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Old 03-02-16, 06:04 AM   #5
Mobile Master Tech
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The 6" around the 4" isn't going to be enough flow area either, and 4" supply duct may not be enough. Manufacturer and code requirements differ but you will probably want to go with the most conservative NFPA 54 requirement of 1sqin of air opening per 2000btu of burner unless you carefully research otherwise, 20sqin per furnace in your case. The area between a 6" pipe and a 4" is about 6.5sqin, and the area of a 4" duct is 12.5sqin.

If you move or replace equipment, make sure to provide external combustion air or have sealed burners that draw only outside air for combustion. The draft caused by drawing in cold air to replace combustion air wastes a lot of heat, and could risk CO poisoning if your house was tightly sealed.
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