How a walk-in cooler works, and what normal looks like
By Vimal Bhaya, Founder and Lead Systems Architect · Last reviewed 4 October 2026
This is part 1 of 4 in How walk-ins work, and how they fail, a short tutorial on how walk-in coolers and freezers run, and what each failure looks like.
A walk-in cooler keeps its air above freezing and at or below 41°F (5°C), or 4°C (39°F) in Ontario, by pumping heat out of the box.
It is two halves joined by copper pipe: a cold coil and fans inside the box, and a compressor and condenser outside it. It has no heaters. Its coil clears its own light frost every time the cooling switches off, so a healthy walk-in cooler never builds ice.
Three layouts cover almost every walk-in cooler
Every walk-in has the same insulated box, the same door and the same cold coil under the ceiling. What changes is where the other half of the machine sits.
By our own estimate, three layouts cover about four in five walk-ins in the US and Canada, and almost every walk-in cooler in restaurants and schools. Nobody publishes a count, so the order below is our estimate too.
Layout | How to tell | How common | What it is prone to |
|---|---|---|---|
Split system, condensing unit outdoors  Coil inside, condensing unit outside, two pipes between. This drawing shows an electronic valve and controller. Most split walk-ins use a thermostatic valve instead. | Two copper pipes leave the box through the roof or a wall. They run to a metal cabinet with a large fan, on the roof or a concrete pad. | The most common | Warm on hot afternoons when the condenser is dirty, and warm on cold nights without winter controls (warm box). |
Split system, condensing unit indoors | The same cabinet sits on top of the box or in a back room. You hear it running indoors. | Common in older city buildings and in Canada | Warm when the kitchen is hot, because it breathes kitchen air, grease and flour (warm box). |
Packaged (self-contained) unit  Ceiling mount Wall mount Side view: half outside, half in | One factory-built unit sits through the roof or a wall, half inside and half outside. No copper pipes run between parts. | Less common, and growing |
A fourth layout feeds the coil from a supermarket's central machine room, shared with the display cases. That is a different system, and this page does not cover it.
A room cooled by a window air conditioner and a controller is not one of these either. It works differently, and CoolBot vs a real walk-in explains how.
A cooler moves heat out of the box, it does not make cold
Cold is only the absence of heat. So the machine's whole job is to pick up heat inside the box and dump it outside, over and over.
It does that with refrigerant, a fluid that boils at low temperatures. Boiling soaks up heat, the way sweat cools your skin. Squeezing the vapour back into a liquid gives that heat up again somewhere else.
Here are the parts, in the order the refrigerant meets them.
- The compressor, outside the box. A pump that squeezes the refrigerant vapour until it is hot enough to give its heat away.
- The condenser coil and its fan, next to the compressor. A radiator that blows the heat into the outdoor air or the room. The refrigerant cools and turns back into a liquid.
- The line set, two copper pipes. The thin one carries liquid into the box. The thick, insulated one brings vapour back out.
- The expansion valve, where the liquid enters the coil. A tiny opening the liquid is squeezed through, so its pressure drops and it can boil cold. Walk-ins use three kinds, set out below.
- The coil inside the box (the evaporator). The liquid boils in its tubes and soaks heat out of the air blown across it.
- The evaporator fans, which pull the box's air through that coil and throw it back out cold.
A thermostat or controller decides when to cool. A drain carries away the water from the coil. The door, its gasket, a closer and a strip curtain keep warm air out.
The expansion valve comes in three kinds, and they behave differently
Every walk-in has one. It decides how much refrigerant reaches the coil. Too little, and the coil is starved: the box runs warm and only part of the coil frosts. Too much, and liquid can flood back to the compressor and damage it.
There are three kinds, and the difference matters. The same warm box has different causes, and a different repair, depending on which one you have.
Kind | How it meters | Adjusts to the load? | Where you find it | What goes wrong |
|---|---|---|---|---|
Capillary tube, or a fixed orifice | A long, very thin tube, or a fixed hole, that lets refrigerant through at a set rate | No. A fixed device will not open more when the load rises | Small packaged units. Daikin's SB ceiling monoblocks, for example, list expansion through a capillary tube | Blocked by moisture or debris. The refrigerant charge has to be exactly right |
Thermostatic expansion valve (TXV) | A valve with a sensing bulb clamped to the pipe leaving the coil. The bulb feels how warm the outgoing vapour is, and opens or closes the valve to match | Yes, by itself | Most split-system unit coolers | A plugged inlet screen, moisture freezing in the valve, or a bulb that has come loose or lost its charge |
Electronic expansion valve (EEV) | A motor-driven valve, moved by a controller that reads a pressure sensor and a temperature sensor | Yes, most precisely | Premium controllers and newer systems, like the split drawing above | A failed sensor, a failed driver, or the wrong setting |
A TXV aims for a target called superheat: how many degrees the vapour has warmed past its boiling point by the time it leaves the coil. A few degrees means the coil is full of boiling refrigerant, with no liquid escaping toward the compressor. Heatcraft gives 6°F to 10°F (3°C to 6°C) for a typical design.
