney flu was large enough to accomodate the appliances. never had a backdraft problem. is this prohibtion of multi appliance venting into the same chimney a relativley recent change in codes/procedure?
            there's a solution to every problem.....you just have to be willing to find it.

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            • #7
              Re: HWH exhaust backdrafts into furnace

              It could be done but can not be tied into the boiler flu unless the boiler flu is increased by 1/2 the size of the second appliance tie-ing into it.

              For example if the boiler's flu is 8" and the water heaters flu is 4"the common piping must be at least 10"

              8" size of the larger of the 2 flu pipes
              plus
              2" half the size of the second flu

              10" common piping required.

              If the common piping is not increased you can not dump a second appliance into the first.

              Comment


              • #8
                Re: HWH exhaust backdrafts into furnace

                Masterplumb:
                Your reply makes the most sense to me. In this case, there is no diameter increase. Just a makeshift "Y" was created by cutting into the bigger vent pipe - it's not even a manufactured part. I don't know if any of the 4" vent pipe juts internally into the 8-9" furnace flue. The furnace was replaced about 10 yrs ago, I assume an inspector approved the work. I would be surprised. With exhaust backing up into the living space, I still cannot understand how the gas company service guy didn't measure any CO however small.
                Thanks for all the feedback.

                Comment


                • #9
                  Re: HWH exhaust backdrafts into furnace

                  I would not assume that the work was filed and inspected.

                  It may not be dumping co at this time but in the future it could very well, and I am sure you are aware of the danger involved in this.

                  I would correct the issue now so I could sleep well at night

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                  • #10
                    Re: HWH exhaust backdrafts into furnace

                    I assume an inspector approved the work.
                    Never assume, a LOT of service/replacement work goes uninpesected unfortunately.

                    Masterplumb's decription is correct afaik.
                    Water Heater Reviews & Water Heater Information

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                    • #11
                      Re: HWH exhaust backdrafts into furnace

                      Originally posted by Service Guy View Post
                      Never assume, a LOT of service/replacement work goes uninpesected unfortunately.

                      Masterplumb's decription is correct afaik.


                      What is "afaik"?

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                      • #12
                        Re: HWH exhaust backdrafts into furnace

                        Originally posted by Masterplumb View Post
                        What is "afaik"?
                        AFAIK = as far as I know.

                        Mark
                        "Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony

                        I never lost a cent on the jobs I didn't get!

                        Comment


                        • #13
                          Re: HWH exhaust backdrafts into furnace

                          You can cut a W/H vent into the furnace vent if it's all gravity, but unless it's a higher BTU (50,000+) you can't go into anything bigger than 8". That's the 7* diameter rule. That said, it still shouldn't be coming out the furnace. Have you looked into the chimney to make sure you don't have a bird's nest or something going?
                          sigpic3:00, I mean 5:00, and work is done. Time to crack a cold one.

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                          • #14
                            Re: HWH exhaust backdrafts into furnace

                            Here we go again,forgetting ,It's a big country,with differend codes and amendments.

                            Hundreds of Homes in S.F. have 2 appliances sharing one flue!!!
                            I can build anything You want , if you draw a picture of it , on the back of a big enough check .

                            Comment


                            • #15
                              Re: HWH exhaust backdrafts into furnace

                              You're right of course. Natl Fuel Gas Code covers venting here.
                              sigpic3:00, I mean 5:00, and work is done. Time to crack a cold one.

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