Auto Air Conditioning & Heater Repair: Heater not working, heater control valve, chevy express

Question
Have a 98 Chevy express van.  The Heater is blowing cold air.  The AC works fine.  Blower fan works fine. The van runs fine.   Looks like I will need to take into shop but was wondering what the troubleshooting process is for a heater blowing cold air.  Could it be the thermostat? heater control valve or Heater core?
If its one of the above which ones would be easy to replace and which ones would you replace first.  Would it be worth the time to backflush the heater core?

Answer
HI Leo

Need to check if your control is doing what you are asking it to do.  In other words is disconnecting the compressor when you put it in heat.  Are your actuators working when you put it in heat .  Is the water circulating in the system the way it should.