The coolant can flow either way through the heater core. Flow direction is out of the thermostat housing to the heater core, and the return line goes to the water pump. If there is a cable controlled valve regulating coolant flow through the heater, it's usually better to plumb the "pressure" hose from the thermostat housing to the valve.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!