Yesterday I noticed that my heat pump was running longer than usual and the aux heat strips kicked in at an outdoor temperature where the heat pump can usually handle the load itself. I checked the heat pump and there's a layer of frost/snow covering the entire coil. My service tech is coming to take a look next week (it's too cold now) and in the meantime I've shut off the heat pump and am running on emergency heat.
What could I be looking at here? Just want to be informed/prepared.