It is normal for outdoor unit of your heat pump to accumulate some frost or even a light layer of ice in the winter. The coils get very cold when the heat pump is operating in heat mode and it is normal for the coils to accumulate a layer of frost or ice.
However, when the coils are iced up, heat transfer cannot occur efficiently and the unit will not operate properly. To remove the ice build-up, a heat pump periodically enters a defrost mode. While in defrost mode, the unit will switch to air conditioning mode but the outside fan will not run. This allows warm refrigerant to circulate through the oudoor unit and melt the ice and frost. When the cycle is complete, the unit will return to heat mode and you will hear a “whoosh” sound and see a cloud of water vapor coming out of the outdoor unit.
If your heat pump is not defrosting and instead allows ice to build up, you should have the system checked out by a good HVAC technician. You could have a faulty reversing valve, low refrigerant levels, or other issues. An ice build-up will cause the unit to lose efficiency and heat poorly. If too much ice builds up, you may hear a loud grinding noise as the fan blades hit the ice.
In some cases, an ice storm or freezing rain can cause a healthy system to ice up. You should remove the ice to help the system operate properly and prevent damage to the fan blades. Turn the system off and carefully remove the ice using a garden hose or a hair dryer. Avoid the temptation to pour hot water on the ice – this can damage the coils.
