what causes an evaporator to ice up
FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner The ice or frost formed on a cooling coil in an air conditioner air handler unit is usually caused by an improper refrigerant charge, possibly by inadequate air flow across the cooling coil, or by a thermostatic expansion valve (TEV) or other air conditioner or heat pump control defect.