Why will my heater only work if car runs above 1500-2000 RPM's

The first thing that comes to mind reading your comments is coolant level. Check the cooling system level, if low, then look closely for a leak. When the cooling system is low, one of the first symptoms is lack of heat due to the reduction of flow.

I agree with Brian. Check the coolant level first. If that is good then look at the condition and tension on the water pump drive belt.