When a freezer defrosts, water runs off the evaporator into a tray and out through a drain pipe. In a cabinet at −20 °C, that water will freeze again before it escapes unless the tray and pipe are kept warm. A drain heater, usually a resistance wire running along the tray or inside the pipe, is switched on during defrost to keep the path open.
If the drain heater fails, ice builds up in the tray and drain over several defrost cycles. Water then overflows into the cabinet, freezes on the floor or product, or backs up around the evaporator. A puddle of ice at the base of the evaporator or a cabinet floor sheet of ice is a typical sign.
Drain lines can also block from food debris, labels or packaging fragments, particularly in busy kitchens. Keeping the cabinet base clean and checking that defrost water flows freely into the condensate pan during routine cleaning prevents most issues.