at what depth should the return line be inserted in a 275 in basement tank? i think it is currently inserted about 1/3rd off the bottom of the tank. is the depth critical? am i correct in assuming that it should be deep enough to always be submerged in oil so it won't loose prime? if not is the worst that could happen that the prime will be lost?
Why are you using a 2 pipe system? It only creates high vacuum, and plugs filters sooner. You won't lose prime if it is not down to the bottom. It should be close to the bottom though, especially if it is an underground tank. Sometimes the return line needed to become a feed line.
If it's a basement 275 and the oil burner is lower than the tank , you don't need to pump all that extra oil with a 2 pipe setup . Even if the line is from the top of the tank . And I agree with Bill about the level of both oil lines . You might need to switch the the 2nd line someday if the first line fails .