Glass-door cooler in Torrance: the coil froze solid because a stuck relay wouldn't let the compressor stop

A cooler that stopped cooling, and a coil you couldn’t see

This call was for a glass-door commercial cooler in Torrance that had stopped cooling. On this unit, everything that makes the cold sits in the bottom compartment, the compressor and the evaporator coil included.

The coil has its own insulated box down there. When I opened the compartment, all I could see was a block of ice.

A coil encased in ice can’t pass air. The fans keep blowing against it and the box warms up, even with the compressor running.

Thawing the coil

I melted the ice with steam. That takes longer than chipping at it, but it doesn’t bend fins or put a hole in a tube. On this cooler that matters more than usual. It runs on R290, a flammable refrigerant, and the label on the unit says in capital letters not to puncture the refrigerant tubing.

A thawed coil only buys time. If the cause is still there, the ice comes back, so the real work was finding out why it froze.

Controller off, compressor still running

The giveaway came when I switched the controller off: the compressor kept running, still pumping refrigerant, and the fans kept spinning.

A healthy controller, switched off, stops the compressor. This one couldn’t, because the part that does the stopping was stuck.

How the controller decides when the compressor runs

The controller reads a temperature probe in the box and switches the compressor through a relay. When the box warms up, the relay closes and the compressor starts. Once the box is cold enough, the relay opens and the compressor stops. On a schedule, the controller also stops the compressor for defrost, so the frost that builds up on the coil can melt off.

With the relay stuck in the on position, none of that happens. The compressor runs around the clock. It doesn’t stop when the box is cold, and it doesn’t stop for defrost. Frost keeps building on the coil until the coil is one piece of ice and air can’t get through. Then the cooler warms up with the compressor still running.

That’s why the ice was only a symptom: melt it and leave, and the coil freezes again.

Proving it with a meter

I pulled the old controller and checked it with a meter. It’s a Carel, with a 2018 date on its label. With the controller disconnected, terminals 4 and 5, the contacts of the compressor relay, should read open. The meter beeped for continuity. That relay was stuck in the on position.

Since the relay is built into the controller, the fix was a new controller.

Wiring the new one by the diagram

The replacement is a Copeland controller, a different make from the Carel, so I wired it by the diagrams: the old wiring on one side and the new controller’s terminals on the other.

One sensor lead on the old diagram, drawn as a dashed line, has no terminal on the new controller. I insulated that lead and left it unconnected.

The settings live in a hidden menu

A replacement controller has to be set up for the cooler it goes into. Setting the temperature takes one key. The rest takes some digging.

A 3-second press opens the basic menu, and a lot of settings aren’t in it. Press and hold again for 7 seconds, and a hidden menu opens with everything from the basic one plus the rest, including the defrost and fan settings. I set those before the cooler went back into service. On a cooler that froze because it skipped defrost, those are the settings you don’t want left at factory defaults.

Cycling on temperature again

Then I let the box pull down. At 37°F the compressor shut off on its own, on temperature, which is exactly what the old controller couldn’t do anymore. The cooler is cycling the way it should.

If your cooler keeps icing up

If the coil freezes again after someone melts the ice, the cause is still there. One easy check before anyone melts it a second time: does the compressor ever shut off? If it runs nonstop, have the controller and its relay tested.

Frozen coils have other causes too. A bakery’s walk-in cooler froze solid twice because its temperature was set far too low. And a replacement control can take more than plugging in: this commercial dishwasher’s universal board needed its wiring and program set for that one machine.

Commercial refrigeration repair in Torrance and across LA

Epic Star Repair handles commercial refrigeration and restaurant equipment repair in Torrance and across Los Angeles. CA BEAR registered (#A 50636), EPA Section 608 Universal certified. We quote a flat rate upfront, and the $85 service call is credited toward the repair. If your cooler is icing up or the compressor won’t shut off, call us before someone melts the ice and calls it fixed.