During defrost the melt water runs from the tray into the drain pipe and leaves the room. If the room temperature is below 0 °C this water freezes in a short time in the part of the pipe inside the room; after a few defrosts the pipe blocks, water overflows from the tray, and icicles form under the evaporator and ice builds up on the floor. The drain line heater keeps this part of the pipe above zero and makes sure the water stays liquid all the way outside. The fault is mostly noticed only when water drips onto the product.
The most common type is a flexible, silicone-sheathed heating cable passed through the inside of the pipe; its power is around 40–50 W per metre and it is sold in ready-made lengths of 1–6 metres. The cable starts at the tray outlet, runs the whole length of the pipe inside the room and projects a little beyond the wall penetration. With types wrapped around the outside of the pipe, the pipe must be metal and insulated over the top; plastic pipe does not conduct the heat inwards. Self-regulating types reduce their own power as they warm up and do not overheat.
However good the heater, the correct pipe layout is essential: the pipe is kept as short as possible inside the room, laid with a fall of at least 5–10% and taken out through the nearest wall. The trap (which cuts off odour and air) must always be placed outside the room, on the side that does not freeze; without a trap, warm, humid outside air is drawn in through the pipe and the tray ices up. The pipe diameter is not chosen smaller than the evaporator outlet. On long horizontal runs pipe hangers are fitted at close intervals; water collects and freezes where the pipe sags.
In most plants the heater is left permanently energised; if it is run only during defrost, the last of the water left in the pipe may freeze. Because its power is small its consumption is insignificant, but its failure causes major problems. Checking it is easy: the current is measured or the pipe is touched by hand. Care is taken that the cable is not folded and does not overlap inside the pipe; the overlapping points overheat and can melt a plastic pipe. In the maintenance carried out going into winter it must always be checked that the heater is working; the problem usually appears on the coldest days.