Yes, that is true but not the way your thinking.
There are two loops: One uses big hoses, the second uses smaller hoses. The big hoses are for the engine cooling to the radiator. Once the T-stat opens that loop takes away all the heat away from the engine starving the loop going to the car heater. Big pipes matter!
So AVGUY asked are the hoses to your heater hot? If so, then there is a problem with the blend door. You need to use SCIENCE (measurement) to properly diagnose car problems. If there are not hot then look at replacing the T-Stat.