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Water Heater venting assistance

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Good day all.

I am in need of help and/or direction to locate information regarding the maximum size of a vent pipe for a water heater. The main vent is 6" "B"; which extends about 10' horizontal and 25' vertical from the appliance location. The original furnace has been removed leaving only the 40k btu water heater on the vent stack. Therefore, does the 7 times diameter rule apply here; which if my math is correct it is OK. Or??

Thanks in advance

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Yes, x7 rule applies. Meets code but may not vent well. Recommend combustion analysis by certified pro. I hope that lateral offset is properly supported and pitched. Now much vent rise from draft hood to breaching at common vent? If less than 3ft., need 4" connector right off draft hood. If you need to reduce the flue size for performance, a lot of guys reline B-vent with flex liners or even smaller B-vent inside the larger but understand neither approach is listed.

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