I live in north Alabama and have a rheem 3 1/2 ton heat pump that is five years old. We are just now getting lows around thirty degrees. My question is how often should this unit go to defrost mode .it seems to be doing it everytime the unit cycles on which it runs about twenty minutes and shuts down for about five minutes constantly in this temperature range. I have a new non programmable Honeywell thormasat . Is it normal for it to defrost every time the unit comes on ?