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Chimney: Is B-vent on top of masonry chimney possible?

RonABCRonABC Member Posts: 24
Is it possible to just add a metal "B-vent" chimney directly on top of an existing masonry chimney -- without the masonry chimney below being changed or any metal liner being placed in the masonry chimney?

The reason that I am asking is that one of the many chimney companies and masons that I had look at my chimney submitted an estimate that included tearing the existing chimney down to the roof line, then just adding a triple wall stainless steel B-vent on top of the existing chimney from the roof line up. According to them, the B-vent would go directly on top of the existing 8"x8" clay flue with an adapter, and no new liner would be placed in the existing chimney. This, of course, would be assuming that the existing chimney and clay flue from the roof line down is all in good condition.

Since they gave their estimate, I have been doing tons of research and I cannot find anything -- no codes, no descriptions, etc. -- that would address this particular situation.

Yes, it would make sense to at least add a liner to the existing chimney if possible, but an issue has come up regarding that. The issue is that going into the chimney is an existing 7-inch metal flue from the gas-fired steam radiator heating unit (187,500 btu) and a 3-inch metal flue from the gas-fired hot water heater (36.000 btu). Apparently, since a 7-inch metal flue is already going into the 8"x8" clay chimney flue, the minimum new metal chimney liner size would have to be at least 7 inches. But, a 7-inch liner will not fit into an 8"x8" clay flue. So, a new metal liner or B-vent cannot be placed inside the existing masonry chimney without removing the existing 8"x8" clay flue all the way down.

So, again, my question is, "Is it possible to just add a metal "B-vent"

chimney directly on top of an existing masonry chimney -- without the

masonry chimney below being changed or any metal liner being placed in the

masonry chimney?"

P.S. This is a side-by-side twin home, so this is actually two chimneys instead

of one -- one for each house. The height of the chimney is about 10 feet up

from the roof line.

Here are two photos: .


  • STEVEusaPASTEVEusaPA Member Posts: 1,498
    My opinion...

    That chimney has to go, and it will, either mechanically, or catastropy.

    It should be taken down, then you could use it as a chase for your vent pipes, following manufactures recommendations for roof terminations (height, distances, etc.
  • HenryHenry Member Posts: 722

    Where are you located as there are different rules for different areas. In Canada, we permit tile lined chimneys and with a certified adaptor going into an insulated B-Vent that is certified for outdoor use. It is a new addition to the latest code. BTW, a 7 inch liner fits in an 8x8 tile chimney. Depending on your area, a truly insulated chimney might be necessary.
  • RonABCRonABC Member Posts: 24
    Chimney: Is B-vent on top of masonry chimney possible?

    Yes, I know the existing chimney has to go.  That's the plan -- take it down above the roof line where the existing chimney is bad.  The question then becomes what to replace that part with -- another masonry chimney, B-vent, etc.
  • RonABCRonABC Member Posts: 24
    Chimney: Is B-vent on top of masonry chimney possible?

    Thanks.  This is located in the U.S.A., East Coast -- New Jersey.


    I have been looking to see if I could find any adaptors that would go from a square clay flue liner to a metal B-vent, and I can't find any online anywhere.  My thinking is that if there is such a thing as a certified adaptor for making this transition, that would mean it must be do-able and okay to do.  But, I don't see any anywhere.  If I could locate such a thing, that would be great.


    When you say, "an insulated B-vent that is certified for outdoor use", that makes me think that is why the company that suggested this approach wrote in their estimate "triple wall stainless steel B-vent".


    The companies that have looked at this chimney so far have said that a 7-inch liner will not fit into an 8 x 8 clay flue because the inside dimension of the 8 x 8 clay flue is less than 7 inches.  Here's a link that indicates that they are correct about that: .  They have said that in order to insert a 7-inch metal liner, the existing 8 x8 clay flue would have to be removed all the way down, which is a big job and a big mess.


    So, that is why I am looking at the possiblity of doing what the one company suggested -- just add B-vent on top of the existing clay flue from the roof line up.  But, I would like to be sure that is okay to do.


