The defrost drain: where the water under a built-in comes from

Where the water is supposed to go

Frost on an evaporator coil is not a fault. It is the moisture that came in with the air, condensing on the coldest surface in the cabinet. The defrost cycle exists to get it off again: a heater near the coil runs for a few minutes on a schedule, the frost melts, and the water falls into a trough directly beneath the evaporator. From there it leaves through a drain tube, and on the freezer side that tube usually runs through the insulated wall of the cabinet down to a pan sitting near the condenser. The pan has no plumbing and no pump. The water simply evaporates, helped by the warmth of the machinery and the air the condenser fan moves across it. That is the entire system. There is no valve and no float anywhere in it, which is why the failure mode is always the same one: a blockage somewhere along a passive path.

The three things that block the path

On the fresh-food side the drain opening sits low at the back of the compartment and it collects what a refrigerator compartment collects, which is crumbs, spilled liquid and the biofilm that grows on any permanently wet surface. That narrows the opening first and closes it eventually, and the tell is water standing under the crisper drawers rather than on the floor. On the freezer side the blockage is ice. The drain tube is the coldest part of the path, so once a small plug forms in it, the next defrost cycle adds to the plug instead of clearing it and it grows on a schedule until nothing gets through at all. Some units run a heater at the freezer drain specifically to stop that happening, and when it fails the drain ices closed within a few cycles even though nothing is physically dirty. The third cause is mechanical: a drain tube kinked or pushed out of position, which is a common finding on a unit that has been moved and pushed back.

• Debris and biofilm narrowing the fresh-food drain opening
• An ice plug in the freezer drain tube, growing one defrost cycle at a time
• A failed drain heater on models fitted with one
• A drain tube kinked, or discharging beside the pan instead of into it
• A cracked or displaced evaporating pan

Reading the puddle: four kinds of water, told apart before anyone opens anything

Not all water under a refrigerator is drain water, and the four kinds are distinguishable from the kitchen floor. Drain water arrives with ice: a sheet across the freezer floor, frost around the bottom of that compartment, or standing water under the crispers, and the puddle appears in pulses a day or two apart rather than continuously, because it is following the defrost schedule. Supply water is clean, turns up at the front or the side, and grows steadily rather than in pulses; that is the ice maker fill line, the inlet valve or a compression fitting behind the unit, and it deserves the shutoff valve and a phone call rather than a towel. Condensation is a door problem, showing as beads along the seal line and a track down the mullion on a humid morning. The fourth kind is the least interesting: something in the cabinet thawed, or a door was left ajar overnight.

• Ice in the freezer plus water under the crispers, arriving in pulses: defrost drain
• Clean water at the front that keeps growing: supply line, inlet valve or fitting
• Beads along the door seal on a foggy morning: gasket
• A puddle once and never again: a door left open, or something that thawed

What is safe to clear, and what is not

The fresh-food drain is the one an owner can reasonably deal with. Find the opening at the base of the rear wall, clear anything visible around it, and flush it with a modest amount of warm water from a turkey baster or a squeeze bottle. If the water goes down, you are done. If it backs up, stop there, because forcing it is where the damage starts. The freezer drain is a different matter and it is genuinely a technician's job. Reaching it means taking the rear evaporator panel off, and directly behind that panel sit the coil, the defrost heater and the wiring for both. A refrigerant tube punctured with a screwdriver while somebody chips at ice turns a drain clearing into a sealed-system repair, which is the single most expensive mistake available in this appliance. There is no version of that story that ends well.

• Safe: clearing and gently flushing the fresh-food drain opening with warm water
• Safe: pulling the crisper drawers out to see whether the water is arriving from above
• Safe: noting the interval between puddles, which tells a technician a great deal
• Not safe: chipping ice anywhere near the evaporator with a screwdriver or knife
• Not safe: boiling water, steamers or heat guns inside the cabinet
• Not safe: pushing stiff wire down a drain tube whose far end you cannot see

The pan at the end of the path, and the puddle that returns in humid weather

If the drain is clear and water still turns up at the toe kick, look at the end of the path. The pan is sized to evaporate what the defrost cycle sends it, and that calculation assumes the surrounding air can accept the moisture. During a run of marine-layer mornings the air is close to saturated for much of the day, and a pan that copes without complaint in October can overflow in June with nothing actually broken anywhere. The other pan faults are simpler. It can crack with age or take a knock during a service visit, and it can sit out of position so the tube discharges beside it rather than into it. A condenser fan that has stopped turning matters here as well, because the pan depends on air moving across it as much as on heat, and water that used to evaporate between cycles begins to accumulate instead.

Drawers, undercounter units and ice machines drain differently

The path described above is the one in a tall combination cabinet. Undercounter refrigerator and freezer drawers are shorter, so the fall from trough to pan is shorter too and the same quantity of debris blocks the tube more easily. Built-in ice machines are a separate case again: many are designed to drain by gravity to a plumbed drain rather than to an evaporating pan, and a slow drain there shows up as cloudy or hollow cubes and a machine that cannot keep up, long before any water reaches a floor. An ice machine built into an outdoor bar has one more problem, which is that an open drain path outdoors eventually collects debris and insects. If the unit that is leaking is not the main refrigerator, say so when you describe the fault, because the route the water was supposed to take is not the same one.

• Refrigerator and freezer drawers: a shorter drain path that blocks with less debris
• Wine cabinets: condensation and door seals account for more water complaints than drains do
• Built-in ice machines: often a gravity drain to plumbing rather than an evaporating pan
• Outdoor bar installations: open drain paths that collect debris over a season