R-Pro › Field cases › Air conditioning
AC Indoor Coil Frost — Insufficient Airflow / Undercharge
Likely causes
Identify frost cause (filter, blower, coil fouling, refrigerant)
Check: Coil surface = white frost or ice thickness. Power OFF 1 hour natural defrost → restore flow. Airflow <50% of normal = filter/blower fault. Normal airflow but frost = undercharge (suction below normal) or EEV stuck closed. Ambient >35°C with indoor setpoint <26°C = operating limit exceeded, frost possible.
What to do: Action per cause: filter/blower = clean/replace. Coil fouling = professional coil cleaner. Undercharge = trace leak then recharge by weight. EEV = diagnose/replace coil. Operating limits = educate user (≥26°C setpoint). Never use dryer/heat gun for defrost = damages aluminum fins.
Recurring frost = accumulating damage (liquid slug, coil corrosion). One occurrence = resolve cause immediately + monitor. ≥2 events per season = full system diagnostic. Educate user: turn OFF immediately when frost seen and call service.
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