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Concentric venting for a 96%er.

STEVEusaPA
STEVEusaPA Member Posts: 5,195
Hello gas gurus. Stepping outside my knowledge with venting for a 96% gas furnace (for one of my investment properties I'm rehabbing and having a relative live in).

My only option is to go thru the roof (existing chimney). There's no option to get it from the middle of the house to an acceptable/approved outside wall.
I have a chimney that has to either be rebuilt (not likely) or I planned to take it down below the the roof line, probably to the ceiling joists. 6" terra cotta lined.

I'm familiar with concentric vents at the roof-run exhaust/intake up to attic separately, then into the roof kit.
What I was wondering was can I (do they make?) a concentric that I can run down the chimney using it as a chase) to the mechanical room (total run <25 feet), then split out my exhaust and intake in the mechanical room? Both pipes wont fit into the chimney separately, and running a separate pipe up next to the chimney requires lots of demo.

Was looking at various furnace manufacturer's install kits. Although they show a concentric under the roof with the kit, I haven't seen one showing it running the entire length and splitting back out at the furnace.

Thanks for your input!
Steve
steve

Comments

  • JUGHNE
    JUGHNE Member Posts: 8,706
    There might be the option of running the single pipe exhaust up the existing chimney, then using the remaining cavity as inlet combustion air raceway. Need a sealed roof cap for the air inlet and then sealed at the bottom to get the inlet air to the furnace.

    Does it have to be 3" exhaust?
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,195
    Thanks @JUGHNE. The one unit I was looking at said 3” exhaust. I’ll keep looking.
    steve
  • HVACNUT
    HVACNUT Member Posts: 4,087
    A few years ago I had to replace the venting on 2 HTP boilers. I dont know the manufacturer but they offered all the fittings and I believe about 5 ft of pipe in a pipe. Additional lengths are field supplied. I remember the PVC wall thickness was very important to accept their fittings properly. It wasn't just schedule 40. We went about 15 ft vertical to a flat roof.
  • JUGHNE
    JUGHNE Member Posts: 8,706
    I have done 100,000 btuh furnaces with 2" going 20' up with minimal fittings.
    Is the larger FAF necessary for the structure?
  • Zman
    Zman Member Posts: 6,807
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    STEVEusaPA
  • RPK
    RPK Member Posts: 90
    Check out this product. I’ve only used it once, it worked out really well.

    http://www.centrotherm.us.com/Products/InnoFlue-Concentric.aspx
    STEVEusaPA
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,195
    edited July 2019
    @RPK Thanks, I'll have to check it out.
    steve
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,195
    edited July 2019
    @Zman Thanks Carl, but I can't get you link to work.
    edit: Actually it does work, but you have to copy the entire link (including the .pdf) then open a new window and paste.
    See if this works...
    https://lochinvar.com/_linefiles/100287110_2000544742_WH56-WH400_Rev D (10778_3000021144).pdf
    Looks like the same idea @JUGHNE mentioned. Run the exhaust up and thru the roof with a concentric fitting. Use the chimney for combustion air.
    steve
  • Zman
    Zman Member Posts: 6,807
    It is nice to have the detail in the I&O, especially when you encounter a skeptical inspector. I did one that way about 6 years ago. I was a little concerned about soot from the old masonry stack. I serviced the unit last year and everything looked great.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    STEVEusaPA