Thanks for the replies guys.... I've heard the 1 foot vertical thing but for some reason in my area the installers don't do it that way and the city lets it go. I see them like this everyday. I see the 90 degree 'T' fairly often but it's usually when the furance vent is not vertical so the overall venting of the water heater is going up. Are either of you guys aware if it's ever okay to have that backdraft for a few seconds when the induction fan is on but the furance is not? I can't imagine it's okay but I'd hate to hang my whole case on that and be wrong... thanks!