Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Condensing boilers - Venting thru existing B-vent chimney

Amir Member Posts: 5
I want to run 2 vent pipes of 2 condensing boilers utilizing existing 8" B-Vent metal liner chimney

Assuming both vents are 3" Diameter, where total output is ~400,000 btu/hr net ( both boilers, at max. output )

Can I secure the vent pipes at the top of the Chimney structure one next to each other ?

I recall reading a code that specifies 10 ft radius clearance for max. 350,000 btu SIDEWALL venting. I would like to know if it applies to Chimney top venting


  • Paul Pollets
    Paul Pollets Member Posts: 3,656

    The (2) boilers (mod-con's I presume) need to be separately vented and supported within the existing chimney. There are special PB kits made for this purpose available from Centrotherm. If you pulled both liners at the same time, it could be done. You could use one PB chimney kit for both boilers, if sized properly, and your local codes allow.

    Running the vents inside of a B-vent would not work.
  • gennady
    gennady Member Posts: 839
    common venting

    Common venting for condensing boiler is not allowed, unless it is a set designed as one boiler from the factory, like EVO boilers. You can run 2 sets of chimney kits, providing they have support every 5', and those vent kits have these supports. vents can be run together, only one limit is distance from exhaust to fresh air intake. There must not be re circulation of exhaust gases into boiler fresh air. Also vent kits must be approved by manufacturer.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    I see NO reason why you can't run both pipes up the existing B vent.

    The 8 inch Bvent pipe is just a chaseway, he will be running PVC pipe up inside the Bvent.

    explain to me what the difference would between the Bvent and a wood construction chaseway. the wood chaseway would be legal without question.

    I would think the B vent would be far safer.

    If you need to seperate the two pipes at the termination point just 45* them away from each other in the attic before you penetrate the roof.

    I have used the existing Bvent many times as a chaseway for both the intake air and the exhaust. I ran the exhaust pipe up through the center of the Bvent cap and used the space arround the PVC exhaust pipe to draw combustion air for the boiler by using a metal T at the bottom of the B vent ( same size as the Bvent ) I put a cap on the bottom of the T and the exhaust pipe goes through it. I connect the boiler combustion air to the side of the T. I make everything air tight. There is no chance of exhaust leakage since it is sealed combustion.

    I have gotten Inspector and factory aproval for this.

    Weil Mclain even has a kit for doing just this.
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Pipe support

    How would you be able to support the PVC within the B-Vent? The weight of the piping would be pressing down on the elbow(s) at the chimney base. Chimney kits for using the existing chimney as a chase come with a base 90 that sits on a shelf and pipe supports every 4 feet to hold the pipe against the chimney walls.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Hard to do in most cases...

    Code requires support either every 8' or 10' depending upon size, and I think the minimum is every floor (hence 10') if memory serves me correctly.

    I have run numerous PVC vents up through an existing B vent as the chase. I support the base of the PVC stacks, and allow the expansion and contraction to go vertically up to the termination on the roof. I use a neoprene plumbing vent flashing on the top which will allow the tube to move without binding the pipe while maintaining a dry penetration.

    If you do decide to open walls and place additional support on the PVC riser, I would NOT suggest it be bound, but instead stabilized using the next bigger pipe as a slide/guide, and again support the base and send expansion growth potential to the roof.

    In my case, we would have had to open finished walls within 3 floors of individual condominiums, which would have created a WHOLE lot of extra work for no real advantage.

    Lochinvar was the first manufacturer that I am aware of that actually began showing the use of a PVC/B vent concentric configuration in their manual, and I hope I had something to do with its getting approved, because I kept bugging Greg Gibbs to get 'er done ;-) They actually will allow the incoming combustion air to go through the annulus between the PVC and the B vent provided that the residual annulus is large enough, thereby increasing the thermal efficiency of the appliance. Hard to put a real number on it, but based on my observations, it is a definite benefit by preheating the incoming combustion air going to the appliance. The first place I tried this was in my own home, hence good field observations.

    In any case, regardless of who's appliance you are using, the installation must comply to the manufacturers requirements as it pertains to vent termination clearances.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
This discussion has been closed.