2003 Ford Explorer ENGINE WAS REPLACE AC BLOWING HOT AIR

HAD A USED ENGINE PUT IN 2003 FORD EXLPORER. I TURNED ON THE AC IT JUST BLOWS OUT HOT AIR. MY QUESTION IS DOES THE HOT AIR BLOWING OUT HAVE ANYTHING TO DO WITH THE REPLACING OF THE ENGINE. I SEE THAT MY WARRANTY SAID VOID IF OVERHEATING OCCURS. THANKS VERY MUCH FOR A ANSWER