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Vexed by Venting – What fits in a 6\" x 6\" chimney?

Uni R_2
Uni R_2 Member Posts: 589
Thanks Jamie, the capping could actually be easy and very functional (and a pain to screen). My concern would be the air quality available on the intake. I'm not sure how much residual it would get from 30 years of burning fuel oil.


  • Uni R_2
    Uni R_2 Member Posts: 589
    Vexed by Venting

    I’m having my oil-fired boiler replaced with a gas burning condensing boiler. My preference for a boiler is the Ultra 80 because it can be wall hung, the controls are ideal for the application and it modulates very low, however the venting is a major issue.

    By far the best option is to exhaust out the soon to be vacant chimney. Alternate options for the exhaust location are over the side walkway and the front driveway but both of those locations have major negative issues.

    The problem with venting through the chimney is that it is 6” by 6”. The Ultra 80 has a 3” intake and exhaust and they both won’t fit in the chimney. It would be a bear trying to follow the intake up beside the chimney because it would require cutting upwards through two soffits/overhangs. The contractor does not want the venting to look like a hack job, nor do I. He also believes that the intake and exhaust need to terminate from one another within W-M’s specs for the boiler to run properly.

    Here are some of the alternatives we discussed:

    A: The Munchkin T80 uses 2” venting which would fit easily through the chimney. The installer is familiar with Munchkins and Vision 1.

    B: Run the Ultra 80 but with dual 2” intake and exhausting subject to W-M’s approval. 2 x 2” gives around 90% of the capacity of 3”. On a boiler that uses the same venting size as a 230 this may be fine, but that’s for W-M to say.

    C: The Viessmann Vitodens 6-24 uses 125mm O.D. coax piping that would fit (but I’d prefer a boiler with more general components and less integration for the long term).

    D: The Baxi Luna and Laars Mascot use 100mm O.D. coax piping that would fit (the installer has no familiarity with these and they are 125k-30k boilers instead of 80k-16k like the Ultra).

    E: The Trinity Ti100 modulates as low as the Ultra 80 but it has the same 3” venting issues. I’m not sure what NY Thermal would say about dual 2” instead of 3” venting.

    I’m very interested in hearing any other ideas or any discussion of these options.

  • Ed_26
    Ed_26 Member Posts: 284

    If you have a 6x6 clay liner in the chimney, can you break it out to give enough room? May be difficult & time consuming, but will result in better job.
  • airman
    airman Member Posts: 10

    How about using the chimney for the exhaust vent and pipe the combustion air from another, easier to get to/from location.

    Robert O'Connor/NJ
  • Uni R_2
    Uni R_2 Member Posts: 589
    Clay it is...

    Yes Ed, it is a 6x6 clay liner. I would have never imagined doing that and would have thought that the clay tiles formed a structural aspect of the chimney. Now the big question is where does one get a 20' chisel? ;-)

    Have you done / seen this done before?
  • Uni R_2
    Uni R_2 Member Posts: 589
    Separate Intake Location

    Thanks Robert,

    As I mentioned in the post, for this to terminate anywhere near the chimney it'll look like a total hack job. For it to terminate 12' lower than the chimney, it would look quite good. However, this would violate the manufacturer's recommendations for venting and I'm not sure what support they'd offer if the boiler ever had problems requiring factory support or any warranty claims. Also, I tend to be under the impression that the boiler needs a fairly equal pressure balance between intake and exhaust though I'm really not sure how much of a factor this is when you factor in combustion being exhausted on one side.
  • Jamie_5
    Jamie_5 Member Posts: 103
    chimney as concentric intake?

    Have you asked Weil McClain whether they will approve using the chimney space remaining (after running the exhaust up through it) as the intake "pipe"? The remaining free area in the chimney after one pipe is run up through it is substantially more than the free area inside a 3" pipe. If W-M permits, you could just run the intake pipe from an appropriately sized hole cut in the chimney to the intake fitting on the boiler. It would require some arrangement at the top of the chimney to get the necessary separation of the intake and exhaust while keeping rain out, but I can't imagine that would be too hard.
  • Ed_26
    Ed_26 Member Posts: 284
    tile chisel

    No, just dreaming. However, I think it could be done with an ice chisel, weld a coupling on, add pipe as necessary for handle???
  • Floyd
    Floyd Member Posts: 429
    Your making a....

    bigger deal of this than need be. I know for a fct that the WM will work just peachy with the intake being anywhere that it can get the air it needs. My Ultra hasn't had the cover on it in probably two years.... just love to be able to watch that thing and be amazed that that chuck of alum. heats mt big old house so well and eff. The intake is coming right outta the basement and the exhaust is still stuck right out the window low enough that my son caught the grandsons trying to use it for... well a toilet!!!!!
    My vote is to run three inch up the chimney and pull intake from wherever... it will work fine. There aren't any press. switches or anything on this boiler that will shut down because of press. diff. like many others that I had had the misfortune to install. I hate press. switches... they are a PITA!!!!

