The pool company we contract every year, closes our pool by blowing out the water from ALL the pipes including the main drain pipe. I learned the hard way that the original "Jandy" valves with their diverter assemblies we had installed when the pool was built in 1996 eventually wore out (due to constant opening and closing of the valves throughout the season) and allowed water to reenter the main drainpipe. During the winter the cold temperatures caused the main drainpipe to crack due to expansion of the frozen water within the pipe. Fortunately, the section of pipe that needed to be replaced was accessible to repair. I replaced the cracked section of pipe and the valves with CMP diverter valves. I ordered xtra diverter assemblies which I replace at the end of the season (and switch back when the pool is opened) to make sure this doesn't happen again.