TL;DR
A frozen coil means either blocked airflow or low refrigerant charge. Run the fan-only defrost cycle first, then address the root cause before restarting cooling.
What you might see
- ice visible on the evaporator coil or suction line
- supply air blowing warm despite compressor running
- very low suction pressure on gauges
- water dripping from unit after defrost
Likely causes
Severely restricted airflow from a blocked filter, closed supply dampers, or a failed blower motor causing the coil to drop below freezing
Low refrigerant charge causing suction pressure to drop below the freezing point of the coil surface moisture
Blower motor failure or belt slip reducing airflow below the minimum needed to absorb the coil's refrigeration capacity
Extended operation in very low ambient conditions outside the equipment's rated operating range
Required tools
- Manifold gauge set
- Clamp ammeter
- Replacement air filter
- LOTO kit
Safety first
- Never chip ice from the evaporator coil with a screwdriver or other tool. Coil tube puncture releases refrigerant.
- Lock out the compressor before accessing the evaporator section for inspection.
- EPA Section 608 certification is required for refrigerant system work.
Procedure
- 1
Switch the thermostat to fan-only mode (no cooling) to begin defrosting the coil. Do not use heat guns or sharp tools on the coil fins.
Warning: Sharp objects will puncture the refrigerant tubing in the coil. Use only warm air defrost. - 2
Allow the coil to fully defrost, typically 30-60 minutes. Verify the drain pan is draining the melt water.
- 3
Replace the air filter and verify all supply and return grilles are open and unobstructed.[1]
- 4
Check the blower motor amperage with a clamp ammeter. Verify the blower is running at rated speed. A failed motor or slipped belt causes low airflow.[1]
- 5
After defrost is complete, restart cooling and measure the suction pressure and superheat. Low suction pressure with high superheat confirms low refrigerant charge.
- 6
If refrigerant charge is confirmed low, locate and repair the leak before adding refrigerant. See the insufficient-cooling guide for the full refrigerant charge procedure.
- 7
Document the defrost event and root cause in the maintenance log. Recurrent freezing requires a root cause analysis, not just repeated defrost cycles.
Sources
Trane Voyager Commercial Rooftop Service Literature, Trane (Trane Technologies)
Trane Voyager Commercial Rooftop IOM, coil freeze-up diagnostic and defrost procedures (general)
View source
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Stop fixing the same fault twice.
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