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Heater core is just an s-shaped pipe with fins so no IN or OUT assigned or needed.
If you are back flushing the heater core so want to see which one is OUT, then start engine and turn heater on.
Feel both hoses at the firewall, the one that warms up first or is hotter is the IN, making the other one OUT
I often reverse the heater hoses each time I change coolant, like back flushing on the fly, lol.
Last edited by RonD; 06-18-2016 at 02:47 PM.