I am concerned that he is worrying about the heater in August. But it must be an issue because it is his first post. If the blower is working, it could be blockage from an air pocket, low coolant level, or sediment in the core.
i saw there is somethign similar to a rats nest inside the heater box. i had one in my drivers side fresh air vent that was totally clogged. take out the heater and see what the problem is, i would change the core while you got it out to