I have a 2017 Elantra and the car gets up to temp, but will not blow out hot or even warm air unless the rpms are up. What could it be?

 

What could it be, maybe anyone of the following ?
Faulty thermostat
low coolant level
restriction in coolant lines to heater core
blocked core due to excessive dust accumulation,
blend door malfunction
Electronic control module for heat/ cooling
Dirty cabin air filter
Water pump
As you can see, it will require some investigating to determine which one or combination could be the cause.