3 inch diameter vent connects

[mathew stouffer]

Single family home built in 05. 40,000 btu water heaters, gas. Does the connector on the draft hood have to be 4 inches?

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[Ken Amelin]

The vent size is determined based on the tables in NFPA 54 and takes into consideration height, lateral lentgh, number of devices and BTUH ratings.

Exception: A three inch dia. vent shall not be connected to a four inch dia draft outlet.

ALSO, Those gas water heaters are piped in series. When piped in series the total pressure drop is greater and the first device will age faster than the second. Not a problem, just something to consider. I prefer to see heaters piped parallel vs. series.