Just saw the note (I don't check messages very often.)
My use of terms like "..3rd order effects..." is probably a first-order effect of my physics & engineering background, eh? :-) Third-order effects are sometimes important (think "Tacoma Narrows bridge collapse" . Who knew that the local wind gusts sometimes had a harmonic that would drive the fundamental vibration frequency of the bridge?), but the basics of designs are still all about the primary & secondary issues. In the condensing boiler combustion efficiency example return water temp is the primary factor, firing rate of the burner secondary, and boiler output temp is a distant third, barely worth mentioning.