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MpjMpj Member Posts: 109
I am looking at a job that needs to have a boiler re-vented. The existing boiler is a W/M CGI-6 (167,000 BTU input) induced draft that was not vent properly through the wall (up against a pool heater). I need to vent it into an existing masonry (30' tall clay liner only) chimney. I spoke to W/M and they said I can increase the boiler vent at the flue outlet from 3" to 5" and it should be fine. I will be having the a liner installed prior to the re vent. There is also a 75,000 BTU water heater that needs to be vented into the chimney also. I have a couple of questions I am not sure about.

When the outside chimney (three sides exposed) gets the new liner does it become a type "B" chimney? If it does I will use table 13.2 (d) of the 2012 NFPA 54 code book.

The rise of the boiler flue will be close to 2' to the chimney breech and the water heater has a 1' rise. It looks like both vent connectors will be 5" single wall connectors. The common vent size (chimney liner) should have an area of 38 sq in which I believe is a 6" pipe.

Please let me know if my math and procedure is correct.



  • Tim McElwainTim McElwain Member Posts: 4,434
    No it does not become a

    "B" vent. You have to use the tables for the particular vent liner manufacturer's liner you install. Keep in mind most liners have 20% less capacity than a "B" vent.
  • MpjMpj Member Posts: 109

    Thanks Tim, I will speak to the chimney person and find out the information.
  • JackJack Member Posts: 1,045
    It might be a good idea

    to reline with B-vent. There are many single wall lined exterior chimneys that still condense like crazy. The double wall construction of the B-vent would better insulate the flue gas temps, and perhaps allow using one size smaller pipe, as you do not have the 20% derating of the corrugated pipe. Refer to your vent tables to confirm. If you go this way do not use Hart & Cooley Metalvent. They have a locking ring that increases the OD of the pipe and can become undone on the way down the flue. I'd suggest Selkirk or Dura-Vent
  • MpjMpj Member Posts: 109

    Thanks for the heads up Jack.

    I spoke the chimney person today and he told me the one that he quoted me on is a six inch smooth liner. he said since it is smooth that there is no need for the 20% reduction.
  • JackJack Member Posts: 1,045
    The smooth wall

    is only one of the benefits of the B-vent liner. Probably the greatest benefit is the double wall construction which will protect the flue gas temp and eliminate or greatly reduce condensation. What happens in a flue is that there is a period on start-up where virtually every appliance will condense. Called the "wet-time" successful vent design with a properly sized flue will allow the vent to warm up and draw more quickly. Exterior chimneys are the worst and a single wall liner can be problematic due to the lower temps it sees. Also, and I do not have a current copy of the sizing guide, but do you really need 6" on a 30' chimney. Tim...Help;)
  • Tim McElwainTim McElwain Member Posts: 4,434
    Using "B" vent all the way

    here is what I came up with:

    Your total BTU is  242,000 and it is Fan Assisted for the boiler for Boiler the water heater is natural draft. They can be vented together according to code and also the venting tables. I agree with Jack using a “B” vent to line the chimney is much better as it has a much lower “wet time” than the liner will have. If you have a  very short run of connector 5” can handle both the boiler and water heater. The double wall common vent if 5” with a FAN/NAT will handle only 210,000 BTU’s a 6 inch will handle 297,000 BTU’s so I would go with a 5” “B” vent connector and a 6” “B” vent common vent.
  • MpjMpj Member Posts: 109

    Thanks to you both. The chimney person said he has to use a flex liner because the chimney has a little twist to it. The water heater flue is about 4'-5' away from the chimney and the boiler is about 3'. Hoping to use two separate flues ( a tee in the chimney). The chimney person said the smooth wall can handle 260,000 BTU's.
  • JulJul Member Posts: 2
    Venting, Concentric venting kit.

    Not sure if a concentric venting kit would help you.  My mom is having the same issue and I came across this video:
  • Bob HarperBob Harper Member Posts: 856
    B-vent vs listed liner

    While most mfrs. allow relining a masonry chimney with B-vent, there are some drawbacks. First of all, look at the warranty. One is only warranted for one year and the best 25 yrs--there are many high grade stainless steel liners that come with a lifetime warranty and some are even transferrable.

    You cannot inspect B-vent outer casings and joints once installed. If you can get a video camera up B-vent, it can not reveal a loose joint.

    When you insulate a liner, it performs just like B-vent only the entire liner is insulated unlike the joint gaps with B-vent.

    A stainless steel liner can serve either gas or oil whereas B-vent can only serve CAT I gas. If you insulate it properly, it could also serve wood or coal if need be.

    A listed liner installed for oil or wood means it can support a 200lb dead load. B-vent is not tested with a top support plate as a liner so it must be supported from below using a listed base support. You can NOT simply cement B-vent into the wall as it puts stress on the tee snout or turnout elbow-liners can be pulled through and cemented to the wall. A B-vent liner would require a B-vent connector to be sleeved so it does not directly contact the masonry wall.
  • MikeGMikeG Member Posts: 154

    What is the best way or type of insulation to use on a SS liner?  My son has a clay flue liner in a masonry chimney up the outside of a house used for a wood stove.  It is cooling down too fast which makes the draft poor at best.  I would like to put a SS liner in it.  Is ist best to use a pre-insulated liner or is there some type pourable material that would work around a liner.  Hard to keep it centered for even coverage I'm sure.  I may also try to insulate the outside with some rigid foam, mesh and cement. Any good brand names.    Just looking for some ideas.   Thanks   MIke
  • JackJack Member Posts: 1,045
    Good points, Bob

    As always, there's more than one way to skin the cat. The base support on the B-vent T can be an issue and in fact when I did my sisters place I opened the wall up a bit. Then I supported the tee with a section of Unistrut. I pop rivet the joints as they are lowered being careful not to penetrate the inner liner.

    I have also done the flex liners and tried to insulate them. Honestly, I can't think of anything related to chimneys that is a bigger pain. The insulation hangs up and tears and I think you end up with more voids. This is especially so if the chimney is tight. I end up looking like Looney Tunes on the roof;) I'm very good at that!

    I haven't done one of these in some time and perhaps someone has figured out a better way of insulating, but I'd rather have the B-vent. It was just cleaner for me. Where oil is and is likely to stay I don't see it as a viable alternative in todays world. Hate to say that, but...!
  • JackJack Member Posts: 1,045
    I have had good luck

    with the Dura-Vent, Dura-liner product. I have it in my house currently. There was an old insert that consumed vast quantities of wood and produced no heat. I replaced that with a VC Encore and was easily able to adapt the existing Dura-liner install to the free standing stove. Excellent product in my experience. I represented Dura-vent back in the 80's when they introduced this product.
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