We have a Trane single package dual fuel heat pump that was installed last year. It is a combination electric heat pump and natural gas heater. These units are shipped in what is called Non-Restricted operation. In this mode, when the second stage of heat is called for, the heat pump is cut off and the gas furnace operates until both stages are satisfied. In the optional, Restricted Mode, the heat pump operates only at an outdoor temperature above a set point. We live in north central Arizona where it can be over 100F in the summer and well below freezing in the winter. It is currently 12F. At these temperatures, the heat pump runs almost continuously as it puts out just enough heat to keep the second stage gas heater from coming on. There has to be a 3-degree temp differential between the indoor thermostat and actual room temperature for the gas heat to kick in. Unfortunately, the Trane is just good enough to keep this from happening except when doing the initial warm-up in the morning. In these times when the heat pump just keeps trying to maintain temp, it would seem to make sense to let the gas heater come on for a while and get it over with. I'm thinking I should have the dealer set the Trane to the Restricted Mode so that the gas heat always comes on below say 32F. Does this make sense?

What causes the outside of the heat pump to freeze up with the heat on? The outside air temp is about 40. I have the thermostat set at 70.