I searched and searched and searched yesterday for an answer to the question "Does altitude affect AFUE?" but did not come up with any definitive answers. Hopefully this discussion will answer that question more easily for folks in the future.
Derating combustion appliances typically applies the equation of about 2% per 1000 ft above sea level (from most manufacturer's specifications). So here in Santa Fe (at 7000 ft ASL), the derate would be 14%. Applying this to a 100 MBTUH input capacity appliance, the "natural" derating would make the appliance's input capacity actually 86 MBTUH. In all of the documentation I could find I could not determine if this derating ever affected the advertised AFUE or SSE rating, only the actual input/output in direct relation to the AFUE.
So my first question is, after an appliance has been naturally derated for altitude, can it be assumed to still function at the manufacturer's AFUE?
And then my second question is, assuming the above is true, and from what I have read here (http://hvac-talk.com/vbb/attachment.php?attachmentid=219042&d=1..., see appendix E), it would seem that Steady State Efficiency (and consequently, AFUE) actually improves with altitude. I know, right? Does anyone know at which rate the SSE (or AFUE) might improve with each 1000 ft increment above sea level?
Putting it the way you spelled it out so well helped me understand the whole process. So thanks also for your comments. I think this question has now been adequately answered! Thanks to everyone in this community. This website is full of great information and great minds!