Read the pattern
Take the fan guard off and actually look at the coil from the face, from the end, and if you can, from behind. Five distinct patterns show up and each sends you somewhere different.
1. Even light frost, clears every defrost
Normal. This is what a healthy coil does. Leave it alone.
2. Even heavy ice across the whole face, does not clear
The defrost system is not working at all. Either it is not initiating — seized timer, wiped controller schedule, blown defrost fuse — or the whole heater circuit is dead. Check whether the box ever gets warm at the coil during what should be a defrost period. If it never does, nothing is initiating.
3. Ice on one section, the rest clear
The most diagnostic pattern there is. Defrost heaters are wired as a bank of elements along the coil. When one opens, its section stops clearing and ice grows outward from it while the neighbouring sections stay clean. The ice is literally drawing you a map of the failed element. This is also why testing elements individually matters — a resistance check across the whole bank can look acceptable while one element is open.
4. Ice on the inlet end only, outlet end dry and warm
A starved coil. The refrigerant is boiling off before it reaches the end of the circuit, so only the first part gets cold enough to collect ice. Causes are a low charge from a leak, a restricted filter drier, or an expansion valve that is underfeeding. You will usually find the box is also not holding temperature.
5. Ice on the face, clear deeper in
Airflow. Moisture is being caught at the face because air is barely moving through. Check the fans are turning and turning the right way, check nothing is stacked against the coil, and check whether the fins are flattened or clogged with grease and dust from a kitchen environment.
What not to do
- Do not chip it. Aluminium fins and thin copper tubing puncture with very little force. We get called to coil replacements that started as someone with a screwdriver and good intentions.
- Do not use hot water. It refreezes in the pan and the drain, and now the next defrost has nowhere to go.
- Do not run a heat gun at it. Localised heat on refrigerant-filled tubing is a genuinely bad idea.
- Do not just clear it and move on. Ice that formed once will form again. Clearing without diagnosing is paying for the same visit repeatedly.
Clearing it safely
If you need the box back before a technician can get there: move product to another box or to ice, switch the system off at the thermostat, leave the fans running if they will run without the compressor, prop the door open and put a fan in the doorway. A fully iced coil takes four to eight hours to clear this way. Put towels down, because all of that ice becomes water and the drain may well be part of your problem.
What we do differently
Two things. We test each defrost element individually with a meter rather than reading the bank as a whole, because the single-open-element fault is extremely common and invisible to a bank test. And we leave you a written record of the coil condition, the element resistances and the termination temperature, so the next time it ices you can see whether it is the same fault returning or a new one. That record is also what the 2026 refrigerant rules expect you to be keeping if a leak ever turns out to be involved.
Questions we get asked
Is any frost on the coil normal?
A light even frost between defrost cycles is completely normal and means the coil is doing its job. The test is whether it clears during defrost. Frost that survives a defrost cycle is the start of a problem.
Can ice on the coil damage anything?
Yes. As ice expands it can bend fins and stress tubing, and a coil heavy with ice puts weight on mounts never designed for it. More commonly, the fan motors burn out trying to push air through a blocked coil, which turns one repair into two.
Why does ice form on the bottom of the coil first?
Meltwater from the previous defrost that did not drain away, refreezing. It points at the drain pan or the drain line rather than at the coil itself.