There is nothing specific in the codes about using a wye versus a tee (actually, it is a "siamese" but who's counting?). If you look in the design manuals of most B-vent mfrs., you will see a similar configuration shown. Yes, having the water heater enter at a little bit of an angle helps in some cases but.....

The ANSI Stds. allow up to 45 seconds of "spillage". After that, it is termed "backdrafting", which is a legal term, if you get my drift. Yes, the std. allows the pre-purge to pressurize the vent for that short period and yes, you may get CO spillage. The codes figure the small amt. of CO spilled will quickly dilute out to where it isn't a problem. There is a growing concern by some very astute practitioners such as DavidR, an HVAC expert who visits this forum, that fan assisted Cat.I appliances should never be allowed to common vent with water heaters. There are many reasons for this but the first is that the draft hood provides a relied opening for spillage. A double acting barometric damper with a spill switch interlocked to the gas controls is a method gaining popularity, though it may not always be recognized.

The pic shown shows no draft hood connector, B-vent larger than draft hood, no 12" rise, no vent support, plenty of room for more vent rise, and is that common vent oversized?
Bob

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disgusted with some people on this forum. Out of here!