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your return line is cloged. if you absoutly dont have a leak. thats the only thing
older returns are made of galv pipe or black and thay get cloged up the water gets soped up in the build up like a sponge. doubble check and make shure your return dont go into the floor as alot of returns do then feal around and see if its hotter in one spot. if it dont go in the ground you need to replace th return line