Walk-In Cooler Glossary: Parts and Terms in Plain English

Refrigeration technicians talk in shorthand. This glossary translates the terms you're most likely to hear on a service call or read on a quote, in alphabetical order. Where the guide has a full page on a part or problem, the term links to it. For how the parts work together, start with the site's overview of how walk-in coolers work.

A

A2L refrigerant. A refrigerant in ASHRAE safety class A2L: lower toxicity and "lower flammability." Newer low-GWP refrigerants such as R-454A, R-454C and R-455A are A2Ls, and systems using them add leak detectors and safety controls. See refrigerant leak safety.

Air defrost (off-cycle defrost). Defrosting by simply stopping refrigeration while the evaporator fans keep blowing box air over the coil. Common on coolers held around 35°F and above. See defrost system.

Alarm delay. The time a temperature must stay out of range before a controller or monitoring system alarms. It stops normal door openings and defrosts from triggering false alarms. See temperature monitoring.

Ambient temperature. The temperature of the air around something: around the condensing unit (which matters for summer and winter performance) or inside the box.

Amp draw. The electrical current a motor or compressor is pulling, measured by a technician with a clamp meter and compared with the nameplate.

Anti-sweat heater. A low-wattage heater in a door frame, glass door or mullion that keeps the surface above the dew point so it doesn't sweat or frost. See door frame heaters.

AWEF (Annual Walk-in Energy Factor). DOE's efficiency rating for walk-in refrigeration systems. Minimum values are set in federal rules. See DOE walk-in regulations.

B

Box. Industry slang for the insulated walk-in enclosure itself.

Box temperature. The air temperature inside the walk-in as measured by the controller or a thermometer.

Buffered probe. A temperature probe surrounded by a liquid or solid that mimics food, so it reads closer to product temperature and ignores brief air spikes.

C

Cam-lift hinge. A door hinge with a ramped surface that lifts the door slightly as it opens and uses gravity to swing it closed. See door closers and hinges.

Capacitor. An electrical part that helps a motor or compressor start (start capacitor) or run efficiently (run capacitor). It can store a dangerous charge after power is off. See contactor, capacitor and relay.

Compressor. The pump at the heart of the system. It draws in low-pressure refrigerant vapor from the evaporator and compresses it to high pressure so it can shed heat in the condenser. See compressor.

Condenser coil. The finned coil in the condensing unit where the refrigerant gives up its heat to outside air. When it's dirty, the whole system suffers. See condenser coil.

Condenser fan motor. The motor that pulls air through the condenser coil. See condenser fan motor.

Condensing unit. The assembly outside the box that holds the compressor, condenser coil, condenser fan and controls. See condensing unit.

Contactor. A heavy-duty electrical switch, controlled by a low-voltage signal, that turns the compressor and condenser fan on and off. Its contacts can pit or weld shut.

Crankcase heater. A small heater on the compressor that keeps its oil warm so refrigerant doesn't collect in it during off cycles, especially in cold weather. See winter and low ambient.

Cut-in / cut-out. The temperatures or pressures at which a control turns equipment on (cut-in) and off (cut-out).

D

Defrost termination. The signal that ends a defrost cycle, usually when a sensor or switch on the coil reaches a set temperature. A time limit (fail-safe) ends it if the sensor never does.

Defrost timer. A clock or controller function that starts defrost cycles at set times or intervals.

Delamination. When a panel's metal skin separates from its foam core, often from water getting in or physical damage. See panels and insulation.

Dew point. The temperature at which moisture in the air condenses into water. Surfaces colder than the dew point sweat. See walk-in sweating outside.

Differential. The gap between the temperature where a thermostat turns cooling on and where it turns it off. Too narrow causes rapid cycling; too wide causes big temperature swings. See thermostat and controller.

Disconnect. The lockable power switch mounted near the condensing unit or evaporator that cuts power to it for service.

Drain line heater (heat tape). An electric heating cable along a freezer's condensate drain line that keeps defrost water from freezing in it. See drain line heater.

Drain pan. The pan under the evaporator coil that catches defrost water and sends it to the drain line. Freezer pans are heated.

E

ECM (electronically commutated motor). An efficient, electronically controlled fan motor. Federal rules require ECM or 3-phase motors for small evaporator fans in walk-ins. See evaporator fan motor.

EEV (electronic expansion valve). An electronically controlled valve that meters refrigerant into the evaporator based on sensor readings, used on newer systems instead of a TXV. See expansion valve.

Electric defrost. Defrosting with electric heaters built into the evaporator coil. Standard on freezers.

