If the coolant temp guage is not going up to a little over 80, you need to replace the thermostat. Not only will this effect your comfort, it's not at all healthy for your engine.

If your coolant temp is coming up okay, but the heater is not making warm air, you have blockage in the heater core or hoses, or the climate control/monovalve are at fault. The mono valve typically fails open, so that is probably not the cause of such a problem.

Good luck,