is your engine warming up completely? how long does that take? if the engine is warming up normally, and your upper hose is hot as well as the lower, then you do not have a thermostat problem. if the t-stat was stuck closed you would overheat fairly quick and boil your coolant. if it was stuck open you would take forever and ever to warm up. if the tstat is fine and you have no hot air then you need to check for a plugged heatercore (not likely) or a malfunctioning blend door (bad lever, blend door motor, or stuck blend door). hope this helps also, make sure both lines going to and from the heater core (the only 2 heater coolant hoses that run into the firewall) are hot.