Frosting and icing are not the same thing. Frost is loose and white and melts at every defrost. Ice, by contrast, is the product of incomplete defrosts: the frost partly melts, the water refreezes before it can run off the fins, and a hard block forms that gets a little thicker with every cycle. The block usually starts from the lower rows and the rear face of the evaporator, fills the tray and finally reaches the fan blades. At this point the evaporator passes no air and the room temperature rises.
Common causes: a burnt-out or loosened defrost heater, the defrost termination sensor fitted in the wrong place or faulty, the defrost time set too short or the number too few, a frozen drain pipe (drain heater failure), a tray with no fall. A door left permanently open, a torn gasket and humid air seeping in through panel joints also carry more moisture than defrost can remove. A fan that is not running and a low charge cause local icing on part of the evaporator.
The symptoms are linked: the room temperature stays above the set point, the compressor runs without stopping, the suction pressure falls below normal, the air coming from the evaporator weakens, the fan makes a noise as it rubs on the ice, and water drips from the ceiling or the tray. The records in the controller show that the defrosts always end on the time limit and that the fin sensor never reaches the termination temperature. This record is the clearest proof that the problem is on the defrost side, and it shortens the search. If there is no record, one defrost is watched from start to finish.
To deal with it the ice is melted by stopping refrigeration and running a long, manually started defrost, or with lukewarm water. Breaking it with a screwdriver, chisel and hammer crushes the fins, punctures the tube and lets the refrigerant escape. If the system is restarted after the ice has melted without the real cause being found, the block comes back in a few days. The heater currents, the termination sensor, the drain line and the door are checked in turn. In frozen storage rooms the drain pipe must always be heated and trapped.