R-Pro › Field cases › Air conditioning
Evaporator Frost Diagnosis — 4 Causes (Undercharge / Overcharge / Airflow / Compressor)
Likely causes
Classify cause by frost location
Check: **Cause by frost location**: ① Front of evaporator (EEV/TXV side) frost = **undercharge** (suspect leak). ② Rear of evaporator + suction line frost = **overcharge** (risk of liquid slug to compressor). ③ Uniform frost over whole evaporator + low airflow = **dirty filter/blower/coil**. ④ Repeated compressor cycling + partial frost = **compressor efficiency drop** (long service life). **Suction pressure**: at 25°C, normal R-410A 130 psi / R-32 105 psi; under 50% = undercharge confirmed. **User action**: turn system OFF, allow 2-hour natural defrost.
What to do: **Per cause**: ① **Undercharge** = trace leak (soap foam/electronic), repair, vacuum, weighed recharge. ② **Overcharge** = recover excess to protect compressor. ③ **Low airflow** = clean/replace filter, coil cleaner spray, check blower motor. ④ **Compressor efficiency low** = compare nameplate RLA, megger insulation. <70% efficiency = consider replacement. **Defrost**: system OFF + breaker OFF + 1–2 hr natural thaw. **NEVER use hair dryer/heat gun directly** = aluminum fin damage.
**Field trick**: photo of frost location = 50% of diagnosis done. Tell user to turn OFF and photograph immediately on first frost. **Recurring frost = damage accumulating** (liquid slug, coil corrosion). One event → resolve root cause + monitor. ≥2 per season = full system inspection mandatory. **High- or low-pressure cutouts also spike** as frost destabilizes pressures.
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