Every refrigeration system has a compressor and a condenser, whether it’s a split system like most walk-ins or self-contained like a reach-in. The compressor pumps refrigerant around the loop, and the condenser (a coil and a fan) gets rid of the heat the refrigerant picked up inside the box. If either one stops doing its part, the box can’t hold temperature, and from the outside the symptoms often look the same.
The compressor won’t start
If the condenser fan is running and the compressor isn’t, or you hear a click every few minutes and then nothing, the compressor itself may be fine. A lot of no-start problems come down to the parts that get it going: the start capacitor and relay, the contactor that switches power to it, or the overload that cuts it off when it gets too hot. Those are small parts, and replacing one costs a fraction of what a compressor does. But compressors do fail, of course. The windings inside can short to ground or open up, and a seized compressor will hum, trip its overload and never turn over. It’s a big repair, so before anyone tells you the compressor is bad, they should check the windings with a meter and show you the readings.
You can check the breaker and the disconnect near the condensing unit yourself. Reset a tripped breaker once, and if it trips again, leave it off.
Short cycling, or running and never stopping
A compressor that starts and stops every minute or two is short cycling. Low refrigerant can do it. So can a dirty condenser coil that trips the high pressure control, a bad thermostat or pressure switch, or an evaporator coil that’s iced over. It’s rough on the compressor either way, since starting up is when it works hardest.
The opposite is a compressor that runs all day while the box sits a few degrees warm. That points to something cutting into capacity. Common culprits are a low charge from a refrigerant leak, a condenser coil caked with dirt, a fan motor that quit, or a door that doesn’t close all the way and keeps letting kitchen air in. On a walk-in, start by checking the door. Our walk-in cooler repair page goes through the box side of things.
The condenser coil and fan
A lot of compressor trouble starts at the condenser. The fan pulls room or outdoor air across a finned coil, and whatever’s in that air ends up stuck to the fins. Indoors that’s mostly grease and flour dust. Outside it’s dirt and leaves, and for a few weeks in spring, cottonwood fluff. Once the coil mats over, the system can’t shed heat, so pressures climb and the compressor runs hot. Cleaning a coil costs very little next to replacing a compressor that overheated because the coil was plugged.
If you can safely see the coil, take a look. Gray, fuzzy fins mean it’s overdue. There’s a longer explanation in our post on dirty condenser coils.
Condenser fan motors are the other common failure. When the fan stops, head pressure shoots up and the compressor trips off on high pressure or overload, sometimes within minutes on a hot day. Then it cools down, restarts, and trips again. That pattern is a good reason to go look at the fan before anything else, and it’s a big part of why a walk-in can run warm on summer afternoons.
Noises
Squealing or grinding is usually a fan motor bearing on its way out, and rattling tends to be a loose fan blade or compressor mount. Knocking from inside the compressor itself is worse news. A loud hum with no start goes back to the start parts above.
When the compressor has to be replaced
Swapping a compressor is a real job. The refrigerant gets recovered, the old compressor comes out, the new one gets brazed in along with a new filter drier, and the system is pulled into a deep vacuum before it’s charged. If the old one burned out, the oil in the system can turn acidic, and that has to be cleaned up or it’ll take out the new compressor too. The tech should also find out why it failed. A dirty condenser or a contactor with pitted, chattering contacts can kill a compressor, and putting a new one in without fixing the cause sets you up to buy another.
On an older system, especially one running R-22, ask for a replacement price next to the compressor price. Sometimes a new condensing unit isn’t that far off, and everything else in the unit would be new along with it. Our page on installation and replacement covers that side, and you’ll get both numbers in writing either way.
