In HEAT operation, frost may occur on the outdoor unit and lower the heating capacity. In that case, the system switches into defrosting operation to take away the frost.HEAT operation stops suddenly and a flowing sound is heard.The outdoor unit is defrosting. HEAT operation starts after the frost on the outdoor unit has been removed. This can take about 4 to 12 minutes.The frost on the outdoor unit melts into water or steam when the air conditioner is in defrosting operation.