If your walk-in cooler is swinging between 28°F and 48°F instead of holding steady, you’ve got a mechanical problem, not a quirk. The most common culprits are a failing thermostatic expansion valve (TXV), a refrigerant issue (overcharge or undercharge), or a dirty condenser coil. Here’s how to tell them apart.
Why Temperature Swings Happen at All
A walk-in should cycle on, pull the box down to setpoint, and hold there within a few degrees. When it swings wide, the refrigeration system is failing to meter refrigerant correctly, remove heat efficiently, or both. The box overshoots in one direction, the controls overcorrect, and you get this seesaw pattern that ruins product and fails health inspections.
The TXV (Most Likely Cause)
The thermostatic expansion valve controls how much refrigerant enters the evaporator coil. It senses the suction line temperature through a sensing bulb strapped to that line. When the bulb loses charge, loses contact with the pipe, or the valve seat wears out, the TXV hunts. It opens too wide, floods the evaporator, then closes too far and starves it. That’s your swing.
Signs pointing to a TXV problem: suction pressure hunting up and down on a gauge set, frost that migrates back down the suction line toward the compressor, and swings that happen even in stable ambient conditions.
A tech will pull superheat readings at the evaporator outlet. Correct superheat for most walk-in evaporators runs roughly 8-12°F, but that varies by system and manufacturer, so always check the equipment specs. If it’s jumping from around 3°F to 25°F on successive readings, the TXV is hunting. Replacement requires a licensed tech (EPA Section 608 certification) because refrigerant recovery is involved.
Refrigerant Charge Problems
Too little refrigerant (undercharge, usually from a leak) and the evaporator runs starved, box temperature rises, compressor cycles short, box gets too warm. Too much refrigerant (overcharge, usually from someone adding refrigerant without checking charge properly) floods the evaporator and can cause liquid slugging back to the compressor.
Both conditions cause temperature instability. The difference: undercharge typically trends warm with short cycling, while overcharge trends erratic with high suction pressure and a compressor that runs hot.
You can’t diagnose charge without gauges and a temperature probe. There’s no reliable way to eyeball it. If a previous tech “just added some refrigerant” without checking subcooling and superheat, that’s worth telling your current tech, because overcharge is surprisingly common after under-qualified repairs.
Dirty Condenser Coil
This one gets overlooked because it comes on gradually. A condenser coil caked with dust, grease, or debris can’t reject heat efficiently. Head pressure climbs. The high-pressure switch trips the compressor off, box temperature rises, compressor restarts, cycle repeats. It looks like a controls problem but it’s a maintenance problem.
Walk-in condensers mounted in kitchen environments (near fryers or exhaust hoods) can foul out fast. A condenser that looks fine on the front can be matted solid from behind.
If the coil is accessible and you can see visible buildup on either side, that’s likely your culprit. Have a tech clean it as part of a maintenance visit, or schedule it before the unit goes back in service. Quarterly is a reasonable baseline for most kitchens. If the condenser is near a fryer or heavy exhaust, every six to eight weeks is smarter.
Defrost Controls and Door Seals (Worth Ruling Out)
Before assuming a refrigerant problem, check two simpler things.
First, door gaskets. A bad seal lets warm, humid air in constantly. The box struggles to recover. If you can see daylight around a closed door, or the gasket tears when you press it, that’s your first fix.
Second, defrost termination. If the defrost cycle isn’t terminating properly, the evaporator either runs too long in defrost (box gets warm) or doesn’t defrost enough (coil ices over, airflow drops, box gets warm). Check that the defrost termination thermostat is clipped securely to the evaporator coil. If the clip is broken or you suspect the thermostat has failed, that’s a quick fix for a tech, not a field DIY.
What a Tech Does to Diagnose This
A refrigeration tech will arrive with a manifold gauge set and a digital thermometer. They’ll check:
- Suction and discharge pressure (and compare to expected values for the refrigerant type and ambient temperature)
- Superheat and subcooling at the evaporator and condenser
- Compressor amp draw
- Defrost cycle timing and termination
This diagnostic usually takes 30-60 minutes. If it’s a TXV, expect the tech to recover refrigerant, swap the valve, pull a vacuum, recharge, and verify superheat before leaving. If it’s a leak, they’ll need to find and repair it before recharging, which adds time.
What You Can Check Right Now
A few things are worth ruling out before you call:
- Check the condenser coil for visible buildup on both front and back
- Check door gaskets for tears or gaps, and look for daylight around the door frame
- Verify the evaporator fan is running (all blades turning, no unusual noise)
- Make sure nothing is blocking airflow inside the box (product stacked too close to the evaporator)
- Review your defrost schedule and confirm it’s set correctly for your usage
If those checks don’t reveal an obvious problem, you’re likely looking at a TXV or refrigerant issue. Both need a licensed tech with gauges and EPA certification.
When to Call a Pro
If the box is swinging more than 5-6°F from setpoint regularly, it’s time for a tech. Temperature swings that cause product to repeatedly hit above 41°F are a food safety issue under the FDA Food Code, and most health departments don’t give much grace on documented temperature logs showing repeated excursions.
We work on walk-ins throughout the Bay Area, from San Jose up through Oakland and Marin. Give us a call and we’ll get you on the schedule fast, often same or next day when we can. We’ll diagnose it properly, not just add refrigerant and hope.