Originally Posted by CivicSiRacer

Also could be a bad heater core (maybe clogged, maybe hose to the core is clogged).

Hrm also could be a bubble in the system too. Who replaced the radiator and filled it? Might have to raise the front by parking it on some wood planks. Make sure the car is cool and pop the radiator cap off. Turn the car on and let it warm up. Let it run for about 10-15 minutes making sure the radiator fan comes on, and turn on the heat to hot. If bubbles come up the neck of the radiator you might have air bubbles in the system. So as the coolant drops add more.