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.
OK, when you start talking about 3rd order effects of parameters, that really make me want to ask about my system and actually pay you for your advice!! Thank you for being here for the community, Dana!