The store manager gets the alert at 7 AM: dairy case reading 8°C. It should be sitting between 2°C and 4°C. By the time they reach the shop floor, the temperature is still climbing. Product is at risk. Food safety regs say they need to act now — log the breach, start pulling stock, and get an engineer on site before the losses get worse.
Supermarket refrigeration isn’t like a domestic fridge. These are complex multi-case systems running off central compressor racks, with individual solenoid valves, expansion valves, and evaporators for each display case. When one case runs warm, the cause could be local to that specific case or systemic across the entire rack. Here’s how to work through it methodically.
Iced-Up Evaporator — Defrost Failure
The most common cause of a single case running warm. Every refrigeration display case runs scheduled defrost cycles — typically electric heater defrosts — to clear frost from the evaporator coil. If the defrost heater element fails, the defrost timer malfunctions, or the termination thermostat is faulty, ice accumulates on the evaporator with every cooling cycle until airflow is completely blocked.
You can often identify this without opening the case panels. If the air curtain across the front of the case feels weak or non-existent, the case temperature is rising steadily, and there’s visible frost on the back panel or around the air discharge grille, you’re almost certainly looking at a defrost failure. The evaporator needs a manual defrost to clear the ice — which takes time and means the case is out of service — followed by diagnosis and repair of whichever defrost component has failed.
Failed Solenoid Valve on the Liquid Line
Each display case on a centralised rack system has a liquid line solenoid valve. When the case thermostat calls for cooling, this solenoid opens and allows liquid refrigerant to flow to the case’s expansion valve and into the evaporator. If the solenoid fails in the closed position, no refrigerant reaches that case. The evaporator warms to ambient temperature, the case temperature climbs, and everything else on the rack continues operating normally — because the fault is isolated to that one valve.
Diagnosis is straightforward. Check for voltage at the solenoid coil when the case thermostat is calling for cooling. Voltage present but the valve not opening means the coil or the valve mechanism has failed. Listen for the click when the solenoid should energise. No click with voltage present confirms a dead coil. Solenoid coils are one of the most common spares a refrigeration engineer carries.
Blocked Expansion Valve
The thermostatic expansion valve (TXV) meters liquid refrigerant into the evaporator at precisely the rate needed for efficient heat exchange. These valves have small internal orifices that can block with wax from oil breakdown, moisture that’s frozen into ice crystals, or metallic debris from compressor wear circulating through the system.
A partially blocked TXV starves the evaporator — the case cools, but not enough to maintain safe temperatures under load. A fully blocked TXV means zero cooling. Wax blockages typically indicate degraded compressor oil in an aging system. Moisture blockages point to a failed or saturated filter drier on the liquid line. The TXV itself can be replaced, but the contamination source needs addressing — otherwise the replacement valve will block in the same way.
Evaporator Fan Failure
The evaporator fans serve two critical functions: they circulate cold air through the case to keep product at temperature, and they maintain the air curtain across the front of the case that prevents warm store air from entering. If a fan motor fails, airflow drops. Cold air pools at the bottom of the case, warm air infiltrates from the top, and the temperature sensor — usually positioned in the return air path — reads high.
Listen to the case. If it’s quieter than the identical cases either side of it, check the fans. Most display cases have multiple small fan motors, and losing one might not be immediately obvious visually. But the temperature impact is measurable within hours. Fan motors on display cases are a standard replacement item and can usually be swapped without recovering refrigerant.
Product Loaded Past the Air Curtain Line
Every open-front refrigerated display case has a load line — a marked position or shelf height that represents the maximum product level. Products stacked above this line physically break through the air curtain. Warm store air flows directly into the case, the evaporator can’t cope with the extra heat load, and the temperature climbs. The refrigeration system is working perfectly — the case is just being asked to do more than it was designed for.
This is an operational problem, not a mechanical one. Staff stacking shelves need to understand that overfilling a case above the load line doesn’t increase sales — it warms the product and creates a food safety risk. Check the load line before calling an engineer. It might save you a call-out charge.
Night Blinds Not Being Deployed
Open display cases are engineered to operate with night blinds — covers pulled down or across the case front when the store is closed. These blinds reduce the refrigeration load dramatically by eliminating warm air infiltration when product visibility is unnecessary. If closing staff aren’t deploying night blinds, the cases work substantially harder overnight. Compressors run longer, energy consumption spikes, and the entire system operates under stress it was never designed for.
The cumulative effect of not using night blinds accelerates compressor wear, increases the risk of mechanical failure, and inflates energy bills. Making night blind deployment part of the closing checklist is a management discipline issue with direct mechanical and financial consequences.
Compressor Failure on the Rack System
If multiple cases are running warm simultaneously, the problem is almost certainly upstream at the compressor rack rather than in any individual case. A failed compressor reduces the rack’s total cooling capacity. The remaining compressors might handle the load during quiet periods, but during peak trading hours — with heavy customer traffic, frequent case door openings, and high ambient temperatures in the store — they can’t keep up. Suction pressure rises, evaporator temperatures climb, and case temperatures follow.
Check the compressor rack. Are all compressors running? Is one tripped on overload or locked out with an error? Rack compressor failures need urgent attention because they affect multiple cases across multiple product categories simultaneously. One failed compressor can put thousands of pounds worth of chilled stock at risk.
Food Safety Compliance
UK food safety regulations require continuous temperature monitoring of chilled display equipment and documented corrective actions when temperatures breach safe limits. If your chilled case is above 8°C, you should already be following your HACCP procedures: recording the breach, pulling affected product, documenting what corrective action was taken, and retaining records for inspection. Environmental health officers check these records, and gaps in your temperature logs raise immediate red flags.
ADK provides emergency refrigeration repair for supermarkets and food retail across London. We understand the urgency — a warm display case isn’t just a repair ticket, it’s a food safety incident with compliance implications. Call us on 0207 801 0808 and we’ll get an engineer to you fast.