Heat pump icing up in summer — Leiden, Zuid-Holland

Ice forming on the outdoor heat pump unit or indoor evaporator coil during summer cooling is a clear sign something is wrong — the system is overcooling the refrigerant below the freezing point of condensation on the coils. The most common cause is restricted airflow: a dirty air filter, blocked return vents, or a failed blower motor. Low refrigerant charge (from a leak) is the second most common cause — less refrigerant means lower coil temperature and ice buildup. Other causes include a stuck or failed defrost board (on reversing models), a dirty evaporator coil, or a malfunctioning thermostatic expansion valve (TXV). An HVAC technician checks refrigerant charge, inspects the filter and coil, measures superheat and subcooling, and fixes the root cause. Running a frozen system damages the compressor — turn it off and let it thaw before a technician arrives.