Maybe too general of a question and since it's usually too late to really do anything about it maybe pointless but I'm curious.
From reading here there are tell tale signs ie short cycling ect. but w/ so many different types of units (1 stage, 2 stage, modulating) sometimes things aren't easy to spot for non-HVAC people. In some sense this might help people who currently are considering replacement. Not to mention giving them some kind of bargaining power..
I just replaced mine and none of the 5 estimates even bothered w/ any sort of analysis (I still think this is the norm in the real world) some "hints" might help people from making a costly mistake.
I actually downgraded my furnace because of all the reading I did here. most estimates wanted to just replace the current BTU rating and in some cases go up a bit. Since my old 1 stage did fine as to heat in my leaky old house (major temp swings of course but never any lack being able to keep up in the coldest of nights) at 90,000BTU's (90 eff duomatic olsen 23 yrs old)
and there are upgrades planned (which btw is also a wrench in any current analysis) on some windows (tons of single pane/alum storm combos probably every outside wall is 50+ glass, oh joy) but not possible for all that this is something homeowners should also take into consideration on a furnace upgrade. What is fine today may be over sized tomorrow.
so maybe homeowner eduction based on what there system is currently doing and how to spot if it's not quite right is helpful. not to mention if a "guy" is just selling a bill of goods. Obviously some people just ignore any advice for a bottom line. some things may never change.
As to my "downgrade" to 80,000btu w/ full mod furnace, sure is more comfortable and w/ these -10 plus nights and single digit days and how the furnace seems to be working it would be helpful to know what to look for... and to figure out if everyone did things right.
As to cost, no major change one way or another but the electric bill is down a bit, even w/ the mod seemly (seemingly because I can't always tell low fire w/ out sticking my face to the duct) running a blower 24/7 lately so that is a hopeful sign..
I suppose I shouldn't just pick on the heating aspect but I guess being a "northerner" it is just way more important....
Pretty sure I could have gone down one more step and still not froze, but oh well....I'll leave that to the next owner.