No Need to Panic

If you have a heat pump, and it is cold outside, you might notice frost or ice forming on your outside unit. There is no need to panic. This could be normal. Heat pumps will ice up when the temperature drops real low. They have a sensor that will tell the unit to defrost itself. Normally the outside fan will stop running, but the compressor will still be running. Sometimes there is a loud "swishing" sound and a loud noise coming from the compressor. This is the reversing valve changing direction allowing the unit to heat up and melt the ice that has formed on itself. This should only last a minute or two. The unit will then switch back into heating mode, and the outdoor fan will turn back on. You may see a large plume of steam come out of the top of the unit. Sometimes this is mistaken as smoke. 

If the unit were to continue to ice up and form large quantities of frost and ice, there may be a problem with the sensor or defrost control board. That is when you want to call for service.