Concentric venting for a 96%er.

STEVEusaPA

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

JUGHNE

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

Thanks @JUGHNE. The one unit I was looking at said 3” exhaust. I’ll keep looking.

HVACNUT

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

I have done 100,000 btuh furnaces with 2" going 20' up with minimal fittings.
Is the larger FAF necessary for the structure?

Zman

Check out page 34.
https://lochinvar.com/_linefiles/100287110_2000544742_WH56-WH400_Rev D (10778_3000021144).pdf

RPK

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

@RPK Thanks, I'll have to check it out.

STEVEusaPA

@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.

Zman

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.