2002 chevy avalanche?

[ERIK H]

My dad has a 2002 chevy avalanche and the heater is not blowing out warm air after a while it will kinda get warm but if you have it on heat and hi it does not blow out the right way what could this be

 

[bowtiebad]

obstruction to the passage way or the water pump may be weak
THE INLET TUBE TO THE HEATER CORE COMES OFF THE ENGINE BELOW THE THERMOSTAT WHICH MEANS IT HEATS UP SIMULTANEOUSLY WITH THE ENGINE AT START UP TO 195 - 210 TEMPERATURE AND THEN THE THERMOSTAT WILL OPEN ALLOWING THE COOLANT TO CIRCULATE THROUGH OUT STAYING APPROX THAT TEMP NO CIRCULATION IS GOING TO BE A SIGN TO START LOOKING AT MAYBE A WEAK WATER PUMP OR AN OBSTRUCTION