EPA Section 608 certification. The federal certification a technician must hold to open a refrigeration system and handle refrigerant. Type II or Universal covers walk-in systems. See hiring a technician.

Evaporator coil. The finned coil inside the box where cold refrigerant absorbs heat from the air. See evaporator coil.

Evaporator fan motor. The motor that blows box air through the evaporator coil.

F

Fail-safe (defrost). The maximum time a defrost is allowed to run before the controller ends it, even if the termination sensor hasn't reached its setting.

Fan cycling. A low-ambient control that stops and starts the condenser fan to keep head pressure up in cold weather.

Fan delay. A switch or controller setting that keeps evaporator fans off after defrost until the coil is cold again, so they don't blow water or warm air into the box. See evaporator dripping or spitting water.

Filter drier. A canister in the liquid line that traps moisture and debris in the refrigerant. It's replaced whenever the system is opened. See filter drier and sight glass.

Flooded head pressure control. A valve that backs liquid refrigerant up into the condenser in cold weather to keep head pressure up. Systems with one need extra refrigerant.

Floodback (liquid slugging). Liquid refrigerant reaching the compressor instead of vapor. It can break valves and wash oil off bearings. See frost on the suction line.

Frost heave. Ground under a freezer floor freezing and expanding, which lifts and cracks the floor. See freezer floor heaving.

Full charge. The amount of refrigerant a system needs to run correctly. Federal leak rules require owners of systems with 15 lb or more of HFC to determine and record it. See walk-in refrigerant rules.

G

Gasket. The flexible seal around a walk-in door, usually magnetic on the hinge and latch sides with a sweep at the bottom. See door gaskets.

GWP (global warming potential). A measure of how strongly a refrigerant warms the climate compared with carbon dioxide. Federal limits on new equipment are written in GWP; R-404A is about 3,922 and R-448A about 1,387.

H

Head pressure (high-side or discharge pressure). The pressure on the hot side of the system, between the compressor and the expansion valve. It rises with a dirty condenser, a failed condenser fan or hot weather. See condensing unit overheating.

Head pressure control. Any method (fan cycling, a flooded valve, variable fan speed) of keeping head pressure high enough in cold weather.

Heat load. The total heat the system has to remove: heat through the walls, from door openings, from product, people, lights and motors.

Hermetic compressor. A compressor sealed in a welded steel shell. It can't be opened for repair, only replaced. Scroll and most small reciprocating compressors are hermetic.

HFC (hydrofluorocarbon). The family of refrigerants, such as R-404A, R-448A and R-449A, being phased down under the federal AIM Act.

High-pressure control. A safety switch that shuts the compressor off if head pressure gets dangerously high. Some reset automatically, others need a manual reset. See pressure controls.

Hot-gas defrost. Defrosting by routing hot refrigerant vapor from the compressor through the evaporator coil.

K

Klixon. A brand name used loosely for small snap-disc temperature switches. On evaporators they often serve as the defrost termination or fan delay switch.

L

Leak detector (refrigerant). A sensor that detects refrigerant in the air. Required on A2L systems, where it triggers fans and shut-off valves.

Leak rate. How fast a system loses refrigerant, calculated each time refrigerant is added. For commercial refrigeration systems covered by federal rules, a leak rate above 20% triggers mandatory repair.

Liquid line. The small refrigerant line carrying warm liquid refrigerant from the condensing unit to the evaporator.

Liquid line solenoid valve. An electrically operated valve in the liquid line that the thermostat opens and closes on pump-down systems. See liquid line solenoid valve.

Lockout/tagout. The OSHA procedure for shutting off and locking equipment's energy source so it can't start while someone is working on it.

Low-ambient control. Any control that keeps an outdoor condensing unit working properly in cold weather.

Low-pressure control. A switch that stops the compressor when suction pressure falls to its setting. On pump-down systems it's the normal off switch; it also protects against loss of charge.

Low-temperature / medium-temperature. Industry shorthand for freezer (low-temp) and cooler (medium-temp) systems.

M

Manual reset. A safety control that stays off after it trips until someone presses its reset button. A trip means something is wrong; repeated resets without a diagnosis are a bad idea.

Moisture indicator. A color-changing element in the sight glass that shows whether the refrigerant contains moisture.

N

Nameplate. The data label on the condensing unit or evaporator listing model, serial number, refrigerant type, voltage and electrical ratings. Photograph it before you call a tech.

Non-condensables. Air or other gases trapped in the refrigerant circuit, usually from poor service practice. They raise head pressure.