You cannot check any of these yourself. What helps is knowing which one you have, from the unit's spec sheet or your technician, because it changes what a low charge diagnosis means. How each one fails, and how a technician tells a blocked valve from a leak, is on the walk-in expansion valve page.
Every part has a healthy look or sound you can check
This table is the page in one place. The last column links to a page on each part. Two are written so far; the rest are marked as links to come.
Part | What it does | Where it sits | Healthy looks or sounds like | Part page |
|---|---|---|---|---|
Compressor | Pumps the refrigerant round the loop | Outdoor split: in the cabinet on the roof or pad. Indoor split: on top of the box or in a back room. Packaged: in the unit on the box | Runs in steady stretches with an even hum, then rests. Not on and off every few minutes | compressor, contactor and starting parts |
Condenser coil and fan | Throws the box's heat away | Next to the compressor, in all three layouts | Fins you can see through. Fan turning whenever the compressor runs, warm air coming off | condenser coil and fan |
Refrigerant and line set | Carry the heat from the coil to the condenser | Split systems: two copper pipes between the halves. Packaged: sealed inside the unit | Nothing to see. A technician checks the charge with gauges | refrigerant leaks and charge |
Expansion valve | Meters liquid into the coil | At the coil inlet, inside the box. Packaged units often use a capillary tube instead | Nothing to see. A technician checks it by measuring | |
The coil (evaporator) | Soaks heat out of the box's air | Under the ceiling inside the box. Packaged: the inside half of the unit | Clear, or a light frost that is gone each time the cooling stops. Never ice | |
Evaporator fans | Push the box's air through the coil, including while the cooling is off | In the unit under the ceiling | All turning, all the time on most coolers | evaporator fans |
Thermostat or controller | Decides when to cool | On the wall inside or outside the box, or on the unit | Still at the setting you chose. The box sits near it | thermostat, controller and pressure switches |
Defrost timer, where fitted | Forces the cooling off for a while on a schedule | On the wall near the box or in the condensing unit. Many coolers have none | Its dial shows the right time of day | defrost timer |
Drain line and trap | Carry the water from the coil out of the box | From the pan under the coil, through the wall, to a trap outside. No heater | No standing water in the pan, no drips from the unit | drain line, trap and drain heater |
Door, gasket, closer and strip curtain | Keep warm, wet air out | The door and its frame | The door swings shut by itself and seals all round. Walls and ceiling dry | door gasket, closer, strip curtain and door heaters |
Winter controls | Keep an outdoor unit working in freezing weather | In the outdoor cabinet. Indoor units usually have none | The box holds on cold nights as well as warm days | winter controls: head-pressure control and crankcase heater |
One cooling cycle runs from the thermostat calling for cold to the compressor stopping
- The box warms a little past its setting, and the thermostat calls for cold.
- On most split systems, that opens a valve on the liquid pipe. Pressure rises in the system and the compressor starts.
- The compressor squeezes the vapour and sends it to the condenser, where the fan blows the heat away.
- The liquid travels into the box through the line set. The expansion valve meters it into the coil.
- The liquid boils in the coil, soaking up heat from the air the fans push through. The vapour returns to the compressor.
- The box reaches its setting, and the thermostat closes the valve on the liquid pipe.
- The compressor keeps running for a moment, then stops on its own. The fans keep running.
The short run at the end is called pump-down, and it is on purpose. The compressor pulls the refrigerant out of the coil and stores it before it stops, so liquid cannot pool in the compressor while it is off and damage it at the next start.
A cooler runs its defrost every time it switches off, using its own air
The coil inside a cooler runs below freezing, so moisture from the box's air can settle on it as a light frost. A cooler does not need heaters to clear it. It uses its own air.
- The box reaches its setting and the compressor stops.
- The evaporator fans keep running.
- They blow the box's air, which is above freezing, across the coil. The light frost melts.
