My 2 ton Trane XL20i heat pump was installed Aug 2010. All OK until temperatures got below 38 degF. At times the unit went into defrost with no ice on coil and times out at 15 minutes in defrost. The installer first replaced the defrost board and when that didn't work (in contact with Trane) finally replaced the entire outdoor unit and the indoor coil and expansion valve. By that time (March, warmer weather) the issue faded. Now (Dec 2011) it's starting all over again. Otherwise, the unit heats and cools fine. Any ideas?