<https://www.inspectorsjournal.com/topic/16195-water-heater-vent-near-wall-fan-assisted-draft/#comment-135662>
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"Hi,  Thanks for the reply but, yes there is a rule: B or L vent termination within 8 feet of a wall or roof obstruction must be 2 feet higher (per IRC 2427.6.4 and UMC 802.6.2-thanks to \"Code check\"). Here in AZ we have lots of flat roofs with parapets and often find an incorrect installation.     What's got me stumped is that this has a forced draft fan into a B vent and that's not typical.     I've got to send the report out tonight and I'm sticking with my opinion that this is not quite right. I'll have to defer to a licensed plumber to evaluate and repair :-C \n"
<https://www.inspectorsjournal.com/topic/16195-water-heater-vent-near-wall-fan-assisted-draft/>