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instaling pvc vent pipe in exsisting 6" bvent for new 96% high efficient furance

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I'm replacing my old furnace with a new 96% efficient furnace. also my hot water tank which the new one will be electric. I would like to run the two 2" pvc pipe up my exsisting 6" b vent pipe. instead of plugging it off as i would not need it. Is this possible and if so is there a special cap for the top of the b vent to seal off the 2 pvc pipe  . I live in kamloops bc canada
asked in Heating System by busterboo (120 points)
edited by busterboo
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1 Answer

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Hi BusterBoo,

Yes, you can use an abandoned B-vent as a chase for the exhausts and combustion air PVC pipes from your new high efficiency furnace. However, being in Kamloops you must be experiencing cold winters and there might be some local laws in your area that would prevent such installation. 
There are also some general requirements (they may slightly vary between manufacturers) that you might need to apply to this installation. 
  1. When the PVC vent pipe is exposed to temperatures below freezing ( i.e., attic, section of the old chimney above the roof) it must be insulated with ½” (22.7 mm) thick Armaflex-type insulation or equal. This is required to prevent condensate from freezing inside the PVC vent pipe and eventually blocking it completely (check the manual that came with the furnace).
  2. There are also requirements for the distances between the exhaust and intake pipe termination above the roof (this is based on American Standard high efficiency furnace, yours might be slightly different) 
  • You need to maintain a minimum of 12” clearance (max 24” above the roof surface) of the air intake pipe termination end above the highest anticipated snow level.
  • Distance from the top of the vent pipe to the bottom of the air inlet must be 12” ±1”
  • Distance between the exhaust outlet and air inlet must be minimum 9” and max 24”
High efficiency furnace PVC vent pipe roof terminations.jpg
  • Other thing to implement would to insulate very well around PVC vent pipes at the B-vent entrance to prevent warm air from penetrating / causing condensation inside the chase.
  • Support and secure PVC pipes at base, top, and section inside the chase.
  • Seal the top opening of the B-vent to prevent rain water from entering but allow any condensation that could develop to evaporate. 

I don't know about any special caps that could be used for this purpose since both pipes will be very close to each other and regular PVC pipe rubber flashing will not work.

Depending on your roof design and attic access (if there's any) you could remove the top section of the B-vent (disconnect it below the roof surface) and re-use existing roof opening (with new rubber flashing) by installing Concentric Vent Kit for Condensing Furnaces and Boilers

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answered by darekrudy (21,730 points)