Refrigeration
1:20 Isn't 120: The Defrost Timer Bug Hiding in Our Controls Migration
On October 1, 2025, I figured out why our freezer defrosts kept running into each other. The answer was a colon.
It’s the kind of mistake that’s embarrassing to write down. I’m writing it down anyway, because anybody moving refrigeration controls from one system to another can make the same one.
Some background
I run an underground freezer in the Valmeyer bluffs. Every evaporator in the freezer has to defrost on a schedule, and our rule is one evaporator in defrost at a time. Several coils defrosting together pushes head pressure up and throws suction pressure around, and that’s how you trip compressors. So the defrosts are spread across the whole day, mostly on the hour or the half hour.
The master copy of that schedule lives in a spreadsheet. Before anybody touches a controller, they check the sheet. One source of truth.
Between late 2024 and early 2025, we moved the refrigeration controls off our old unit controllers and onto a new platform. The first stretch of any migration like that is rough. Graphs and alarms couldn’t be fully trusted at first.
The bug
Our defrost sheet was written the way the old controllers wanted it. A defrost length on the sheet looked like 1:20.
On the old boxes, 1:20 meant one hour and twenty minutes. 80 minutes.
On the new system, the field is just minutes. Type 120 and you get exactly what it says: 120 minutes.
I’d been copying the sheet into the new controls, and “1:20” had quietly become “120.” Every one of those defrosts was running 40 minutes longer than intended, half again as long as it should have been. The same math catches any length over an hour: an hour and a half written “1:30” becomes 130 minutes instead of 90.
It had been that way for a long time before I caught it.
Why it showed up as overlap
When the schedule puts defrosts every hour or half hour, an extra 40 minutes means a unit is still in defrost when the next one starts. Now you have two coils defrosting at once, which is exactly what the one-at-a-time rule is there to stop.
That’s what I kept seeing: defrosts overlapping far more than the schedule said they should. It’s easy to treat that as a scheduling problem and keep moving times around. Rearranging times doesn’t help when every defrost is running 40 minutes long.
How I found it
At the end of September I was checking whether a round of defrost schedule changes had taken, and I had to adjust some. The next day it finally clicked: the two systems were counting time differently, and I’d been copying the sheet without realizing it.
It took a bit, but I got the entries changed over to match, so 80 minutes is entered as 80 minutes.
The next day I checked, and the defrosts were hitting the right times. A couple of days later I wrote in my notes that the new defrost schedule was “working good.” Around here, that’s high praise.
It wasn’t the last unit mix-up
In 2026 we found the same kind of problem somewhere else. The compressor load and unload timers had been entered in minutes instead of seconds. Fixing those timers, along with a staging glitch, smoothed out pressure swings that had been tripping the plant.
Two different settings, two different systems, same root cause: a number got carried over and nobody checked what unit it was in.
Lessons learned
Check the units on every migrated setting. Don’t just check the number. Ask what the field actually means on the new system: minutes, seconds, hours and minutes, or tenths of something.
Write the units in your master sheet. “80 min” can’t be misread. “1:20” can be read two ways depending on which box you’re standing in front of.
Watch one real cycle after a change. After a migration, pick a unit, watch its defrost start, and time it to the end. If 80 minutes on paper turns out to be 120 on the clock, you’ll know in an afternoon.
When the schedule looks wrong, check the inputs before you reschedule. Moving defrost slots around won’t fix it when the real problem is the length of every defrost.
Assume there’s more than one. If one setting carried over in the wrong unit, others probably did too. Ours showed up again in the compressor timers.
For other operators
If you’re planning a controls migration, make a list of every setting that carries a time or a rate: defrost lengths, drip and fan delays, load and unload timers, alarm delays. Next to each one, write the unit the old system used and the unit the new one expects. It’s a boring afternoon. It’s a lot less boring than chasing overlapping defrosts.
This is from my own operation. Your controllers will have their own formats, so check your manufacturer’s documentation before changing any settings.
Related: The Defrost Game, on how I stagger defrosts across 17+ freezer units.
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