During the winter we noticed a musty smell from a hanging locker and once cleared out, saw that the locker panelling was wet at the bottom. Behind the panel was a void space against the hull which contained 3 overboards (2 bilge, 1 shower), electrical cable and the A/C liquid and gas lines, one line being insulated. After checking the overboards for leaks, looking for any deck fittings above the locker that were leaking decided that all looked good and watertight. The A/C is reverse cycle and we happened to be inside the locker when the heating kicked in. It appears that when the A/C is in reverse cycle, the liquid and gas lines reverse, and the un-insulated gas line now becomes the (cold) liquid line. The observation was that water was condensing out on the cold liquid line behind the panel and running down to wet the very bottom of the wooden panel, causing after time a musty smell.(and a rotten panel) The fix was to cover both lines with pipe insulation and once done now have a dry locker. I wonder how many other boat A/C installations have caused the same problem?