Defrost cycle

A defrost cycle is an automatic operating period used by an air source heat pump to clear frost or ice from the outdoor heat exchanger. Frost can form when moisture in cold outside air freezes on the coil while the unit is extracting heat. During defrost, heating output may briefly pause or change, and product manuals can link defrost operation to water volume, flow rate, tank temperature or commissioning checks. The term names an operating mode; it is not a fault by itself, a planning verdict or proof that a heat pump is correctly sized.