Heater cores can leak and still provide hot air. Have you checked if it's plugged? Feel the two heater hoses after the engine is warmed up. They should be too hot to hold onto for very long. If they are not, no coolant is circulating through the heater core. They can often be unplugged by running water through it from a garden hose, but be aware clearing away sediment often uncovers a leak.
If the heater hoses are real hot, something is wrong with the temperature door in the heater box.