What is the most likely failure mode of a compression Tee? Why would tying into an existing main with a union be better? Background: A recent thread wandered a little into the subject of Teeing into an existing mainline. The OP had said he used a compression Tee, and board members quickly claimed that he'd be fixing that in a few years. Obviously, the best solution (when possible) is to expose enough pipe to bend it just enough to slip in a standard Tee. Presumably, the 2nd best solution might be the 4 ells trick. But one of the other suggested alternatives was a union. I'm curious as to why a union would be better than a compression tee. In both cases, you are relying on a rubber seal (so if that is the typical point of failure, the union wouldn't seem to obviously be better). Additionally, the unions seem like something of a negative because from my (extremely limited) experience, PVC unions must be lined up perfectly or they will leak. I guess another question would be, if using a union, how do you install that? The only solution I can think of (assuming a situation where the existing pipe can not be bent at all) requires two unions, one on each side. Glue the female sides to the two sides of the existing main line. Then, after getting the length of the pipe in the middle just right (perhaps 1/8" too short), quickly glue the male ends to the pipe to insert, quickly insert it, and close the unions before the glue sets.