Aircon Frozen Coil, Low Gas Cause

Case Details

What We Checked

• Blower wheel was heavily caked with dust and debris. Airflow at the supply vent was barely perceptible.
• Ice had formed across the evaporator coil surface, concentrated where airflow was lowest.
• After defrosting, the coil fins showed no damage, corrosion, or deformation.
• Refrigerant pressures were not tested at this stage — airflow restriction was the clear primary suspect.

The Diagnosis

Nine years of operation without a chemical overhaul had left the blower wheel too clogged to move adequate air across the evaporator coil. In normal operation, warm room air passing over the coil absorbs cold and keeps the coil surface above freezing. When airflow drops below a critical threshold, the coil surface temperature falls past zero degrees because there is not enough warm air to balance the refrigerant temperature. Moisture in the remaining trickle of air condenses and freezes on contact, and the ice layer further blocks airflow — creating a self-reinforcing cycle. The coil itself, the refrigerant charge, and the entire sealed circuit were intact. The problem was purely mechanical: a clogged blower upstream.

What Fixed It

We recommended a full chemical overhaul rather than a simple filter clean, because the caking on the blower wheel was too compacted for dry brushing to remove. The overhaul covered the blower wheel, fan barrel, drainage tray, and coil surface. After reassembly, we ran the unit through a complete cooling cycle and monitored suction pressure to confirm it had returned to the normal operating range without any refrigerant work. We also showed the client the tissue test — holding a tissue at the supply vent to feel airflow strength — as a simple way to spot future blower degradation before it reaches the freezing point again.

The bedroom reached its set temperature with no ice forming on the coil. No parts were replaced and no refrigerant was added.

Why This Happens

Two reasons a coil freezes — and how to tell them apart.

• Low refrigerant drops coil pressure and temperature below freezing. Low airflow does the same thing by removing the warm air that normally keeps the coil above zero. The symptom is identical but the fix for each is completely different — one requires sealing a leak and recharging gas, the other requires cleaning.
• A blower check takes minutes and costs nothing. It should come before any refrigerant testing or coil replacement quote, because airflow restriction is the more common cause in units that have not had a chemical overhaul in several years.
• If cleaning the blower stops the freezing, the refrigerant circuit and coil were never at fault. Ask your technician to confirm this by monitoring suction pressure after the overhaul — normal readings prove the sealed system is intact.
• Ice on the coil creates a self-reinforcing cycle: the ice layer further blocks airflow, which drops the coil temperature further, which freezes more moisture. Turning the unit off to defrost before inspection is important because running it with ice on the coil strains the compressor.