My blazer has very little heat. Acts like it is drawing cold outside air. There is no switch to change from fresh air to inside air.

Is there something that changes from outside to inside air?

Has anyone changed from the clutched fan to an electric? What all is involved and how much cfm should the fan move?

 

What engine do you have? Does the vehicle have air conditioning?

 

4.3 vortec. Has towing package with big radiator. Heard that the orange long life antifreeze can setup and plug radiator and heater core if not changed often. Don't know how often the previous owner changed it.

 

Originally GM recommended 5 years or 150,000 mile interval for coolant service. Experience has found that 3 years or 36,000 is more like it. If it's not serviced often enough, yes, it can cause problems. Over time it can clog radiator and heater core passages. If the green ethylene glycol is introduced it will compound the problems.

Normal engine operating temp is just under 200F. Make sure your engine is at proper temp. Are both of the heater hoses hot and approximately the same temp where they enter and exit the heater core? If there is a noticeable difference, the core is clogged.

If you have A/C you should be able to select outside air or recirculate. Without A/C, IIRC, you're limited to outside air intake only.

Fan clutch: Provided it's operating properly, there's no need for an electric cooling fan.