A walk-in cools by moving heat, not making cold. The compressor raises refrigerant pressure, the condenser releases heat outdoors, the expansion valve drops the pressure and the evaporator absorbs heat from the box. The cycle repeats continuously. When any step is weak, the box warms.
The same four steps run in every walk-in, from a small restaurant cooler to a large freezer. The names of the parts change, but the sequence does not.
The four steps
| Step | Part | What happens |
|---|---|---|
| 1. Compression | Compressor | Low-pressure vapor is squeezed into hot high-pressure gas |
| 2. Condensation | Condenser coil | The gas releases heat to outdoor air and becomes a liquid |
| 3. Expansion | Expansion valve | Pressure drops, so the liquid becomes very cold |
| 4. Evaporation | Evaporator coil | The cold refrigerant absorbs heat from box air and boils into vapor |
Follow one molecule around
Start at the evaporator. Warm air from the box passes over the coil, and the cold refrigerant inside absorbs that heat and turns to vapor. The vapor travels through the suction line to the compressor. The compressor raises its pressure and temperature, sending hot gas through the discharge line to the condenser. A fan blows outdoor air over the condenser, the gas gives up heat and turns to liquid, and the liquid flows through the liquid line, past the drier and sight glass, to the expansion valve. The valve meters it into the evaporator, and the cycle begins again.
High side and low side
- The high side runs from the compressor outlet to the expansion valve. It carries hot, high-pressure refrigerant.
- The low side runs from the expansion valve to the compressor inlet. It carries cold, low-pressure refrigerant.
- The compressor and expansion valve are the dividing points.
How faults show up
| Weak step | What you notice |
|---|---|
| Compression | Compressor hums, trips or runs without cooling |
| Condensation | Hot unit, high pressure, trips on hot days, dirty coil |
| Expansion | Warm box with low suction pressure, frost at the valve |
| Evaporation | Iced coil, weak airflow, slow cooling |
| Low charge | Bubbling sight glass, oil stains, long run times |
Where defrost fits
A freezer coil collects frost that blocks airflow. Defrost temporarily reverses the heat flow with electric heaters or hot gas, melts the frost and drains the water away. See electric, off-cycle and hot-gas defrost for how each method works.
Why this helps you
When you describe a problem, saying which part of the cycle seems weak helps a technician. "The condensing unit is hot and tripping" points to the high side. "The coil is iced and the box is warm" points to the low side or defrost. It also helps you read quotes and understand why a technician measures pressures and temperatures before recommending anything.
Read the systems guide, the parts guide and the troubleshooting guide for more.
Reading pressures
Technicians read two pressures, suction on the low side and discharge on the high side, and compare them with temperatures at the same points. Together with superheat and subcooling, these numbers show whether the charge, the valve and the airflow are right. That is why a technician who arrives with gauges and thermometers is not wasting time. They are following the cycle around the system to see where it breaks.
Questions about this topic
Does a walk-in make cold?
No. It moves heat from inside the box to the outside. What feels like cold is the absence of heat.
Why does the outdoor unit blow hot air?
That is the heat removed from the box, plus the heat of compression, being released.
Is it the same for a freezer?
The cycle is the same. Freezers run at lower pressures and need defrost.