Heater not blowing hot air

Most commonly there is an air pocket in the cooling system. First be sure the engine isn't overheating due to lack of coolant flow. Air pockets like to develop right under the thermostat, and thermostats don't open in response to hot air. They have to be hit with hot liquid. Look for a bleeder screw, threaded plug, or temperature sensor on or near the thermostat housing that can be removed to burp the air out of the system.

Once that is done, the top radiator hose should be hot indicating coolant is circulating. At that point, feel the heater hoses. They should be too hot to hold onto for very long.