still/always learning - i attended a seminar this week, specific to oil burners if it matters, and when the discussion turned to net stack temp, there wasn't an explanation for it's value regarding the overall performance, and the instructor, in an aside, didn't have a valid answer. the book even had an asterisk after saying that the subtracted value must not be missed. well....the book also didn't provide a use for the value. ie - that value tells you xyz. or, now you can plug that value into x formula. asking questions of vets and searching the various net sites didn't help. with a typical hand-held or big $ box, the needed values are already provided. what say yee about the value of this subtraction lesson?