Paul: There is no difinitive answer to this one. If you are experiencing heavy frosting on the evaporator it indicates that warm, humid air is entering the box, and that further indicates that colder, denser (i.e. heavier) air is escaping from somewhere. The colder the air, the denser and heavier it becomes, and the more it is likely to force its way out of a weak seal, especially low down in the box. Make sure that all drains and access holes for tubing and wiring
are sealed and not allowing air to escape or enter. If you have a front opening door also, this is a very common place for cold air to escape, and warmer air will then get sucked in through the weaker top seals
. If you have a fan anywhere in the box, either as a spillover control or to assist the natural convection, make sure that this is not overpowered and forcing air out of one section of the seals and allowing warmer, humid air to be sucked in from another location. To evaluate the integrity of your seals, tape them over temporarily after defrosting the evaporator and while you are away from the boat for a decent time, say a week, and see if that cures the frost build-up. Excessive frost is a result of weak box/seal integrity and is in no way influenced by the size, type, or manufacturer of the refrigeration
Hope this helps.