Van heater not warming up-any auto mech types?

[forgop]

So, my wife's van isn't getting hot despite running for quite some time. On occasion, it may finally begin to produce hot air after 15 minutes of driving. If it matters at all, the engine temp got over the summer related to a stuck thermostat and it was fine after that. I'm not a heating or cooling system guy at all to know if one would have caused another. Is this likely a bad heater core? I haven't noticed a coolant smell at all. Blower is fine-the air just isn't getting warm except the rare occasion it may start warming up after driving several minutes even after the engine temp is fine.

Thanks for any suggestions you may have.