What is actually happening
There are two very different versions of this call and they lead in opposite directions. Work out which one you have before anything else, because it changes what we bring on the truck.
Version one: nothing is running. Stand next to the condensing unit, outside the box or on the roof. Silence. No compressor hum, no condenser fan. This is an electrical or safety-control fault, not a refrigerant problem. Something has opened the circuit deliberately.
Version two: it is running and the box still climbs. The compressor is working, you can hear it, and the temperature keeps going the wrong way. This is a capacity problem — the system is running but not moving heat.
Causes, in the order we find them
If nothing is running
- High-pressure switch tripped. The most common cause by a distance, and it is usually a dirty condenser that caused it. The switch is doing its job: protecting the compressor from pressures that would wreck it.
- Contactor coil failed or contacts welded. A cheap part that fails constantly. You can often hear the difference — a healthy contactor pulls in with a solid click.
- Control circuit fuse or transformer. The 24-volt side is dead, so nothing is being told to start.
- Low-pressure switch locked out. Which means the charge is gone, which means a leak. The switch is the symptom, not the fault.
If it is running but not cooling
- Condenser coil blocked. In a Boston kitchen the enemy is airborne grease, which bonds dust to the fins into a mat you cannot see through. Head pressure climbs, capacity collapses.
- Refrigerant charge low. A system does not use refrigerant. If it is low, it leaked, and under the rules that took effect in January 2026 that leak has to be found and repaired rather than topped up.
- Liquid line solenoid stuck shut or a blocked filter drier starving the evaporator.
- Evaporator fans not turning — the coil gets cold, nothing blows the cold anywhere.
- Compressor running but not pumping. Valves gone. It sounds normal and does nothing. This is usually a replacement conversation.
Three checks worth making yourself
- Look at the condenser coil. Shine a light through it. If you cannot see light through the fins, you have found at least part of your problem. Do not pressure-wash it — you will flatten the fins and make it permanently worse.
- Check the evaporator fans are spinning inside the box. If the coil is cold and the fans are still, the fault is in front of you.
- Check the breaker and the disconnect. Both. The disconnect next to the condensing unit gets switched off by cleaners, roofers and delivery crews more often than anyone admits.
What this usually costs
A contactor is a same-visit repair. A condenser clean is routine work and often turns into a maintenance conversation, because a coil that bad did not get that way in a month. A leak repair depends entirely on where the leak is — a flare joint is cheap, a pinholed evaporator in a ten-year-old box is the point where replacing the equipment starts making more sense than chasing it.
The local angle
Two Boston-specific causes come up often enough to mention. Rooftop condensers near the harbour in East Boston, the Seaport, Chelsea and Revere corrode from salt air far faster than inland units, and a corroded condenser loses capacity long before it looks broken. And basement condensing units in the older restaurant stock around the North End and Chinatown frequently sit in a space with no real ventilation, so on a warm week they are recirculating their own hot discharge air and tripping on high pressure every afternoon.
Questions we get asked
How long does product stay safe in a dead walk-in cooler?
A loaded cooler with the door kept shut holds below 41F for roughly four to six hours. An empty one loses it in under two, because product mass is what carries the cold. Massachusetts cold-holding rules give you no grace period once you cross 41F, so start moving high-risk items early rather than waiting to see what happens.
Should I prop the door open to check on it?
No. Every door opening dumps the remaining cold air out of the bottom and pulls humid kitchen air in at the top, which both warms the box and feeds ice onto the coil. Check it once, note the temperature, and keep it shut.
Is it worth hitting the reset before calling?
Once. If a high-pressure switch has tripped because the condenser was dirty, cleaning the coil and resetting it can genuinely fix it. If it trips again within an hour, stop. Repeated resets on a pressure fault are how a compressor gets destroyed.