nestork wrote:
Yes, it couldn't be a 4X4X4 wye, it'd have to be a 4X4X6 inch wye
connected to a 6 inch flue connector so the flue connector could
handle all of the flue gas going through it when both water heaters
are firing.

Without that, or maybe even with it, you could end up with hot exhaust gasses
coming out at the top of one heater when the other is running.