Have you made sure the heater core is clean? They can get just as dirty of the rest of the coolant system, and they don't create much heat when dirty.
This is why when people clean out their cooling system, they turn the heat to the max so cooling system cleaner circulates through the heater core.
This is why when people clean out their cooling system, they turn the heat to the max so cooling system cleaner circulates through the heater core.