I have a heat pump system everything works great. However, why does the fan come on when the system is off? It will run about every 20 minutes and it acts like its going through a cycle. Is this normal? I thought maybe a contact switch but I don't think that it has one. Any suggestions or tell me what I need to do!

For starters, provide more info about the system (brand/model #'s) and the thermostat.