Why Undercounter Chiller Won't Defrost: Quick Overview & Fix Tips

Undercounter chillers are the perfect cold storage solution, especially for commercial kitchens dealing with small footprints. But when they stop working correctly, like when the defrost fails, things get chaotic fast. Why should you care so much? In fact, a recent industry report states that defrost failure causes nearly 28% of all service calls in foodservice operations! That's huge! That’s why learning a bit about proper defrost troubleshooting is so important; it drastically cuts down on expensive breakdown time and unnecessary repair costs. Let's dive in and fix this!

How an Undercounter Chiller Defrost System Works

Before you troubleshoot, it helps to know the logic behind the system.

Most commercial undercounter chillers use one of three defrost types:

• Automatic (Electric or Hot Gas Defrost): Common in premium units such as Furnotel refrigeration series; uses heating elements or compressor pressure to melt frost periodically.
• Manual Defrost: Requires turning off the unit for a few hours.
• Off-Cycle Defrost: Relies on compressor rest intervals to let frost melt naturally.     

The defrost cycle keeps the evaporator coil free of ice, allowing cool air to circulate evenly. Without it, ice creates a barrier that traps cold air and forces the compressor to work overtime, leading to energy waste and early failure.

Common Reasons Why Your Chiller Won’t Defrost

Here’s a quick breakdown of the most frequent culprits behind a non-defrosting unit, from technical to environmental factors.

Pro Tip: If your chiller forms thick frost within 12–24 hours after cleaning, chances are high your defrost heater or timer has failed.

Technical Breakdown: Understanding the Defrost Components

1. Defrost Timer

Controls when and how long the system defrosts. Usually programmed for 15–30 minutes every 6 hours. A worn-out timer may freeze in one position, keeping your system “stuck in cooling.”

2. Defrost Heater

Located beneath or around the evaporator coil. Uses electrical resistance to melt accumulated frost. In an undercounter unit, the wattage typically ranges from 200–600W, depending on size.

3. Defrost Thermostat

Terminates the defrost cycle once the coil temperature reaches around 10°C. A faulty thermostat leads to either incomplete or continuous defrosting.

4. Drain Line & Pan

Collects and disposes of melted frost water. Clogging here is one of the most overlooked maintenance issues.

Step-by-Step Troubleshooting Guide

Follow these tested diagnostic steps to pinpoint and fix the issue efficiently.

Step 1: Check Power and Timer

1. Unplug the chiller and locate the defrost timer module (often behind the kick plate).
2. Manually rotate it until you hear a click; the unit should enter defrost mode.
3. If nothing happens, the timer is likely defective.

Step 2: Test the Defrost Heater

1. Disconnect power.
2. Use a multimeter to test the heater’s resistance (should read 10–50 ohms).
3. If the circuit is open, replace the heater assembly. ∗Click here for how to replace the defrost heater manually! 

Step 3: Inspect the Thermostat

1. Remove the thermostat from the evaporator.
2. Test resistance at 0°C (should read 0 ohms).
3. If it reads infinity, the thermostat has failed.

Step 4: Clear the Drain and Coils

1. Melt any visible ice manually using warm water.
2. Clean the coil fins using a soft brush and coil cleaner.
3. Check that the drain hose is not kinked or frozen.

Step 5: Recalibrate the Temperature Sensor

If your model uses a thermistor, compare its resistance reading with manufacturer's specs. For most undercounter chillers:

• 10°C = 4.9kΩ
• 0°C = 9.8kΩ
• –10°C = 18.6kΩ

Out-of-range readings indicate the sensor should be replaced.

Preventive Maintenance & Professional Recommendations

What to Avoid When Troubleshooting

• Never use sharp tools to remove ice; you risk puncturing the evaporator coil.
• Don’t bypass the thermostat unless temporarily for diagnostic purposes.
• Avoid overloading the chiller; poor air circulation accelerates icing.
• Never pour boiling water directly on the coils; rapid thermal shock can warp metal parts.

When to Call a Technician

If the defrost issue recurs within days or the compressor overheats, stop DIY repairs. Persistent frost formation could indicate:

• Refrigerant undercharge or overcharge
• Shorted defrost relay
• Faulty control board

A licensed commercial refrigeration technician can perform amp draw tests and refrigerant pressure checks that aren’t safe for untrained personnel.

FAQs About Undercounter Chillers and Defrosting

Q1: How often should an undercounter chiller defrost?
Every 6–8 hours under normal use. Frequency may vary depending on humidity and door openings.

Q2: Can I defrost a chiller manually?
Yes. Turn it off, remove food, and leave the door open for 3–4 hours. Clean up meltwater afterward.

Q3: Why does ice return right after defrosting?
Likely due to a broken door seal or humidity infiltration. Check gaskets and drain.

Q4: How do I know if my defrost timer is bad?
If the compressor runs continuously and no heat cycle activates, the timer is stuck.

Q5: Can a defrost issue increase electricity use?
Absolutely. A frozen evaporator coil can make your compressor run 40–60% longer.

Q6: Should I replace or repair my chiller?
If the repair cost exceeds 40% of the replacement price or the unit is over 8 years old, replacement is more cost-effective.