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What are the safety/legal considerations for safely re-routing gas combustion ducts?

I have a two-story house with two gas combustion air ducts. The left duct serves a fireplace that is not in use and will be removed. The right duct serves both the water heater and furnace. I would like to remove the highlighted yellow sections and re-route the water heater + furnace exhaust duct to the left duct, and cap/kill the fireplace. The right duct is in the middle of a room on the second floor; the main intent is to remove the right duct that was in a removed wall to be completely removed. There is currently 24" between the 6" round ducts.

Section 304.11 of the 2015 Washington State Fuel Gas Code doesn't specify the required slope for horizontal runs, only:

  1. Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air.

project diagram

Make/Model info

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    These are exhaust vents, not fresh air ducts, right? Also, what make and model are the current water heater and furnace, and is replacing either or both appliances a possibility? Aug 31 '20 at 0:52
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    @ThreePhaseEel: Exhaust vents, yes. All pictured vents carry exhaust fumes from natural gas combustion. None of the appliances need replacing and all are functioning normally. Question updated with make model info, thx.
    – JJ Zabkar
    Aug 31 '20 at 14:57
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Consulting with an HVAC technician advised:

  1. disconnect the left duct from the fireplace just below the first floor subfloor
  2. disconnect the right duct just above the wye on the first floor
  3. route the furnace + water heater duct to the left duct using 6" round B-Vent. The technician pointed out that it's key to match the existing brand (I have Ameri-Vent) so that the turn-and-click attachments are the same.
  4. remove the right duct and fireplace, capping each with a sheet metal & silicone end cap.

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