My Carrier unit gets frozen up a lot because I tent to forget to change over to all gas when temperatures get colder.  I can have some big freeze over on the unit but it still manages to get heat out of the air. 

I have electric resistance heater in furnace also.  My problem is that I would rather use air source heat pump or go directly to propane and not use resistance heat due to costs.  At about $.06 for resistance heat, I can heat cheaper now with prepaid propane.  Problem is that I do not have a heating system that allows me to enter the price of propane into system so I have to make my own judgement call and switch it over manually.