I was having major problems with mine towards end of last week, when it was only -2/3. Locks frozen, then when the locks opened the dead-lock mechanism was frozen so was locked out for half hour!
However the solution as said before is WD40. Take your door card off, take your handle off. Spray as much as you can inside the door on the lock mechanism box and where everything goes into it, like wise with the handle and the back of the locking barrel and generally cover it in wd40!
The last week when its been -8 and below it hasnt even stuck let-alone frozen. And it would of frozen without the wd40 as the whole door was one sheet of ice including the handle - but it operated perfectly.