@ScottG, I have to agree with you as if I remember correctly, you & I had this discussion last year during the "winterizing" season. I tried it both ways & was SHOCKED at how much came out of the hot water reservoir tank when I opened the yellow flip valve. It was at LEAST 2 gallons of water. Myself, I'd tend to be on the cautious side of things & flip the valve open. I KNOW there's a "check valve" in there somewhere, because without pressure in the system, the tank won't drain properly. I had to do mine twice, and the second time I opened the flip valve when there was still pressure in the system, and it drained just fine. The first attempt without pressure, it just dribbled a little. Just trying to relay MY experience with the yellow fippy-doodle!