Usually they stop working as a heat pump around 35 degrees Fahrenheit, then your furnace/aux heat will take over. Depends on how it was set up by the installer.
__________________ Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?