- The water drips into the pan and runs out through the drain to a trap outside the box.
- The box warms past its setting, and cooling starts again with a clear coil.
This is called off-cycle or air defrost. Some coolers add a timer or controller that forces the cooling off on a schedule, for boxes that might otherwise run for hours without a break. One controller maker ends those air defrosts when the coil reaches 40°F (4°C).
Why a cooler should never ice. Off-cycle defrost only works while the box air is above freezing. Emerson's refrigeration manual uses it for rooms of 35°F (2°C) or warmer. If the thermostat is set colder than that, or the cooling never switches off, the coil never gets its chance to clear. Frost turns to ice, and ice on a cooler's coil is always a fault.
Here are the vital metrics of a healthy walk-in cooler that you can check
These are typical figures from manufacturers' manuals and the food code. Your installer's settings come first.
| What | Normal | Where the number comes from |
|---|---|---|
| Box temperature | Above 32°F (0°C). Systems are commonly sized to hold a 35°F (2°C) room | 10 CFR 431.302, Heatcraft sizing guide |
| Food safety line | 41°F (5°C) or colder under the FDA Food Code. 4°C (39°F) or colder in Ontario | FDA, O. Reg. 493/17 |
| Defrost | Every time the compressor stops, with the fans still running. Where a controller forces one, it can end when the coil reaches about 40°F (4°C) | Emerson manual, KE2 manual |
| Ice on the coil | None. A light frost that clears each off cycle is fine | Emerson manual |
| Run time | About 16 hours in every 24 for a 35°F (2°C) room. Older guidance says 16 to 18 | Heatcraft sizing guide, Emerson manual |
| Coil compared with the box air | About 10°F to 16°F (6°C to 9°C) colder. A technician measures this | Heatcraft unit cooler manual, Emerson manual |
| On and off cycles | No single normal number, because it depends on the load. Expect steady runs with real rests, adding up to about 16 hours of running a day. More than 10 to 12 starts an hour is short cycling, by one compressor maker's limit | Heatcraft sizing guide, Tecumseh compressor manual |
What the technician will measure: refrigerant pressures, superheat at the coil and the compressor's current draw. You do not need those numbers. You need to know whether your box behaves like this table.
Here is what a well-functioning cooler looks like in terms of air temperature
These are real readings from a walk-in cooler in a busy quick-service coffee shop in Ontario, taken every 5 minutes from 1 March to 15 April 2026. Staff are in and out of it all day.
At night, the cooler ripples. The air drifts up while the cooling rests and drops when it runs, about four times an hour, by about 1°F (0.5°C). On nine nights in ten it sat between 34°F and 37°F (1°C and 3°C). Each rest is also the coil's chance to clear its light frost, as the defrost section above explains.
By day, it spikes. Over the 46 days there were about four rises a day of 2.7°F (1.5°C) or more, four in five of them between 8 AM and 8 PM. The typical one added about 5°F (3°C) and was back down within 40 minutes. Nearly three in four were back within the hour.
So normal, for a busy cooler, is not a flat line. It is a steady ripple at night, and spikes by day that come back down. A struggling cooler looks different: the spikes take longer to come down, or the night ripple stops. There is a real example of that on walk-in cooler troubleshooting.
The same idea, on real readings from a walk-in freezer, is on how a walk-in freezer works. There the bumps are bigger and come on a schedule, because a freezer has to heat its coil.
This is the air, not the food. The air in this cooler stayed above 41°F (5°C) for at least half an hour on 39 of the 46 days, almost always after the door had been open. A loaded cooler's food changes temperature far more slowly than its air. That is why a sensor in a small bottle of glycol reads like the food rather than the air, as glycol buffers and thermobuffers explains, and why a short spike from the door is not a reason to throw anything away. The same point, made with a freezer's defrost, is on why temperature alarms cry wolf.
Five symptoms cover most of what goes wrong
Each one has its own section, with what it usually is and what to check, on walk-in cooler troubleshooting.
- The box is warm and will not hold temperature
- Condensation, wet walls or ceiling, or a door that will not seal
- Ice on the coil
- Freezing the food: the box is too cold
- Runs all the time, or clicks on and off
This page describes the common case, not your exact box
Your cooler may differ. Some have an electronic controller with its own defrost settings, and some sit on a supermarket rack. Your installer's settings win over the typical numbers here.
Our readings come from one busy cooler, in one shop, over six weeks of a Canadian spring. A quieter box spikes less. A box in a hot kitchen, or the same box in summer, takes longer to come back down.