Chimney: B-vent on top of masonry possible?

I decided to start a new thread for this question. I had originally posted questions about this chimney on 4/7/2012 using the subject heading "Chimney - is repair possible?".
Here is the specific question that I have now:
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 the chimney in question submitted an estimate that included tearing the existing chimney down to the roof line, then 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 then, I have been doing tons of research and I cannot find anything -- no codes, no descriptions, etc. -- that would address this particular situation.
And, 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 and a 3-inch metal flue from the gas-fired hot water heater. 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. For those who may not have seen my earlier thread about this chimney, it is actually two chimneys in one:
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: http://tinypic.com/r/t8rtac/5 http://tinypic.com/r/9unmvb/5.
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I'm not sure I understand the geometries here. I would think a 8 x 8 flue is measured on the inside. In which case, I would think a 7" round stainless steel liner would fit. I would think they might want more space to make installation easier, allow for difference along the way. But if the flue is aligned, straight, not screwed up, I would think you could get a 7" liner in.

That's one that you'll probably have to ask your local code official about as it sounds like an uncommon situation. If it has to pass inspection, I'd make sure the inspector agrees it's OK. Has the company proposing this done it before? The obvious question with part B pipe, part chimney is how you correctly connect the two at the transition point. Not only from a flue gas point of view, but also so that it's something sturdy to hold the B pipe, keep water out, rugged enough to last, etc. You don't want a shyster joing them together with just some sheet metal that's going to crap out in 5 years.
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snipped-for-privacy@optonline.net wrote:

Unfortunately, the 8 x 8 is the exterior size. The inside dimensions on these are variously listed on different websites as being 6 1/2, 6 5/8, or 6 7/8 -- but none say 7 inches. All of the chimney people that I talked with so far say that a 6-inch metal liner will fit inside and 8 x 8 clay flue but a 7-inch metal liner will not.

They claim that they have done this and it's no problem, but I am not convinced about that and would really like to be able to find anywhere that describes this type of setup and says whether it is okay.
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snipped-for-privacy@optonline.net wrote:

P.S. Here's one link I found about clay flue outside and inside dimensions:
http://www.koppclay.com/clay_flue_lining.html
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TomR wrote:

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: http://inspectapedia.com/chimneys/Chimney_Flue_Size.htm .
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).

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responding to http://www.homeownershub.com/maintenance/chimney-b-vent-on-top-of-masonry-possible-693699-.htm hoodzies wrote: Did you end up getting this work done? I have a setup similar where I have a masonry chimney in the house, with a metal direct vent chimney above the roof line. I actually want to replace the smaller direct vent gas stove I have with a b-vent stove (which I already own), but it's hard to find anyone that will do the work, even though I am fairly certain it can be done, even if I had to put class-A chimney pipe above the roof and run a 4 inch liner through the masonry chimney up through the new metal chimney. Anyway, just wondering how you made out?
-Mike TomR wrote:

--

Mike



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