2001 Honda Civic Overheating when you turn Heater on?
 

I have a Honda Civic that is overheating when I turn the defrost / heater on. I've had the water pump, timing belt, therm. all have been replaced and it is still over heating. The dealership has ran several test and can't seem to figure it out either. Test ran are: pressure test, they have actually flushed the system and re-ran test and all comes back ok. I'm hope someone can help me.