    By any chance, would you happen to have any kind of link to the new code in Canada that you are referring to that specifically allows that?
  • SlimpickinsSlimpickins Member Posts: 314
    What SteveusaPa said

    I agree with Steve. In the 2nd pic, the chimney has a definite curve away from the roof. I wouldn't want to be the guy to lean a ladder against it to drop a liner. 
  • RonABCRonABC Member Posts: 24
    Chimney: Is B-vent on top of masonry chimney possible?

    Yes, the existing chimney will be taken down to at or near the roof line.
  • RobGRobG Member Posts: 1,850

    I understand what you are trying to do. Take down the existing chimney(s) to roof level, place a b-vent to clay transition fitting at roof line and run b-vent up to correct height to create proper draft. (this is all assuming that the liner is in good shape from the roof down) I would ask the company proposing this if:

    A. Is it code approved? Can you show me the code?

    B. Can you provide me with information on the transition fitting so that I may research it.

    C. Have you done this before, If so, where

    D. Ensure that a permit is pulled and all inspections are performed by the AHJ.

    If you do go this route I would also suggest that you build an insulated box around the B-vent to prevent potential flue gas condensation and for aesthetic purposes.

  • HenryHenry Member Posts: 722

    I saw two flues. It is a 7 inch liner that is needed but won't fit in an 8x8 exterior dimension. We always measure the interior as there has been variances on the thickness.

    To adapt the B-Vent or insulated S/S vent, we make a piece of square sheetmetal with a round piece. The rond piece gets to be assembled with a B-Vent or insulated chimney adaptor. It is a legal way to proceed. B.149 is not available online. You fall under NFPA 54 and the AHJ.
  • RonABCRonABC Member Posts: 24
    Chimney: Is B-vent on top of masonry chimney possible?

    Thanks Henry.  I think that's the ticket.  This morning I heard from two other sources that said that, while it is preferred to add a metal liner to the chimney if that is possible, it is also acceptable to just replace the chimney above the roof line with metal B-vent.  Then I went to a B-vent supply place.  They couldn't find a clay-flue-to-B-vent adapter piece in their catalogs, but they also do custom sheetmental fabrication.  They said they can custom make two sheetmetal adapter fittings (one for each flue) with a square end to slide over the clay flue, and a round end to go inside the two 7-inch metal B-vents that will replace the existing chimney.  
  • JackJack Member Posts: 941

    This flue can be downsized, depending upon height of flow and whether the vent connector is insulated. I believe that once you start dickering around with that flue it should be brought up to current code, and I am unaware of any code that authorizes simple extension. Maybe it will work, but I don't think it meets the code.

    Whenever you get into a venting re-model on an old appliance you have to ask if you are throwing good money after bad. I would suggest that you look at the cost/benefit of upgrading your system. Now is the time, before you spend a pile on an old system that it sounds like you are band-aiding together. Get the whole picture before you move ahead.

    Perhaps Tim will chime in on this. I'd like to hear from him on this one.
  • HenryHenry Member Posts: 722
    Chimney height

    He has to extend the height of the chimney to meet code regulations regarding the slope and distance from the roof to the chimney.
  • RonABCRonABC Member Posts: 24
    Chimney: Is B-vent on top of masonry chimney possible?

    Just an update in case anyone is interested:


    I later found out that, with natural gas appliances (my steam heat boiler and hot water heater), it is permissible to use a smaller liner than 7 inches inside the original chimney chase -- even if the connector pipe going into the chimney now is 7 inches -- as long as the input BTU's of the appliances going into the chimney, and the height of the chimney/vent etc., are calculated correctly using a standard chart regarding the code requirements. Here's one link with those requirements:


    In my case, the total BTU's going into the chimney/vent are 224 thousand.  And, given all of the other height specifications etc., I can use a 6 inch liner in the existing chimney -- which will easily fit inside the 8"x8" clay flue that has an inside dimension of 6-5/8 inches across. That means that I can have a 6 inch aluminum liner installed inside the existing flue and a 6 inch double wall "Type B" (B-vent) coming out of the top of the existing chimney (after the existing chimney is torn down to just above the roof line). 
  • HenryHenry Member Posts: 722

    Don't use an aluminium liner! They can be damaged during installation. Use only a S/S liner!
  • STEVEusaPASTEVEusaPA Member Posts: 1,498
    Henry is spot on...good point!

    After all of that, go with the SS. Aluminum can even be damaged during transport
This discussion has been closed.


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