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Sweating vent issue

Weezbo Member Posts: 6,232
or tape all the joints and fix the roof terminations rain collar.


  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Problem we've never seen...

    We have a Buderi boiler, atmospheric, Tall stack around 30 feet, with about 20 feet of not quite (43 Degrees)horizontal in the attic. Single wall leader going to the B vent is 4 foot long. Leader is showing some SUBSTANTIAL signs of sweating, and the boiler is not being allowed to run below recommended operating temperatures.

    Anyone else experienced this? If so, what's the cure?

    I have my ideas, but thought I'd check with my peers first.

    Any one?


  • Constantin
    Constantin Member Posts: 3,796
    I presume there is a reason...

    ... the stack is being run at that angle through the attic. My first guess would be to replace the thing with something watertight that can handle the condensate, then insulate the heck out of it to keep the flue gasses as warm as possible until they exit.

    In an ideal world, I'd pipe a insulated AL24-9 flue straight through the roof, but I suppose there is a reson the thing is installed the way it is, right?
  • Marty_2
    Marty_2 Member Posts: 10
    cold attic

    the air gap in bvent is great for reducing clearances but not so hot at keeping the inner liner warm enough to prevent condensation. An all fuel vent with insulation in it might have been a better choice with that long run through an unconditioned attic.
  • Bob Harper
    Bob Harper Member Posts: 1,030
    more data

    ME, can you share the BTU input, SSE, and appliance collar size? Maybe downsizing the B-vent and re-configuring it might help. Being in the attic is better than being exposed outside but you might even consider building a chase in the attic (with clearances) to keep the pipe even warmer. Solid packed chimneys are ok for burn cycles that can warm them up and keep them warm. However, low stack temps with short cycles will suffer a similar fate. By the time you have warmed the inner liner, insulation, and outer casing, it has cycled off. If it continues to be problematic, I'd lean towards installing a Suck-O-Matic at the termination.
  • Nick W
    Nick W Member Posts: 200

    suck-o-matic?can you please explain-is this a draft fan.
    and would you require the manufactures consent.

  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542

    I haven't seen the specifics yet, only what my partner tells me. It is my parctice (and the companies) to size the stack to the lowest possible size in an effort to maintain good flow and high velocity to avoid this very problem, and trust me, this is not the first vent stack we've done like this. The offset is to keep the penetration away from the visual impact zone on the front of the house.

    It is my suspicion that due to the free flowing nature of the fixed draft hood that we are drawing in so much relief air that we are causing the flue gasses to cool down and condense. Before I go to the expense of building a chase to insulate the stack in the attic, I'd prefer to retrofit a suck control agent. I will block the open draft hood and install a barometric damper to control the dillution air and maintain proper -.02" WC over fire draft.

    This should make a marked difference. I've had to do this before, and in fact ended up having to install a guillotine blade in the beaching to acheive proper draft.

    Thanks for your input.

  • Michal
    Michal Member Posts: 213
    low flue gasses

    well it sounds like your running the flue gasses too low. what size flue are you running and does the boiler cycle long enough to keep the stack warm enough, and do not tape the joints, totaly not the right thing to do. Make sure you have a weather tight end cap. The stack will be eaten up in a few years from the corrosive nature of the condensate from the flue gasses. What you might have to do is down size the flue pipe, which can be done if certain parameters are met, and with a 30 foot high chimney you might be able to, call tech support at buderuc, and I think you should be using 403 stainless on the inner linner
  • Jack_21
    Jack_21 Member Posts: 99
    Some B-vent is better than others

    What I am alluding to is the propensity of some B-vent to allow air circulation between the walls. I think it is a funciton of how the fitting is cut and how tight a crimp they manuf on the two pipes. I like Selkirk's pipe better than others for this reason. Personally, I think the Amerivent is the worst. I don't think it is a bad idea to drop a bead of hi temp silicone in the joint prior to assembly, especially in unheated spaces. I think you could modify (restrict) the diverter to set it also rather than ging to a barometric. Please let us know how it works out.
  • Mark is that

    single wall leader(connector I presume) which is 4 feet long horizontal? If so is it pitched a 1/4" per foot? You might try before removing the draft hood going with double wall all the way I have found that many times singlw wall into double wall with poor draft can be solved with simply changing the connector to double wall. You may be loosing a lot of flue gas temp through that single wall. By going to double wall you can then pick up some flue gas temp. What is the flue gas temp by the way? If the flue gas temp is staying below 300 degrees no matter what then I would say the draft hood may be the problem. I measure temps both sides of the diverter along with draft just to see what affect the draft hood has. If you are going to use a adjuster my way of doing it is to put it after the draft hood and just before you go vertical into the common vent. That way you can control total draft.

    I have not had a lot of experience on that boiler so I would have to do further research to be sure.
This discussion has been closed.