What is a defrost drain?

Meltwater from each defrost cycle runs down a channel into a tube and out to an evaporation pan above the compressor. That tube clogs with lint and debris and then freezes solid, so the water has nowhere to go — it sheets across the freezer floor and eventually seeps forward under the crisper drawers. Clearing it fixes the symptom, but fitting the small drain heater clip is what stops it coming back in three months.