Need 12" rise straight off draft hood
need draft hood connector
support of B-vent in 3 axis
need 6" clearance off draft hood and any single walled pipe
need minimum vent height per WH mfr.
Is that ceiling joist system capable of supporting ~400 lbs?
pan?
TPR discharge?
straps?
Access with plywood decking in front of WH?

Mounting of expansion tank may comply with mfr--note mfr. model, serial and get manual before you shoot it down. That tank has a diaphragm and is not full of water unless the diaphragm ruptured. Should be a shutoff to tank so it can be isolated if need be. Did you note the pressure at the Shraeder valve with an ordinary air gauge? Does no good if not properly charged.

What is the house water pressure? Is there a pressure reducing valve or check valve at the water supply just downstream of the meter?

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