The day has been really cold, and it has been below freezing for a week, when Bob gets a call from the dispatcher telling him that a customer has a very frozen heat pump outdoor coil.
Bob arrives at the house and talks to the homeowner who informs him, "The auxiliary heat light on the thermostat has been on for most of three days and the outdoor unit has a huge sheet of ice on it. I noticed that the neighbor's heat pump doesn't have ice on it."