    My .02...

  • Uni R_2
    Uni R_2 Member Posts: 589
    More than 2¢

    Floyd thanks... you know that your opinion is a very valuable to me. This is the kind of feedback I'm looking for. I wonder if the grandkid's ph is lower than the condensate? ;-)
  • Constantin
    Constantin Member Posts: 3,796
    Haven't you answered your own question?

    Unless WM gives you factory approval, the Munchkin looks like your best bet for a non-concentric solution. It's approved, warranteed, etc. in ways that the WM solution may not be, even if they give you verbal approval... Plus, your installer is familiar with the boiler and is Vision 1 certified, which means you can enjoy all the benefits a T80 has to offer.

    The Viessmann way is another possibility, as long as you don't have more than 20' of eqv. pipe length. This may or may not be an issue but getting to 20' is pretty easy once you have a 90° or two. A concentric intake/vent would always be my first choice due to ΔP concerns, but other styles (like the dual pipe system used by the Munchkin) also seem to work well with the equipment that was designed for such a flue termination style.
  • Uni R_2
    Uni R_2 Member Posts: 589
    re: Haven't you answered your own question?

    Thanks Constantin but my answer(s) was nowhere as good as Floyd's. You have to love the Wall! The installer was the one that balked at doing the air out beside the chimney rather than going through it. According to the venting supplement, I actually think that it isn't a big issue and that it would be absolutely fine.

    In Chapter 2 of the supplement, they talk about not locating the air inlet in an area likely to have products that could contaminate the air. "Do not operate an Ultra boiler if the boiler combustion air inlet is located in a laundry room or pool facility, for example." It makes the whole situation gray as far as W-M even saying that outside air is required. Are they condoning inside air being used as long as it isn't from a place where contaminants are present? That's how it reads. Regardless, I think the essence of chapter 3 is that you set it up so that the exhaust stays out of the air inlet and getting air from beside the chimney seems fine per #3. I've noticed in the Buderus GB142 install guide that they even show using indoor air as an install method.

    From chapter 3...

    #2. “The air piping must terminate in a down-turned 180-degree return bend as shown in Figure 1. Locate the air inlet no further than 2 feet from the center of the vent pipe. This placement avoids recirculation of flue products into the combustion airstream.” [check]

    #3. ... “The top of the (vent) coupling must be at least 1 foot above the air intake.” ... [no maximum... check]

    I'm going to mention this to the installer. He was opposed to any method where the air and the vent couldn't be run together to near the point of termination. With this in mind, he can call W-M to get any further clarification that he might want.

    I didn't say the installer was Vision certified. I'm not even sure if HTP offers certification training for the Cdn market. I think the only mfr that does much for training north of the border is Viessmann and I applaud them for that.

    The smaller Vitos will actually go 10M/33' vertically which is good because it's over 20' for this run. Also, I'm not so sure that concentric venting has any major tangible performance advantages, but I could be wrong. One thing that I've thought would be both a plus and a minus on continuous coax venting is that if the inner vent ever developed a small leak, it would just harm the boiler by getting sucked back in rather the poisoning the occupants.
  • Constantin
    Constantin Member Posts: 3,796

    I thought one could only get access to a Vision 1 system/programming instructions once one has been certified/trained. But I'm happy to be wrong.

    I don't think that concentric venting is that much of a benefit performance-wise, except that pre-warming the incoming air has some benefit, sealed combustion of this sort is very safe, and the metal/polypropylene vent used by Viessmann is a step up in terms of temperature resistance from the usual PVC mod/con fare we see in the US. The only other manufacturer I know of to specify coax PP or stainless venting is Burnham in their upcoming Freedom model.

    For me, the other big benefit is that a coaxial intake/exhaust can offer the lest problematic installation, if the vent terminal is designed right, since there will be no ΔP between the two terminals - wind gusts hit them at the same time and with the same ferocity, etc.

    Presumably, this is why I have a barometric damper for the oil boiler in my basement. When the wind gusts, that baro snaps open and shut quite a bit, a reflection of how the pressure is changing inside the chimney as a result of the air flow across the chimney hole at the top.

    Without the barometric damper, those pressure changes could influence the combustion process, by causing the flames to fan out and impinge (CO, NO and other nasties result), or even backdraft and come out the intake. While developing an outdoor model, I burned up several water heaters that way.

    So, in my mind, a good vent/intake terminal that is coaxial is the way to go. The pipes going into said terminal do not have to be coaxial, but given the benefits, why not?
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