My accord 2003 heater compartment heater only kicks in at at least 2000 rpm. The only way to get heat is if the engine runs faster. What could be the solution?

. Reverse flush the heater core. When the heater is on and the engine is at normal operating temperature, compare the heat in the lines going into and out of the heater core. That should tell you if the core needs flushing.

. Ensure that the blend door actuator is working properly.