Motor will not start up (humms):

 There were 2 types of motors used. I suspect your heater has a starter relay. The solid state relay was used to allow the energizing of the start winding which would allow the motor to achieve approx 80% of rated speed, then the relay would de-energize the start winding to prevent overheating. There is no attachment between the motor and the relay, except for the wiring.

This is what the starter relay looks like. If your heater has one, check to see if it is energizing the starter windings at start-up. If not it is probably defective. The part is still available at http://www.reddyparts.com/, I think, but today's replacement part looks slightly different.

      or     

If your motor does not have the relay, then the motor has an internal start-up system, which is not serviceable, but the motor is probably still available.

The motor usually has the part number printed on it, but the number on the label may not be the current part number.  

Which motor does your heater have?