why don't my heat in my 2004 Buick get hot?

there's a door in the vent system that may not be opening all the way , if you can reach the heater hoses under the hood at the firewall you can if if they are both nice and hot , if not the heater core may be plugged , the contacts on the selector switch can be cleaned up if things aren't working at all , getting to these things is an ordeal