My teacher, the late, great Gil Carlson, used to talk about The Law of the Tee, which states: “Whatever goes into a tee must come out.” To which you’re probably saying, “Duh!”
I know. It does sounds obvious and a bit silly, but it’s the hinge pin of most hydronic systems, and that’s why Gil always brought it up. If the water comes out of a tee the wrong way, you’re in trouble.