O

Overload (thermal overload). A protective device that shuts a compressor or motor off when it overheats or draws too much current, then resets after it cools.

P

Panel. A prefabricated insulated wall, ceiling or floor section of a walk-in, usually foamed polyurethane between metal skins, locked together with cam locks. See panels and insulation.

Pressure relief port. A small, usually heated vent in a freezer wall that equalizes air pressure so the door can be opened after it closes. See pressure relief port.

Pressure transducer. An electronic sensor that reports refrigerant pressure to a controller.

Probe. The temperature sensor wired to a controller. Probes can fail, drift or be placed where they read the wrong air.

PSC motor (permanent split capacitor). An older, less efficient type of fan motor that uses a run capacitor. Still allowed for condenser fans under federal rules.

Pump down. A control method where the thermostat closes the liquid line solenoid, the compressor pulls refrigerant out of the evaporator and then shuts off on the low-pressure control. Protects the compressor from liquid at start-up.

R

R-22. An older HCFC refrigerant. Production and import ended in 2020; existing systems may still run and be serviced with recovered or reclaimed R-22.

R-404A. The most common walk-in refrigerant of recent decades. Existing systems can still be serviced, but it can't be used in new remote condensing units installed from 27 July 2026. See retrofitting R-404A.

R-448A / R-449A. Lower-GWP HFC blends (each just under 1,400) used in many current walk-in systems and as R-404A replacements.

Reclaimed refrigerant. Used refrigerant that has been reprocessed to purity standards by an EPA-certified reclaimer.

Refrigerant. The fluid that carries heat out of the box, boiling in the evaporator and condensing in the condenser.

Relay (start relay). A switch that brings a compressor's start capacitor into the circuit for a moment at start-up.

Remote condensing unit. A condensing unit installed away from the box, often outdoors or on a roof, connected by refrigerant lines.

S

Schrader valve. A small spring-loaded service port, like a tire valve, where technicians connect gauges. Its core can leak.

Scroll compressor. A hermetic compressor that compresses refrigerant between two spiral scrolls.

Self-contained system. A refrigeration system with the condensing unit and evaporator built into one package, often mounted on the walk-in's wall or roof.

Semi-hermetic compressor. A larger compressor with a bolted housing that can be opened and rebuilt in the field.

Setpoint. The temperature the controller is trying to hold.

Short cycling. The compressor starting and stopping too often. It wears out compressors and contactors. See short cycling.

Sight glass. A small window in the liquid line where a technician can see the refrigerant. Bubbles can mean low charge but not always.

Strip curtain. Overlapping vinyl strips hung in the doorway to limit warm air entering when the door is open. See strip curtains.

Subcooling. How far liquid refrigerant has been cooled below its condensing temperature. Technicians use it to judge refrigerant charge on some systems.

Suction line. The larger, insulated refrigerant line carrying cold vapor from the evaporator back to the compressor.

Suction pressure (low-side pressure). The pressure on the cold side of the system, between the expansion valve and the compressor.

Superheat. How far refrigerant vapor leaving the evaporator is warmed above its boiling temperature. It shows whether the evaporator is getting the right amount of refrigerant. Heatcraft says systems with a 10°F design TD should generally run 6 to 10°F of superheat at the evaporator.

T

TCS food (time/temperature control for safety). Food that needs temperature control to limit bacterial growth, such as meat, dairy and cut produce. The FDA Food Code limit for cold holding it is 41°F or below. See food safety when a walk-in fails.

TD (temperature difference). The difference between the box air temperature and the refrigerant's boiling temperature in the evaporator. A lower TD keeps humidity higher. Heatcraft balances general-purpose coolers at 10 to 12°F TD.

Thermostat / temperature controller. The control that turns cooling on and off to hold the setpoint. Modern walk-ins use electronic controllers that also run defrost and alarms. See thermostat and controller.

TXV (thermostatic expansion valve). A valve that meters liquid refrigerant into the evaporator based on a sensing bulb clamped to the suction line, keeping superheat steady. See expansion valve (TXV).

U

Unit cooler. The industry term for the evaporator assembly: coil, fans, drain pan and defrost parts in one housing.

V

Vapor-proof light fixture. A sealed fixture that keeps moisture away from the lamp and wiring, used inside walk-ins. See lights not working.

W

Walk-in (federal definition). An enclosed storage space refrigerated above 32°F (cooler) or at or below 32°F (freezer) that can be walked into and has less than 3,000 square feet of chilled storage area.

Wet insulation. Panel foam that has absorbed water through damaged skins or seams. It insulates poorly, can freeze inside freezer panels and is a common reason panels are replaced.