I believe it takes 4" vent pipe up to 100,000 BTU
BTUs are shown on label side of tank.http://waterheatertimer.org/pdf/Gas-venting-specs.pdf
Does new water heater have power vent? What does manual say?
Are you experiencing vent problem now? When you light a match and put it under vent stack, does smoke immediately get pulled upward? Are furnace and waster heater connected to same venting. Is there dark exhaust stain around vent hood? Does exhaust vent back out of vent hood when furnace kicks on? Are flue pipes stained with rust and white coating? Are vent pipes vertical, or are there level runs? Are the pipes made of galvanized and screwed together with 3 screws at each joint?
Frankly I would double-triple-check any internet answer dealing with possible CO poisoning at your home.
1) Ask local plumber for specific code in your area.
2) Call gas company
3) Post question on pro water heater forum:http://www.thetankatwaterheaterrescue.com/forums/forum3/
4) Read whole manual for new water heater before buying.
5) Contact manufacturer.
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