Exactly what is "low heat" for making cheese sauce? Having a poor track record of making cheese sauces, I'm determined to do it right at least once in this decade.   I read that one should use "low heat" only.  Too much heat causes the cheese to go stringy or clumpy.  Okay - but just what is "low heat"?  Warm but not beyond what I can stick my finger in comfortably?  Can someone put a number on it, degrees centigrade?
Cheese have enzymes. Enzymes break quickly and easily at 150F/65 C. 50 Celsius for a short period of time should suffice to melt you cheese without breaking the protein structures.

I have committed the same mistake many times. Actually the fist day my in laws where over i tried to impress them with homemade pasta and cheese sauce. My Cheese sauce clumped.   My back up plan is to add milk tomato sauce put it on the blender until it liquifies.