2005 impala ls

Just some simple things to check first: Is your A/C on? A lot of cars, when in defrost mode, will override the user controls to apply heat to the glass. So when you switch out of defrost, if you have the A/C still on, it will go back into cooling mode. Also, do you have the physical dial set to warm? I know these are basic questions but you'd be surprised how often people don't think of these things. It does happen.

Thanks for your response. Yes I have checked all the simple things. With heat on vent it blows cold. But if I just switch to defrost it blows hot.