Why this fault compounds
Most refrigeration faults are static: something is broken and the box behaves badly until it is fixed. A frozen drain is different because it feeds itself. Defrost produces meltwater. The water cannot leave. It refreezes, usually in the drain pan and at the base of the coil. The next defrost has to melt the old ice as well as the new frost, takes longer, often fails to finish, and adds more water that cannot leave.
Two weeks of that and a coil that was clear is a solid block. Operators frequently call us about an iced coil when the actual fault is six feet away in a drain line.
The four causes
Heat trace failed
The drain line from a freezer runs through sub-freezing space for at least part of its length, so it carries a heater cable to keep the water liquid until it is out of the cold. These fail like any heating element, and because they are usually hidden under insulation nobody notices until there is ice on the floor. Current-testing the heat trace is a thirty-second check that almost nobody does.
Line routed through unheated space
The classic version is a drain that exits the freezer, runs across an unheated storeroom, a loading dock or an outside wall, and only then reaches a drain. Whatever happens inside the box, the line freezes where the ambient is below freezing. In Boston that includes a lot of back-of-house spaces from December through March.
Not enough fall, or a sag
Water sitting still freezes; water moving does not get the chance. Any low point in the line is where it will block, every single time, and a line that was correct at installation can sag over a decade of being leaned on, hung from and knocked.
P-trap inside the freezer
A trap exists to stop warm air being drawn back up the drain into the box, and it does that by holding standing water. Standing water inside a freezer is ice. The trap has to be in warm space outside the box. We still find them installed internally, and when we do, no amount of clearing will produce a lasting fix.
What it looks like in the box
- Ice building on the floor directly beneath the evaporator.
- A drain pan with standing ice in it.
- Icicles hanging from the coil or the pan.
- Coil ice that returns from the bottom upward rather than evenly.
- Water appearing outside the box where it overflows and finds another route.
The safety part
Ice on a freezer floor is one of the more common staff injury sources in a commercial kitchen, and it is one that sits squarely with the operator. Someone carrying a case of product onto a sheet of ice in a cold, poorly lit box will fall eventually. If you have ice underfoot in a walk-in, that moves this up the priority list regardless of what the temperature readings say.
How we fix it properly
Clearing the blockage is the easy part and it is not the job. We current-test the heat trace rather than assuming it works, follow the line end to end looking for sags and for sections running through cold space, confirm the trap is outside the box and actually holding, and check the fall. Then we tell you which of those four causes you have. If the answer is routing — and on older installations it often is — rerouting or properly insulating and tracing the line is the fix, and clearing ice every few weeks is not.
Questions we get asked
Where should the P-trap on a walk-in drain be?
Outside the box, in warm space, always. A trap inside a freezer holds standing water in a sub-freezing environment, which freezes solid and blocks the line permanently. We still find traps installed inside freezers and it is one of the most reliable causes of a recurring frozen drain.
Can I pour hot water down the drain to clear it?
It may clear it once, and it will refreeze, because you have not changed anything about why it froze. Treat it as a temporary measure to get through service, not a repair.
How much fall does a drain line need?
A consistent slope the whole way to the trap, with no sags and no low points. A sag holds water, water freezes, and the line blocks at the sag every time. Many drain faults we find are not heat trace failures at all, just a line that has drooped over the years.