The normal flow of coolant is from the top of the engine into the heater core and back to the water pump. It's important to identify the inlet hose because you'll want to reverse flush the heater core. By reversing the hoses, the coolant will flow in reverse direction through the core, but it'll still be flowing in the same direction as far as the engine is concerned. This may not prevent it from carrying debris from the core into the engine cooling system.
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