During months with low outside temperature, frost or ice may appear on the outside coils of a heat pump. This is normal on our chilly Arizona morning, and may cause the heat pump to go into its designed "defrost cycle." The defrost cycle is programmed to melt the frost or ice on the outdoor coil. During this cycle of a few minutes, the heat pump will make a different sound as it "reverses" itself. You may notice steam coming off the coil, or water draining off the heat pump. All this is normal and means the unit is operating properly. Note: NEVER turn your thermostat off during the defrost cycle. During defrost, you may notice some cooler air coming from your supply air registers. This is normal and will only occur for a few minutes.
And remember, regular maintenance on your heating and air conditioning equipment it the best way to minimize breakdowns.