The chef opens the cold room first thing in the morning and the evaporator coil is a solid block of ice. Airflow is blocked. The room temperature has climbed overnight — the probe says 8°C and climbing. Product is at risk. This is a refrigeration emergency, and it’s one of the most common cold room faults we deal with in London restaurants, hotels, and food businesses.
Every cold room evaporator accumulates frost during normal operation. That’s physics — warm, moist air meets a sub-zero coil and moisture freezes on the surface. But a properly functioning system has a defrost cycle that melts this frost periodically, keeping the coil clear. When that defrost cycle fails, ice builds up fast and the consequences cascade quickly.
How the Defrost Cycle Works
Before we get into fault causes, it’s worth understanding how defrost works. Most commercial cold room evaporators use electric defrost heaters — resistive elements embedded in or strapped to the coil. A timer or controller initiates defrost at set intervals (typically two to four times per day), the compressor stops, and the heaters switch on to melt accumulated frost. The meltwater drains into a tray and out through a heated drain line to a collection point outside the cold room.
A defrost termination thermostat monitors the coil temperature. Once the coil reaches a set point — usually around 10°C to 15°C — the thermostat signals that defrost is complete, the heaters switch off, and the compressor restarts. The whole cycle takes 15 to 30 minutes. When any part of this sequence fails, ice accumulates with every operating cycle.
Failed Defrost Timer or Controller
This is the most common cause of evaporator icing. The defrost timer — whether a mechanical clock timer or an electronic controller — is responsible for initiating the defrost cycle. If it fails, defrost never starts. Frost builds from the first hour of operation and gets worse by the day.
Mechanical timers are notorious for sticking. The motor that drives the timer cam can fail, or the contacts can burn and weld shut in the wrong position. Electronic controllers can lose their programming after a power cut, or the controller itself can fail. Either way, no initiation signal means no defrost and a steadily growing block of ice on the coil.
Defrost Heater Element Burned Out
Even if the timer initiates defrost perfectly, the heaters have to actually work. Electric defrost heaters operate in a harsh environment — cycling between sub-zero temperatures and high heat, surrounded by moisture. Over time, the elements corrode and burn out. If one or more heater elements have failed, the defrost cycle runs but doesn’t generate enough heat to melt the frost completely.
You’ll often see a pattern: the part of the coil closest to a working heater is clear, while the rest is iced. A resistance check on each heater element quickly identifies which ones have failed. Replacement heaters are specific to the evaporator model but are generally a straightforward swap.
Defrost Termination Thermostat Failed
The termination thermostat tells the system when defrost is complete. If it fails closed (always satisfied), the defrost cycle terminates immediately after starting — the controller thinks the coil is already warm when it isn’t. The heaters run for seconds instead of minutes, nowhere near long enough to clear the frost.
If it fails open (never satisfied), the defrost runs for the maximum time but that’s less of an icing problem and more of an energy and temperature-control issue. The failed-closed scenario is the one that causes rapid ice build-up. Testing the thermostat with a multimeter at ambient temperature versus coil temperature confirms whether it’s switching correctly.
Low Refrigerant Charge
A refrigerant leak causes the evaporator to operate at a lower pressure and temperature than designed. The coil surface drops well below zero — potentially to -25°C or colder — which means frost forms faster and denser than the defrost cycle can cope with. Even a fully functional defrost system can’t keep up if the coil is running 10 degrees below its design temperature.
Signs of low refrigerant alongside icing include: the compressor running continuously, the cold room struggling to pull down to temperature between defrost cycles, and frost or ice forming unevenly on the coil (concentrated at the inlet). A pressure check confirms the charge level, and the leak needs to be found and repaired before recharging.
Damaged Door Gasket
A torn, compressed, or missing door gasket allows warm, moisture-laden air from the kitchen to constantly infiltrate the cold room. That moisture has to go somewhere — and it ends up as frost on the coldest surface in the room, which is the evaporator coil. The defrost system may be working correctly but simply cannot keep up with the volume of moisture entering through the bad seal.
Close the cold room door and look for daylight around the edges. Feel for cold air escaping. If the gasket is damaged, replacing it is cheap and quick — and it solves the moisture problem at source rather than trying to defrost it away after the fact.
Evaporator Fan Not Running
The evaporator fan circulates cold air around the room and across the coil. If the fan motor fails, air stagnates around the coil surface. Without airflow, moisture in the immediate vicinity of the coil freezes and stays frozen. Ice builds from the coil outward in a dense, solid mass rather than the light frost you’d normally see.
A failed evaporator fan also means the room warms unevenly — the area near the coil may stay cold while the far end climbs in temperature. Listen for the fan when the compressor is running. If the cold room is quiet, the fan has likely failed and needs replacing.
Emergency Response
A fully iced evaporator is an emergency for any food business. Product temperatures climbing above safe limits means potential stock losses and food safety risk. Step one is to move critical stock to another cold storage if available. Step two is to call an engineer.
ADK provides emergency cold room repair across London. We carry defrost timers, heater elements, termination thermostats, fan motors, and refrigerant for the most common cold room systems. We diagnose the root cause — not just defrost the ice and leave — because if the underlying fault isn’t fixed, the ice will be back within 24 hours. Call us day or night.