A heater core will flow either way. The pressurized coolant comes from the thermostat housing, and returns to the water pump. That also keeps coolant circulating through the engine during warmup before the stat opens, unless someone has installed a cutoff valve in one of the heater hoses.
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!